Dimosthenis Sokaras

All Publications

• Deciphering decomposition pathways of high explosives with cryogenic X-ray Raman spectroscopy. Proceedings of the National Academy of Sciences of the United States of America Paredes Mellone, O. A., Nielsen, M. H., Babicz, J. T., Vinson, J., Willey, T. M., Sokaras, D. 2025; 122 (23): e2426320122

  Abstract

  We employed cryogenic X-ray Raman spectroscopy to investigate the early-stage decomposition of the high explosive molecule hexanitrohexaazaisowurtzitane (CL-20). By systematically varying the radiation dose under cryogenic conditions, we induced the decomposition of the molecule using ionizing radiation and observed the evolution of spectral features at the carbon, nitrogen, and oxygen K edges. Through extensive first-principles calculations, we identified key intermediates in the early stages of the decomposition process, resulting from C-C and C-N bond cleavage which leads to the opening of the internal cage structure. A detailed analysis of spectral trends and fingerprints provided evidence supporting N-NO2 homolytic cleavage as the primary initial decomposition pathway. The combination of advanced core-level spectroscopy methods and state-of-the-art theoretical calculations enabled a comprehensive characterization of the molecular changes induced by controlled radiation dose exposures. Our findings establish a benchmark for understanding the decomposition chemistry of high-explosive materials, offering important insights into their stability and reactivity under extreme conditions.

  View details for DOI 10.1073/pnas.2426320122

  View details for PubMedID 40460126

• Reaction Mechanism of the Synthesis of a Disordered Rock Salt Cathode Material CHEMISTRY OF MATERIALS Liang, Z., Marques, O., Mellone, O., Masina, S. M., Cao, C. L., Sokaras, D., Weker, J., Stone, K. H. 2025

  View details for DOI 10.1021/acs.chemmater.4c03071

  View details for Web of Science ID 001500035400001

• Dissolved Fe species enable a cooperative solid-molecular mechanism for the oxygen evolution reaction on NiFe-based catalysts NATURE CATALYSIS Kuai, C., Liu, L., Hu, A., Zhang, Y., Zhang, Y., Xia, D., Nordlund, D., Sokaras, D., Decarolis, D., Gianolio, D., Xin, H., Li, L., Lin, F. 2025

  View details for DOI 10.1038/s41929-025-01342-5

  View details for Web of Science ID 001491355800001

• Selenium Distribution and Speciation in Tissues from Rats Administered with Non-Native Selenotrisulfides INORGANIC CHEMISTRY Baker, A. T., Kidman, C. J., Vogt, L. I., George, G. N., Sokaras, D., Howard, D. L., Pape, V. S., Kulik, K., Yoshida, S., Harris, H. H. 2025; 64 (20): 9961-9983

  Abstract

  Selenotrisulfides (STS, R-S-Se-S-R) are metabolic intermediates in the bioconversion of inorganic Se species to organoselenium compounds. These Se species are reactive with a variety of endogenous molecules, particularly thiol-containing proteins, with this reactivity facilitating Se transport and subsequent utilization within the body. In this study, X-ray fluorescence microscopy (XFM) and high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) were applied to investigate Se distribution and speciation in vivo. Male rats administered with 1 mg Se/kg b.w. as selenious acid (SA), L-penicillamine selenotrisulfide (PenSSeSPen) or selenenyl penicillamine bound to rat serum albumin (RSA-SSeSPen) showed statistically significant elevations in Se concentrations in the kidney, liver, and blood after 48 h treatment; however, no change in Se concentration was observed in the testes. Notably, XFM revealed a strong colocalization of Se and Cu in the renal cortex, a phenomenon previously observed in cultured cells and in rats fed diets supplemented with 5 mg Se/kg as selenite. Linear combination and principal component analyses of Se Kα1 HERFD-XAS spectra revealed marked differences in Se speciation between the renal cortex and medulla and between red blood cells and plasma for all groups, including the control. STS were identified in linear combination fits of spectra from all tissues, except the testes. These results highlight the vital roles of STS in the intracellular reduction and transport of Se throughout the bloodstream and various tissues.

  View details for DOI 10.1021/acs.inorgchem.5c00323

  View details for Web of Science ID 001487296100001

  View details for PubMedID 40358531

• The Role of Cu3+ in the Oxygen Evolution Activity of Copper Oxides. Journal of the American Chemical Society Basera, P., Zhao, Y., Garcia-Esparza, A. T., Babbe, F., Bothra, N., Vinson, J., Sokaras, D., Yano, J., Boettcher, S. W., Bajdich, M. 2025

  Abstract

  Cu-based oxides and hydroxides represent an important class of materials from a catalytic and corrosion perspective. In this study, we investigate the formation of bulk and surface Cu3+ species that are stable under water oxidation catalysis in alkaline media. So far, no direct evidence existed for the presence of hydroxides (CuOOH) or oxides, which were primarily proposed by theory. This work directly places CuOOH in the oxygen evolution reaction (OER) Pourbaix stability region with a calculated free energy of -208.68 kJ/mol, necessitating a revision of known Cu-H2O phase diagrams. We also predict that the active sites of CuOOH for the OER are consistent with a bridge O* site between the two Cu3+ atoms with onset at ≥1.6 V vs the reversible hydrogen electrode (RHE), aligning with experimentally observed Cu2+/3+ oxidation waves in cyclic voltammetry of Fe-free and Fe-spiked copper in alkaline media. Trace amounts of Fe (2 μg/mL (ppm) to 5 μg/mL) in the solution measurably enhance the catalytic activity of the OER, likely due to the adsorption of Fe species that serve as the active sites . Importantly, modulation excitation X-ray absorption spectroscopy (ME-XAS) of a Cu thin-film electrode shows a distinct Cu3+ fingerprint under OER conditions at 1.8 V vs RHE. Additionally, in situ Raman spectroscopy of polycrystalline Cu in 0.1 mol/L (M) KOH revealed features consistent with those calculated for CuOOH in addition to CuO. Overall, this work provides direct evidence of bulk electrochemical Cu3+ species under OER conditions and expands our longstanding understanding of the oxidation mechanism and catalytic activity of copper.

  View details for DOI 10.1021/jacs.4c18147

  View details for PubMedID 40311110

• The electrode-electrolyte interface of Cu <i>via</i> modulation excitation X-ray absorption spectroscopy ENERGY & ENVIRONMENTAL SCIENCE Garcia-Esparza, A. T., Li, X., Babbe, F., Lim, J., Skoien, D., Simon, P. S., Yano, J., Sokaras, D. 2025

  View details for DOI 10.1039/d5ee01068c

  View details for Web of Science ID 001478165100001

• X-ray Absorption Spectroscopy of Dilute Metalloenzymes at X-ray Free-Electron Lasers in a Shot-by-Shot Mode. The journal of physical chemistry letters Bogacz, I., Szilagyi, E., Makita, H., Simon, P. S., Zhang, M., Doyle, M. D., Chatterjee, K., Kretzschmar, M., Chernev, P., Croy, N., Cheah, M. H., Dasgupta, M., Nangca, I., Fransson, T., Bhowmick, A., Brewster, A. S., Sauter, N. K., Owada, S., Tono, K., Zerdane, S., Oggenfuss, A., Babich, D., Sander, M., Mankowsky, R., Lemke, H. T., Gee, L. B., Sato, T., Kroll, T., Messinger, J., Alonso-Mori, R., Bergmann, U., Sokaras, D., Yachandra, V. K., Kern, J., Yano, J. 2025: 3778-3787

  Abstract

  X-ray absorption spectroscopy (XAS) of 3d transition metals provides important electronic structure information for many fields. However, X-ray-induced radiation damage under physiological temperature has prevented using this method to study dilute aqueous systems, such as metalloenzymes, as the catalytic reaction proceeds. Here we present a new approach to enable operando XAS of dilute biological samples and demonstrate its feasibility with K-edge XAS spectra from the Mn cluster in photosystem II and the Fe-S centers in photosystem I. This approach combines highly efficient sample delivery strategies and a robust signal normalization method with high-transmission Bragg diffraction-based spectrometers at X-ray free-electron lasers (XFELs) in a damage-free, shot-by-shot mode. These photon-out spectrometers have been optimized for discriminating the metal Mn/Fe Kα fluorescence signals from the overwhelming scattering background present on currently available detectors for XFELs that lack suitable energy discrimination. We quantify the enhanced performance metrics of the spectrometer and discuss its potential applications for acquiring time-resolved XAS spectra of biological samples during their reactions at XFELs.

  View details for DOI 10.1021/acs.jpclett.5c00399

  View details for PubMedID 40193717

• Revealing the Moon's Taurus-Littrow Landslide via Integrated Analysis of Pristine Apollo 17 Soil Core 73001/2 JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS Neuman, M., Jolliff, B. L., Wang, K., Petro, N., Valenciano, J., Neal, C. R., Eckley, S., Kent, J., Sun, L., Lucey, P., Bell, S., Joy, K., Tartese, R., Jones, R., Carpenter, P., Morris, R. V., Haney, N. C., Simon, S., Cato, M., Shearer, C. K., Welten, K. C., Nishiizumi, K., Caffee, M. W., Colina-Ruiz, R. A., Kroll, T., Sokaras, D., Ishii, H. A., Bradley, J. P., Gillis-Davis, J., Mcfadden, J. A., Thompson, M. S., Christoffersen, R., Keller, L. P., Simon, J. I., Mccubbin, F. M., Zeigler, R. A., Gross, J., Ketcham, R. A., Hanna, R. D., Edey, D. 2025; 130 (4)

  View details for DOI 10.1029/2024JE008556

  View details for Web of Science ID 001474034400001

• Determination of Thiol Protonation States by Sulfur X-ray Spectroscopy in Biological Systems JOURNAL OF PHYSICAL CHEMISTRY LETTERS Ribson, R. D., Follmer, A. H., Babicz, J. T., Alfaro, V., Hadt, R. G., Hunter, M. S., Wilson, M. A., Sokaras, D., Alonso-Mori, R. 2025; 16 (9): 2401-2408

  Abstract

  Cysteine is one of the most functionally diverse of the proteinogenic amino acids, owing to its reactive thiol side chain that can undergo deprotonation to form a strongly nucleophilic thiolate. However, few techniques can directly interrogate sulfur charge and covalency in cysteine, particularly in proteins. X-ray spectroscopies provide an element specific probe of sulfur. We demonstrate the sensitivity of S Kβ and Kα X-ray emission spectroscopy (XES) to cysteine ionization and compare it to S K-edge X-ray absorption spectroscopy (XAS) in the physiologically relevant biomolecules l-cysteine and N-acetyl-l-cysteine at room temperature in solution phase. Kβ XES and K-edge XAS are most sensitive to chemical changes at the cysteine thiol and can be used to evaluate the composition of thiol/thiolate mixtures. These results provide a foundation for assessing the pKa of functionally significant cysteine residues in proteins and open the door to time-resolved studies of cysteine-dependent enzymes.

  View details for DOI 10.1021/acs.jpclett.4c03247

  View details for Web of Science ID 001433819500001

  View details for PubMedID 40012333

  View details for PubMedCentralID PMC11892467

• Experimental electronic structures of the FeIV=O bond in S=1 heme vs. nonheme sites: Effect of the porphyrin ligand. Proceedings of the National Academy of Sciences of the United States of America Braun, A., Gee, L. B., Waters, M. D., Jose, A., Baker, M. L., Mara, M. W., Babicz, J. T., Ehudin, M. A., Quist, D. A., Zhou, A., Kroll, T., Titus, C. J., Lee, S. J., Nordlund, D., Sokaras, D., Yoda, Y., Kobayashi, Y., Tamasaku, K., Hedman, B., Hodgson, K. O., Karlin, K. D., Que, L., Solomon, E. I. 2025; 122 (8): e2420205122

  Abstract

  High-valent FeIV=O species are common intermediates in biological and artificial catalysts. Heme and nonheme S=1 FeIV=O sites have been synthesized and studied for decades but little quantitative experimental comparison of their electronic structures has been available, due to the lack of direct methods focused on the iron. This study allows a rigorous determination of the electronic structure of a nonheme FeIV=O center and its comparison to an FeIV=O heme site using 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy (XAS). Further, variable temperature magnetic circular dichroism (VT-MCD) of the ligand field transitions, combined with nuclear resonance vibrational spectroscopy of the two S=1 FeIV=O systems show that the equatorial ligand field decreases from a nonheme to a heme FeIV=O site. Alternatively, RIXS and Fe L-edge XAS combined with MCD show that the Fe dπ orbitals are unperturbed in the FeIV=O heme relative to the nonheme site because the strong axial Fe-O bond uncouples the Fe dπ orbitals from the porphyrin π-system. As a consequence, the thermodynamics and kinetics of the H-atom abstraction reactions are actually very similar for heme compound II and nonheme FeIV=O active sites.

  View details for DOI 10.1073/pnas.2420205122

  View details for PubMedID 39982745

• Structural Changes at the Zinc Active Site of ACE2 on Binding the SARS-CoV-2 Spike Protein Receptor Binding Domain. Inorganic chemistry Dolgova, N. V., Qureshi, M., Latimer, M., Grishin, A., Cygler, M., Vogt, L. I., Cotelesage, J. J., Sokaras, D., Kroll, T., Pickering, I. J., George, G. N. 2025

  Abstract

  The causative agent of Covid-19 is the SARS-CoV-2 virus. Initiation of cell entry by SARS-CoV-2 is critically dependent upon binding of the SARS-CoV-2 spike protein to angiotensin-converting enzyme 2 (ACE2, EC 3.4.17.23). The mechanism of binding of the SARS-CoV-2 spike receptor binding domain to ACE2 is among the most intensively studied infection mechanisms of any pathogen, including a very large number of structural studies. ACE2 is a membrane-associated zinc carboxypeptidase, comprising three domains, the protease domain, a neck domain, and a membrane-spanning α-helical domain. In addition to its role as a carboxypeptidase, ACE2 is also a chaperone for a Na+-amino acid cotransporter called B0AT1, and in the presence of B0AT1, full-length ACE2 forms dimers. Most studies to date related to Covid-19 have employed just the ACE2 protease domain and have neglected any possible roles of the Zn2+-containing ACE2 active site. We show here that ACE2, including the neck domain in addition to the protease domain (and in the absence of B0AT1), is dimeric and shows distinctive allostery in its catalytic activity. In contrast, the intensively studied protease domain is monomeric and shows no allostery. Binding of the spike receptor binding domain (RBD) to dimeric ACE2 eliminates its allostery. X-ray absorption spectroscopy of Zn2+ ACE2 shows distinctive changes in the active site structure upon binding of spike RBD but only in the dimeric form. Taken together, our results indicate that the Zn2+-containing active site exhibits a notable level of flexibility and that the dimeric form of ACE2, including both protease and neck domains, likely presents a superior model for the study of ACE2-spike interactions than the monomeric ACE2.

  View details for DOI 10.1021/acs.inorgchem.4c04974

  View details for PubMedID 39962897

• Identifying a highly efficient molecular photocatalytic CO₂ reduction system via descriptor-based high-throughput screening NATURE CATALYSIS Hu, Y., Yu, C., Wang, S., Wang, Q., Reinhard, M., Zhang, G., Zhan, F., Wang, H., Skoien, D., Kroll, T., Su, P., Li, L., Chen, A., Liu, G., Lv, H., Sokaras, D., Gao, C., Jiang, J., Tao, Y., Xiong, Y. 2025

  View details for DOI 10.1038/s41929-025-01291-z

  View details for Web of Science ID 001415630200001

• Platinum hydride formation during cathodic corrosion in aqueous solutions. Nature materials Hersbach, T. J., Garcia-Esparza, A. T., Hanselman, S., Paredes Mellone, O. A., Hoogenboom, T., McCrum, I. T., Anastasiadou, D., Feaster, J. T., Jaramillo, T. F., Vinson, J., Kroll, T., Garcia, A. C., Krtil, P., Sokaras, D., Koper, M. T. 2025

  Abstract

  Cathodic corrosion is an electrochemical phenomenon that etches metals at moderately negative potentials. Although cathodic corrosion probably occurs by forming a metal-containing anion, such intermediate species have not yet been observed. Here, aiming to resolve this long-standing debate, our work provides such evidence through X-ray absorption spectroscopy. High-energy-resolution X-ray absorption near-edge structure experiments are used to characterize platinum nanoparticles during cathodic corrosion in 10 mol l-1 NaOH. These experiments detect minute chemical changes in the Pt during corrosion that match first-principles simulations of X-ray absorption spectra of surface platinum multilayer hydrides. Thus, this work supports the existence of hydride-like platinum during cathodic corrosion. Notably, these results provide a direct observation of these species under conditions where they are highly unstable and where prominent hydrogen bubble formation interferes with most spectroscopy methods. Therefore, this work identifies the elusive intermediate that underlies cathodic corrosion.

  View details for DOI 10.1038/s41563-024-02080-y

  View details for PubMedID 39843682

  View details for PubMedCentralID 7768681

• When Photoelectrons Meet Gas Molecules: Determining the Role of Inelastic Scattering in Ambient Pressure X-ray Photoelectron Spectroscopy ACS CENTRAL SCIENCE Li, H., Jana, A., Garcia-Esparza, A. T., Li, X., Kaminsky, C. J., Hamlyn, R., Prabhakar, R., Atwater, H. A., Ager, J. W., Sokaras, D., Yano, J., Crumlin, E. J. 2024

  View details for DOI 10.1021/acscentsci.4c01841

  View details for Web of Science ID 001381708500001

• Experimental Definition of the S = 1 π vs S = 2 σ Reactivity and S = 2 Character in the Ground State of an S = 1 FeIVO Complex. Journal of the American Chemical Society Braun, A., Gee, L. B., Waters, M. D., Baker, M. L., Mara, M. W., Zhou, A., Kroll, T., Nordlund, D., Sokaras, D., Hedman, B., Hodgson, K. O., Que, L., Solomon, E. I. 2024

  Abstract

  Iron(IV)-oxo intermediates found in iron enzymes and artificial catalysts are competent for H atom abstraction in catalytic cycles. For S = 2 intermediates, both axial and equatorial approaches are well-established. The mechanism for S = 1 sites is not as well understood: an equatorial approach is more energetically favorable, and an axial approach requires crossing from the S = 1 to the S = 2 surface. In this study, we use 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy on the S = 1 [FeIVO(TMC)(CH3CN)]2+ and observe both S = 2 and S = 1 final states, which enables the experimental evaluation of the energetics of the axial and equatorial reactivity of an S = 1 FeIVO center on its S = 2 vs S = 1 surface. The observation of S = 2 final states in the RIXS spectrum demonstrates significant S = 2 character spin-orbit mixed into the S = 1 ground state.

  View details for DOI 10.1021/jacs.4c11034

  View details for PubMedID 39668699

• Isolating multiplet structure in 5f inverse photoemission SOLID STATE SCIENCES Tobin, J. G., Ramanantoanina, H., Daul, C., Roussel, P., Yu, S., Sokaras, D., Kutepov, A. 2025; 160

  View details for DOI 10.1016/j.solidstatesciences.2024.107779

  View details for Web of Science ID 001382318800001

• Direct observation of ultrafast cluster dynamics in supercritical carbon dioxide using X-ray Photon Correlation Spectroscopy. Nature communications Majumdar, A., Li, H., Muhunthan, P., Späh, A., Song, S., Sun, Y., Chollet, M., Sokaras, D., Zhu, D., Ihme, M. 2024; 15 (1): 10540

  Abstract

  Supercritical fluids exhibit distinct thermodynamic and transport properties, making them of particular interest for a wide range of scientific and engineering applications. These anomalous properties emerge from structural heterogeneities due to the formation of molecular clusters at conditions above the critical point. While the static behavior of these clusters and their effects on the thermodynamic response functions have been recognized, the relation between the ultrafast cluster dynamics and transport properties remains elusive. By measuring the intermediate scattering function in carbon dioxide at conditions near the critical point with X-ray photon correlation spectroscopy, we directly capture the cross-over dynamics between 4 and 13 picoseconds, revealing the transition between ballistic and diffusive motion. Complementary analysis using large-scale molecular dynamics simulations reveals that this behavior arises from collisions between unbound molecules and clusters. This study provides direct evidence of the ultrafast momentum exchange between clusters, which has significant impact on transport properties, solvation processes, and reaction kinetics in supercritical fluids.

  View details for DOI 10.1038/s41467-024-54782-1

  View details for PubMedID 39627208

  View details for PubMedCentralID PMC11615208

• Metal-Ligand Covalency in the Valence Excited States of Metal Dithiolenes Revealed by S 1s3p Resonant Inelastic X-ray Scattering. Journal of the American Chemical Society Larsen, C. B., Ledbetter, K., Nascimento, D. R., Biasin, E., Qureshi, M., Nowak, S. H., Sokaras, D., Govind, N., Cordones, A. A. 2024

  Abstract

  Metallo dithiolene complexes with biological and catalytic relevance are well-known for having strong metal-ligand covalency, which dictates their valence electronic structures. We present the resonant sulfur Kβ (1s3p) X-ray emission spectroscopy (XES) for a series of Ni and Cu bis(dithiolene) complexes to reveal the ligand sulfur contributions to both the occupied and unoccupied valence orbitals. While S K-edge X-ray absorption spectroscopy played a critical role in identifying the covalency of the unoccupied orbitals of metal dithiolenes, the present focus on XES explores the occupied density of states. For a series of [Cu(mnt)2]n- and [Ni(mnt)2]n- anions and dianions, a comparison of the nonresonant and resonant S Kβ XES spectra highlights the dramatic improvement in spectral resolution and corresponding ability to differentiate subtle changes in occupied electronic structure across the series. Furthermore, the use of resonant inelastic X-ray scattering (RIXS) probes the valence excited states and the core-valence couplings of the complexes. By employing a theoretical approach based on time-dependent density functional theory to interpret the RIXS spectra, we reveal how metal-ligand covalency influences the excited state energies and covalencies. We identify the low energy excited states as having the same symmetry as the nominal "ligand field" or "d-d" states that typically dominate the photophysics of 3d metal complexes but with significant metal-ligand charge transfer character dictated by their covalency. These results suggest that strong metal-ligand covalency can be used to influence the charge-transfer photochemistry of first row transition metal complexes.

  View details for DOI 10.1021/jacs.4c11667

  View details for PubMedID 39377493

• Structural evolution of liquid silicates under conditions in Super-Earth interiors. Nature communications Morard, G., Hernandez, J. A., Pege, C., Nagy, C., Libon, L., Lacquement, A., Sokaras, D., Lee, H. J., Galtier, E., Heimann, P., Cunningham, E., Glenzer, S. H., Vinci, T., Prescher, C., Boccato, S., Chantel, J., Merkel, S., Zhang, Y., Yang, H., Wei, X., Pandolfi, S., Mao, W. L., Gleason, A. E., Shim, S. H., Alonso-Mori, R., Ravasio, A. 2024; 15 (1): 8483

  Abstract

  Molten silicates at depth are crucial for planetary evolution, yet their local structure and physical properties under extreme conditions remain elusive due to experimental challenges. In this study, we utilize in situ X-ray diffraction (XRD) at the Matter in Extreme Conditions (MEC) end-station of the Linear Coherent Linac Source (LCLS) at SLAC National Accelerator Laboratory to investigate liquid silicates. Using an ultrabright X-ray source and a high-power optical laser, we probed the local atomic arrangement of shock-compressed liquid (Mg,Fe)SiO3 with varying Fe content, at pressures from 81(9) to 385(40) GPa. We compared these findings to ab initio molecular dynamics simulations under similar conditions. Results indicate continuous densification of the O-O and Mg-Si networks beyond Earth's interior pressure range, potentially altering melt properties at extreme conditions. This could have significant implications for early planetary evolution, leading to notable differences in differentiation processes between smaller rocky planets, such as Earth and Venus, and super-Earths, which are exoplanets with masses nearly three times that of Earth.

  View details for DOI 10.1038/s41467-024-51796-7

  View details for PubMedID 39362851

  View details for PubMedCentralID PMC11452200

• Apollo Next Generation Sample Analysis (ANGSA): an Apollo Participating Scientist Program to Prepare the Lunar Sample Community for Artemis SPACE SCIENCE REVIEWS Shearer, C. K., Mccubbin, F. M., Eckley, S., Simon, S. B., Meshik, A., Mcdonald, F., Schmitt, H. H., Zeigler, R. A., Gross, J., Mitchell, J., Krysher, C., Morris, R. V., Parai, R., Jolliff, B. L., Gillis-Davis, J. J., Joy, K. H., Bell, S. K., Lucey, P. G., Sun, L., Sharp, Z. D., Dukes, C., Sehlke, A., Mosie, A., Allton, J., Amick, C., Simon, J. I., Erickson, T. M., Barnes, J. J., Dyar, M. D., Burgess, K., Petro, N., Moriarty, D., Curran, N. M., Elsila, J. E., Colina-Ruiz, R. A., Kroll, T., Sokaras, D., Ishii, H. A., Bradley, J. P., Sears, D., Cohen, B., Pravdivseva, O., Thompson, M. S., Neal, C. R., Hana, R., Ketcham, R., Welten, K., ANGSA science team 2024; 220 (6): 62

  Abstract

  As a first step in preparing for the return of samples from the Moon by the Artemis Program, NASA initiated the Apollo Next Generation Sample Analysis Program (ANGSA). ANGSA was designed to function as a low-cost sample return mission and involved the curation and analysis of samples previously returned by the Apollo 17 mission that remained unopened or stored under unique conditions for 50 years. These samples include the lower portion of a double drive tube previously sealed on the lunar surface, the upper portion of that drive tube that had remained unopened, and a variety of Apollo 17 samples that had remained stored at -27 °C for approximately 50 years. ANGSA constitutes the first preliminary examination phase of a lunar "sample return mission" in over 50 years. It also mimics that same phase of an Artemis surface exploration mission, its design included placing samples within the context of local and regional geology through new orbital observations collected since Apollo and additional new "boots-on-the-ground" observations, data synthesis, and interpretations provided by Apollo 17 astronaut Harrison Schmitt. ANGSA used new curation techniques to prepare, document, and allocate these new lunar samples, developed new tools to open and extract gases from their containers, and applied new analytical instrumentation previously unavailable during the Apollo Program to reveal new information about these samples. Most of the 90 scientists, engineers, and curators involved in this mission were not alive during the Apollo Program, and it had been 30 years since the last Apollo core sample was processed in the Apollo curation facility at NASA JSC. There are many firsts associated with ANGSA that have direct relevance to Artemis. ANGSA is the first to open a core sample previously sealed on the surface of the Moon, the first to extract and analyze lunar gases collected in situ, the first to examine a core that penetrated a lunar landslide deposit, and the first to process pristine Apollo samples in a glovebox at -20 °C. All the ANGSA activities have helped to prepare the Artemis generation for what is to come. The timing of this program, the composition of the team, and the preservation of unopened Apollo samples facilitated this generational handoff from Apollo to Artemis that sets up Artemis and the lunar sample science community for additional successes.

  View details for DOI 10.1007/s11214-024-01094-x

  View details for Web of Science ID 001294423600001

  View details for PubMedID 39176178

  View details for PubMedCentralID PMC11335912

• Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes. Journal of the American Chemical Society Reinhard, M. E., Sidhu, B. K., Lozada, I. B., Powers-Riggs, N., Ortiz, R. J., Lim, H., Nickel, R., Lierop, J. v., Alonso-Mori, R., Chollet, M., Gee, L. B., Kramer, P. L., Kroll, T., Raj, S. L., van Driel, T. B., Cordones, A. A., Sokaras, D., Herbert, D. E., Gaffney, K. J. 2024

  Abstract

  To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

  View details for DOI 10.1021/jacs.4c02748

  View details for PubMedID 38889309

• Observation of a Picosecond Light-Induced Spin Transition in Polymeric Nanorods. ACS nano Reinhard, M., Kunnus, K., Ledbetter, K., Biasin, E., Zederkof, D. B., Alonso-Mori, R., van Driel, T. B., Nelson, S., Kozina, M., Borkiewicz, O. J., Lorenc, M., Cammarata, M., Collet, E., Sokaras, D., Cordones, A. A., Gaffney, K. J. 2024

  Abstract

  Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction could enable faster switching, but the photoinduced dynamics at the nanoscale remains poorly understood. Here, we report a femtosecond optical pump multimodal X-ray probe study of polymeric nanorods. Simultaneously tracking the ST order parameter with X-ray emission spectroscopy and structure with X-ray diffraction, we observe photodoping of the low-spin-lattice within ∼150 fs. Above a ∼16% photodoping threshold, the transition to the high-spin phase occurs following an incubation period assigned to vibrational energy redistribution within the nanorods activating the molecular spin switching. Above ∼60% photodoping, the incubation period disappears, and the transition completes within ∼50 ps, preceded by the elastic nanorod expansion in response to the photodoping. These results support the feasibility of ST material-based GHz optical switching applications.

  View details for DOI 10.1021/acsnano.3c10042

  View details for PubMedID 38833689

• The Liquid Jet Endstation for Hard X-ray Scattering and Spectroscopy at the Linac Coherent Light Source. Molecules (Basel, Switzerland) Antolini, C., Sosa Alfaro, V., Reinhard, M., Chatterjee, G., Ribson, R., Sokaras, D., Gee, L., Sato, T., Kramer, P. L., Raj, S. L., Hayes, B., Schleissner, P., Garcia-Esparza, A. T., Lim, J., Babicz, J. T., Follmer, A. H., Nelson, S., Chollet, M., Alonso-Mori, R., van Driel, T. B. 2024; 29 (10)

  Abstract

  The ability to study chemical dynamics on ultrafast timescales has greatly advanced with the introduction of X-ray free electron lasers (XFELs) providing short pulses of intense X-rays tailored to probe atomic structure and electronic configuration. Fully exploiting the full potential of XFELs requires specialized experimental endstations along with the development of techniques and methods to successfully carry out experiments. The liquid jet endstation (LJE) at the Linac Coherent Light Source (LCLS) has been developed to study photochemistry and biochemistry in solution systems using a combination of X-ray solution scattering (XSS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). The pump-probe setup utilizes an optical laser to excite the sample, which is subsequently probed by a hard X-ray pulse to resolve structural and electronic dynamics at their intrinsic femtosecond timescales. The LJE ensures reliable sample delivery to the X-ray interaction point via various liquid jets, enabling rapid replenishment of thin samples with millimolar concentrations and low sample volumes at the 120 Hz repetition rate of the LCLS beam. This paper provides a detailed description of the LJE design and of the techniques it enables, with an emphasis on the diagnostics required for real-time monitoring of the liquid jet and on the spatiotemporal overlap methods used to optimize the signal. Additionally, various scientific examples are discussed, highlighting the versatility of the LJE.

  View details for DOI 10.3390/molecules29102323

  View details for PubMedID 38792184

• Iron Kβ X-ray Emission Spectroscopy: The Origin of Spectral Features from Atomic to Molecular Systems Using Multi-configurational Calculations. The journal of physical chemistry. A Guo, M., Braun, A., Sokaras, D., Kroll, T. 2024

  Abstract

  Kβ X-ray emission spectroscopy (XES) is widely used to fingerprint the local spin of transition-metal ions, including in pump-probe experiments, to identify excited states or in chemical and biological reactions to characterize short-lived intermediates. In this study, the spectra of ferrous and ferric complexes for various spin states were measured experimentally and described theoretically through restricted active space (RAS) calculations including dynamic correlations. Through the RAS calculations from simple atomic models to complex molecular systems, spectral effects such as the exchange interactions, crystal-field strength, and covalent orbital mixing were evaluated and discussed. The calculations find that only the spectral features of low-spin cases show a dependence on the crystal-field strength, particularly for ferrous low spin. The effect of the covalent orbital mixing strength on the first moment of the Kβ1,3 main line and the Kβ1,3-Kβ' energy splitting is quantitatively described. Clear relationships are found within a given nominal spin but less between different spin states, which calls for careful selection of reference spectra in future experiments. This study further advances our understanding of the correlation between changes in experimental spectral features and their corresponding electronic structure information.

  View details for DOI 10.1021/acs.jpca.3c07949

  View details for PubMedID 38329897

• A versatile pressure-cell design for studying ultrafast molecular-dynamics in supercritical fluids using coherent multi-pulse x-ray scattering. The Review of scientific instruments Muhunthan, P., Li, H., Vignat, G., Toro, E. R., Younes, K., Sun, Y., Sokaras, D., Weiss, T., Rajkovic, I., Osaka, T., Inoue, I., Song, S., Sato, T., Zhu, D., Fulton, J. L., Ihme, M. 2024; 95 (1)

  Abstract

  Supercritical fluids (SCFs) can be found in a variety of environmental and industrial processes. They exhibit an anomalous thermodynamic behavior, which originates from their fluctuating heterogeneous micro-structure. Characterizing the dynamics of these fluids at high temperature and high pressure with nanometer spatial and picosecond temporal resolution has been very challenging. The advent of hard x-ray free electron lasers has enabled the development of novel multi-pulse ultrafast x-ray scattering techniques, such as x-ray photon correlation spectroscopy (XPCS) and x-ray pump x-ray probe (XPXP). These techniques offer new opportunities for resolving the ultrafast microscopic behavior in SCFs at unprecedented spatiotemporal resolution, unraveling the dynamics of their micro-structure. However, harnessing these capabilities requires a bespoke high-pressure and high-temperature sample system that is optimized to maximize signal intensity and address instrument-specific challenges, such as drift in beamline components, x-ray scattering background, and multi-x-ray-beam overlap. We present a pressure cell compatible with a wide range of SCFs with built-in optical access for XPCS and XPXP and discuss critical aspects of the pressure cell design, with a particular focus on the design optimization for XPCS.

  View details for DOI 10.1063/5.0158497

  View details for PubMedID 38170817

• A multimodal flow reactor for photocatalysis under atmospheric conditions JOURNAL OF CHEMICAL PHYSICS Garcia-Esparza, A. T., Qureshi, M., Skoien, D., Hersbach, T. P., Sokaras, D. 2023; 159 (24)

  Abstract

  Photocatalysis is a promising concept for the direct conversion of solar energy into fuels and chemicals. The design, experimental protocol, and performance of a multimodal and versatile flow reactor for the characterization of powdered and immobilized photocatalysts are herein presented. Ultimately, this instrument enables rigorous evaluation of photocatalysis performance metrics. The apparatus quantifies transient gas-phase reaction products via online real-time gas analyzer mass spectrometry (RTGA-MS). For H2, the most challenging gas, the photocatalytic system's RTGA-MS gas detection sensitivity spans over three orders of magnitude and can detect down to tens of parts per million under atmospheric conditions. Using Pt nanoparticles supported on anatase TiO2 photocatalyst via wet impregnation, the instrument's capability for the characterization of photocatalytic H2 evolution is demonstrated, resulting in an apparent quantum yield (AQY) of 48.1% ± 0.9% at 320 nm, 45.7% ± 0.3% at 340 nm and 31% ± 1% at 360 nm. The photodeposition of Pt on anatase TiO2 was employed to demonstrate the instrument's capability to track the transient behavior of photocatalysts, resulting in an improved 55% ± 2% AQY for H2 evolution at 340 nm from aqueous methanol. This photocatalytic instrument enables systematic study of a wide variety of photocatalytic reactions such as water splitting and CO2 reduction to valuable C2+ fuels and chemicals.

  View details for DOI 10.1063/5.0179259

  View details for Web of Science ID 001133660000005

  View details for PubMedID 38153150

  View details for PubMedCentralID PMC10756709

• Synchrotron speciation of umbilical cord mercury and selenium after environmental exposure in Niigata NEUROTOXICOLOGY Weng, M., Dolgova, N. V., Vogt, L. I., Qureshi, M., Sokaras, D., Kroll, T., Saito, H., O'Donoghue, J. L., Watson, G. E., Myers, G. J., Sekikawa, T., Pickering, I. J., George, G. N. 2024; 100: 117-123

  Abstract

  The insidious and deadly nature of mercury's organometallic compounds is informed by two large scale poisonings due to industrial mercury pollution that occurred decades ago in Minamata and Niigata, Japan. The present study examined chemical speciation for both mercury and selenium in a historic umbilical cord sample from a child born to a mother who lived near the Agano River in Niigata. The mother had experienced mercury exposure leading to more than 50 ppm mercury measured in her hair and was symptomatic 9 years prior to the birth. We sought to determine the mercury and selenium speciation in the child's cord using Hg Lα1 and Se Kα1 high-energy resolution fluorescence detected X-ray absorption spectroscopy, the chemical speciation of mercury was found to be predominantly organometallic and coordinated to a thiolate. The selenium was found to be primarily in an organic form and at levels higher than those of mercury, with no evidence of mercury-selenium chemical species. Our results are consistent with mercury exposure at Niigata being due to exposure to organometallic mercury species.

  View details for DOI 10.1016/j.neuro.2023.12.011

  View details for Web of Science ID 001156245200001

  View details for PubMedID 38128735

  View details for PubMedCentralID PMC11682714

• Tracking Active Phase Behavior on Boron Nitride during the Oxidative Dehydrogenation of Propane Using Operando X-ray Raman Spectroscopy. Journal of the American Chemical Society Cendejas, M. C., Paredes Mellone, O. A., Kurumbail, U., Zhang, Z., Jansen, J. H., Ibrahim, F., Dong, S., Vinson, J., Alexandrova, A. N., Sokaras, D., Bare, S. R., Hermans, I. 2023

  Abstract

  Hexagonal boron nitride (hBN) is a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP) to propylene. Using a variety of ex situ characterization techniques, the activity of the catalyst has been attributed to the formation of an amorphous boron oxyhydroxide surface layer. The ODHP reaction mechanism proceeds via a combination of surface mediated and gas phase propagated radical reactions with the relative importance of both depending on the surface-to-void-volume ratio. Here we demonstrate the unique capability of operando X-ray Raman spectroscopy (XRS) to investigate the oxyfunctionalization of the catalyst under reaction conditions (1 mm outer diameter reactor, 500 to 550 °C, P = 30 kPa C3H8, 15 kPa O2, 56 kPa He). We probe the effect of a water cofeed on the surface of the activated catalyst and find that water removes boron oxyhydroxide from the surface, resulting in a lower reaction rate when the surface reaction dominates and an enhanced reaction rate when the gas phase contribution dominates. Computational description of the surface transformations at an atomic-level combined with high precision XRS spectra simulations with the OCEAN code rationalize the experimental observations. This work establishes XRS as a powerful technique for the investigation of light element-containing catalysts under working conditions.

  View details for DOI 10.1021/jacs.3c08679

  View details for PubMedID 37931025

• Ultrafast x-ray detection of low-spin iron in molten silicate under deep planetary interior conditions. Science advances Shim, S. H., Ko, B., Sokaras, D., Nagler, B., Lee, H. J., Galtier, E., Glenzer, S., Granados, E., Vinci, T., Fiquet, G., Dolinschi, J., Tappan, J., Kulka, B., Mao, W. L., Morard, G., Ravasio, A., Gleason, A., Alonso-Mori, R. 2023; 9 (42): eadi6153

  Abstract

  The spin state of Fe can alter the key physical properties of silicate melts, affecting the early differentiation and the dynamic stability of the melts in the deep rocky planets. The low-spin state of Fe can increase the affinity of Fe for the melt over the solid phases and the electrical conductivity of melt at high pressures. However, the spin state of Fe has never been measured in dense silicate melts due to experimental challenges. We report detection of dominantly low-spin Fe in dynamically compressed olivine melt at 150 to 256 gigapascals and 3000 to 6000 kelvin using laser-driven shock wave compression combined with femtosecond x-ray diffraction and x-ray emission spectroscopy using an x-ray free electron laser. The observation of dominantly low-spin Fe supports gravitationally stable melt in the deep mantle and generation of a dynamo from the silicate melt portion of rocky planets.

  View details for DOI 10.1126/sciadv.adi6153

  View details for PubMedID 37862409

• Solution phase high repetition rate laser pump x-ray probe picosecond hard x-ray spectroscopy at the Stanford Synchrotron Radiation Lightsource STRUCTURAL DYNAMICS-US Reinhard, M., Skoien, D., Spies, J. A., Garcia-Esparza, A. T., Matson, B. D., Corbett, J., Tian, K., Safranek, J., Granados, E., Strader, M., Gaffney, K. J., Alonso-Mori, R., Kroll, T., Sokaras, D. 2023; 10 (5): 054304

  Abstract

  We present a dedicated end-station for solution phase high repetition rate (MHz) picosecond hard x-ray spectroscopy at beamline 15-2 of the Stanford Synchrotron Radiation Lightsource. A high-power ultrafast ytterbium-doped fiber laser is used to photoexcite the samples at a repetition rate of 640 kHz, while the data acquisition operates at the 1.28 MHz repetition rate of the storage ring recording data in an alternating on-off mode. The time-resolved x-ray measurements are enabled via gating the x-ray detectors with the 20 mA/70 ps camshaft bunch of SPEAR3, a mode available during the routine operations of the Stanford Synchrotron Radiation Lightsource. As a benchmark study, aiming to demonstrate the advantageous capabilities of this end-station, we have conducted picosecond Fe K-edge x-ray absorption spectroscopy on aqueous [FeII(phen)3]2+, a prototypical spin crossover complex that undergoes light-induced excited spin state trapping forming an electronic excited state with a 0.6-0.7 ns lifetime. In addition, we report transient Fe Kβ main line and valence-to-core x-ray emission spectra, showing a unique detection sensitivity and an excellent agreement with model spectra and density functional theory calculations, respectively. Notably, the achieved signal-to-noise ratio, the overall performance, and the routine availability of the developed end-station have enabled a systematic time-resolved science program using the monochromatic beam at the Stanford Synchrotron Radiation Lightsource.

  View details for DOI 10.1063/4.0000207

  View details for Web of Science ID 001094050700002

  View details for PubMedID 37901682

  View details for PubMedCentralID PMC10613086

• Unraveling Metal-Ligand Bonding in an HNO-Evolving {FeNO}6 Complex with a Combined X-ray Spectroscopic Approach. Journal of the American Chemical Society Gee, L. B., Lim, J., Kroll, T., Sokaras, D., Alonso-Mori, R., Lee, C. 2023

  Abstract

  Photolytic delivery of nitric oxide and nitroxide has substantial biomedical and phototherapeutic applications. Here, we utilized hard X-ray spectroscopic methods to identify key geometric and electronic structural features of two photolabile {FeNO}6 complexes where the compounds differ in the presence of a pendant thiol in [Fe(NO)(TMSPS2)(TMSPS2H)] and thioether in [Fe(NO)(TMSPS2)(TMSPS2CH3)] with the former complex being the only transition metal system to photolytically generate HNO. Fe Kbeta XES identifies the photoreactant systems as essentially Fe(II)-NO+, while valence-to-core XES extracts a NO oxidation state of +0.5. Finally, the pre-edge of the Fe high-energy-resolution fluorescence detected (HERFD) XAS spectra is shown to be acutely sensitive to perturbation of the Fe-NO covalency enhanced by the 3d-4p orbital mixing dipole intensity contribution. Collectively, this X-ray spectroscopic approach enables future time-resolved insights in these systems and extensions to other challenging redox noninnocent {FeNO}x systems.

  View details for DOI 10.1021/jacs.3c04479

  View details for PubMedID 37610249

• X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity. Journal of the American Chemical Society Braun, A., Gee, L. B., Mara, M. W., Hill, E. A., Kroll, T., Nordlund, D., Sokaras, D., Glatzel, P., Hedman, B., Hodgson, K. O., Borovik, A. S., Baker, M. L., Solomon, E. I. 2023

  Abstract

  Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy, providing structural insight and allowing the distinction of 6- vs 5-coordinate active sites as shown through application to the Fe(IV)═O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)═O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3dxz/yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.

  View details for DOI 10.1021/jacs.3c06181

  View details for PubMedID 37590931

• Kβ X-ray Emission Spectroscopy of Cu(I)-Lytic Polysaccharide Monooxygenase: Direct Observation of the Frontier Molecular Orbital for H2O2 Activation. Journal of the American Chemical Society Lim, H., Brueggemeyer, M. T., Transue, W. J., Meier, K. K., Jones, S. M., Kroll, T., Sokaras, D., Kelemen, B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2023

  Abstract

  Lytic polysaccharide monooxygenases (LPMOs) catalyze the degradation of recalcitrant carbohydrate polysaccharide substrates. These enzymes are characterized by a mononuclear Cu(I) active site with a three-coordinate T-shaped "His-brace" configuration including the N-terminal histidine and its amine group as ligands. This study explicitly investigates the electronic structure of the d10 Cu(I) active site in a LPMO using Kβ X-ray emission spectroscopy (XES). The lack of inversion symmetry in the His-brace site enables the 3d/p mixing required for intensity in the Kβ valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kβ XES data are correlated to density functional theory (DFT) calculations to define the bonding, and in particular, the frontier molecular orbital (FMO) of the Cu(I) site. These experimentally validated DFT calculations are used to evaluate the reaction coordinate for homolytic cleavage of the H2O2 O-O bond and understand the contribution of this FMO to the low barrier of this reaction and how the geometric and electronic structure of the Cu(I)-LPMO site is activated for rapid reactivity with H2O2.

  View details for DOI 10.1021/jacs.3c04048

  View details for PubMedID 37441786

• Uncovering the 3d and 4d Electronic Interactions in Solvated Ru Complexes with 2p3d Resonant Inelastic X-ray Scattering. Inorganic chemistry Poulter, B. I., Biasin, E., Nowak, S. H., Kroll, T., Alonso-Mori, R., Schoenlein, R. W., Govind, N., Sokaras, D., Khalil, M. 2023

  Abstract

  The electronic structure and dynamics of ruthenium complexes are widely studied given their use in catalytic and light-harvesting materials. Here we investigate three model Ru complexes, [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4-, with L3-edge 2p3d resonant inelastic X-ray scattering (RIXS) to probe unoccupied 4d valence orbitals and occupied 3d orbitals and to gain insight into the interactions between these levels. The 2p3d RIXS maps contain a higher level of spectral information than the L3 X-ray absorption near edge structure (XANES). This study provides a direct measure of the 3d spin-orbit splittings of 4.3, 4.0, and 4.1 eV between the 3d5/2 and 3d3/2 orbitals of the [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4- complexes, respectively.

  View details for DOI 10.1021/acs.inorgchem.3c00919

  View details for PubMedID 37314410

• Energy conversion and storage via photoinduced polarization change in non-ferroelectric molecular [CoGa] crystals. Nature communications Sadhukhan, P., Wu, S. Q., Kanegawa, S., Su, S. Q., Zhang, X., Nakanishi, T., Long, J. I., Gao, K., Shimada, R., Okajima, H., Sakamoto, A., Chiappella, J. G., Huzan, M. S., Kroll, T., Sokaras, D., Baker, M. L., Sato, O. 2023; 14 (1): 3394

  Abstract

  To alleviate the energy and environmental crisis, in the last decades, energy harvesting by utilizing optical control has emerged as a promising solution. Here we report a polar crystal that exhibits photoenergy conversion and energy storage upon light irradiation. The polar crystal consists of dinuclear [CoGa] molecules, which are oriented in a uniform direction inside the crystal lattice. Irradiation with green light induces a directional intramolecular electron transfer from the ligand to a low-spin CoIII centre, and the resultant light-induced high-spin CoII excited state is trapped at low temperature, realizing energy storage. Additionally, electric current release is observed during relaxation from the trapped light-induced metastable state to the ground state, because the intramolecular electron transfer in the relaxation process is accompanied with macroscopic polarization switching at the single-crystal level. It demonstrates that energy storage and conversion to electrical energy is realized in the [CoGa] crystals, which is different from typical polar pyroelectric compounds that exhibit the conversion of thermal energy into electricity.

  View details for DOI 10.1038/s41467-023-39127-8

  View details for PubMedID 37296168

  View details for PubMedCentralID PMC10256679

• Tracking the Dynamics of a Ag-MnO x Oxygen Reduction Catalyst Using In Situ and Operando X-ray Absorption Near-Edge Spectroscopy ACS ENERGY LETTERS Schroder, J., Zamora Zeledon, J. A., Kamat, G. A., Kreider, M. E., Wei, L., Mule, A. S., Torres, A., Yap, K., Sokaras, D., Gallo, A., Stevens, M., Jaramillo, T. F. 2023

  View details for DOI 10.1021/acsenergylett.3c00823

  View details for Web of Science ID 001015819200001

• The Local Electronic Structure of Supercritical CO2 from X-ray Raman Spectroscopy and Atomistic-Scale Modeling. The journal of physical chemistry letters Muhunthan, P., Paredes Mellone, O., Kroll, T., Sokaras, D., Ihme, M. 2023: 4955-4961

  Abstract

  Supercritical CO2 is encountered in several technical and natural systems related to biology, geophysics, and engineering. While the structure of gaseous CO2 has been studied extensively, the properties of supercritical CO2, particularly close to the critical point, are not well-known. In this work, we combine X-ray Raman spectroscopy, molecular dynamics simulations, and first-principles density functional theory (DFT) calculations to characterize the local electronic structure of supercritical CO2 at conditions around the critical point. The X-ray Raman oxygen K-edge spectra manifest systematic trends associated with the phase change of CO2 and the intermolecular distance. Extensive first-principles DFT calculations rationalize these observations on the basis of the 4ssigma Rydberg state hybridization. X-ray Raman spectroscopy is found to be a sensitive tool for characterizing electronic properties of CO2 under challenging experimental conditions and is demonstrated to be a unique probe for studying the electronic structure of supercritical fluids.

  View details for DOI 10.1021/acs.jpclett.3c00668

  View details for PubMedID 37216638

• Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy. Nature communications Reinhard, M., Gallo, A., Guo, M., Garcia-Esparza, A. T., Biasin, E., Qureshi, M., Britz, A., Ledbetter, K., Kunnus, K., Weninger, C., van Driel, T., Robinson, J., Glownia, J. M., Gaffney, K. J., Kroll, T., Weng, T. C., Alonso-Mori, R., Sokaras, D. 2023; 14 (1): 2443

  Abstract

  Reliably identifying short-lived chemical reaction intermediates is crucial to elucidate reaction mechanisms but becomes particularly challenging when multiple transient species occur simultaneously. Here, we report a femtosecond x-ray emission spectroscopy and scattering study of the aqueous ferricyanide photochemistry, utilizing the combined Fe Kβ main and valence-to-core emission lines. Following UV-excitation, we observe a ligand-to-metal charge transfer excited state that decays within 0.5 ps. On this timescale, we also detect a hitherto unobserved short-lived species that we assign to a ferric penta-coordinate intermediate of the photo-aquation reaction. We provide evidence that bond photolysis occurs from reactive metal-centered excited states that are populated through relaxation of the charge transfer excited state. Beyond illuminating the elusive ferricyanide photochemistry, these results show how current limitations of Kβ main line analysis in assigning ultrafast reaction intermediates can be circumvented by simultaneously using the valence-to-core spectral range.

  View details for DOI 10.1038/s41467-023-37922-x

  View details for PubMedID 37147295

  View details for PubMedCentralID 16604

• Sulfur X-ray Absorption and Emission Spectroscopy of Organic Sulfones. The journal of physical chemistry. A Vogt, L. I., Cotelesage, J. J., Dolgova, N. V., Boyes, C., Qureshi, M., Sokaras, D., Sharifi, S., George, S. J., Pickering, I. J., George, G. N. 2023

  Abstract

  The sulfones are a widespread group of organo-sulfur compounds, which contain the sulfonyl SO2 group attached to two carbons and have a formal sulfur oxidation state of +2. We have examined the sulfur K near-edge X-ray absorption spectroscopy (XAS) of a range of different sulfones and find substantial spectroscopic variability depending upon the nature of the coordination to the sulfonyl group. We have also examined the sulfur Kβ X-ray emission spectroscopy (XES) of selected representative sulfones. Density functional theory simulations show satisfactory reproduction of both absorption and emission spectra while enabling assignment of the various transitions comprising the spectra. The correspondence between observed and simulated spectra shows promise for ab initio prediction of sulfur X-ray absorption and emission spectra of sulfones of any substituent. The absorption spectra and, to a lesser extent, the emission spectra are sensitive to the nature of the organic groups bound to the sulfonyl (SO2) moiety, clearly showing the potential of X-ray spectroscopy as an in situ probe of sulfone chemistry.

  View details for DOI 10.1021/acs.jpca.2c08647

  View details for PubMedID 36912654

• Understanding the Stability of Manganese Chromium Antimonate Electrocatalysts through Multimodal In Situ and Operando Measurements. Journal of the American Chemical Society Kreider, M. E., Kamat, G. A., Zamora Zeledón, J. A., Wei, L., Sokaras, D., Gallo, A., Stevens, M. B., Jaramillo, T. F. 2022

  Abstract

  Improving electrocatalyst stability is critical for the development of electrocatalytic devices. Herein, we utilize an on-line electrochemical flow cell coupled with an inductively coupled plasma-mass spectrometer (ICP-MS) to characterize the impact of composition and reactant gas on the multielement dissolution of Mn(-Cr)-Sb-O electrocatalysts. Compared to Mn2O3 and Cr2O3 oxides, the antimonate framework stabilizes Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore, dissolution of Mn and Cr from Mn(-Cr) -Sb-O is driven by the ORR reaction rate, with minimal dissolution under N2. We observe preferential dissolution of Cr totaling 13% over 10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating an enrichment of Mn at the surface of the particles. Despite this asymmetric dissolution, operando X-ray absorption spectroscopy (XAS) showed no measurable changes in the Mn K-edge at comparable potentials. This could suggest that modification to the Mn oxidation state and/or phase in the surface layer is too small or that the layer is too thin to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS was used to assess the effects of applied potential, scan rate, and current on Mn-Cr-Sb-O during cyclic voltammetry and accelerated stress tests. With this deeper understanding of the interplay between oxygen reduction and dissolution, testing procedures were identified to maximize both activity and stability. This work highlights the use of multimodal in situ characterization techniques in tandem to build a more complete model of stability and develop protocols for optimizing catalyst performance.

  View details for DOI 10.1021/jacs.2c08600

  View details for PubMedID 36453840

• Investigating the electronic structure of high explosives with X-ray Raman spectroscopy. Scientific reports Paredes-Mellone, O. A., Nielsen, M. H., Vinson, J., Moua, K., Skoien, K. D., Sokaras, D., Willey, T. M. 2022; 12 (1): 19460

  Abstract

  We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures ([Formula: see text] 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

  View details for DOI 10.1038/s41598-022-24066-z

  View details for PubMedID 36376464

• 5f Covalency from X-Ray Resonant Raman Spectroscopy. Journal of physics. Condensed matter : an Institute of Physics journal Tobin, J. G., Nowak, S., Yu, S. W., Alonso Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2022

  Abstract

  X-Ray Resonant Raman Scattering (XRRS), a variant of Resonant X-ray Inelastic Scattering (RXIS), has been used to investigate the two prototype systems, UF4 and UO2. Both are U5f2 and each is an example of 5f localized, ionic behavior and 5f localized, covalent behavior, respectively. From the M5 XRRR measurements, the 5f band gap in each can be directly determined and, moreover, a clear and powerful sensitivity to 5f covalency emerges.

  View details for DOI 10.1088/1361-648X/ac9bbd

  View details for PubMedID 36261038

• Alzheimer's Drug PBT2 Interacts with the Amyloid β 1-42 Peptide Differently than Other 8-Hydroxyquinoline Chelating Drugs. Inorganic chemistry Summers, K. L., Roseman, G., Schilling, K. M., Dolgova, N. V., Pushie, M. J., Sokaras, D., Kroll, T., Harris, H. H., Millhauser, G. L., Pickering, I. J., George, G. N. 2022; 61 (37): 14626-14640

  Abstract

  Although Alzheimer's disease (AD) was first described over a century ago, it remains the leading cause of age-related dementia. Innumerable changes have been linked to the pathology of AD; however, there remains much discord regarding which might be the initial cause of the disease. The "amyloid cascade hypothesis" proposes that the amyloid β (Aβ) peptide is central to disease pathology, which is supported by elevated Aβ levels in the brain before the development of symptoms and correlations of amyloid burden with cognitive impairment. The "metals hypothesis" proposes a role for metal ions such as iron, copper, and zinc in the pathology of AD, which is supported by the accumulation of these metals within amyloid plaques in the brain. Metals have been shown to induce aggregation of Aβ, and metal ion chelators have been shown to reverse this reaction in vitro. 8-Hydroxyquinoline-based chelators showed early promise as anti-Alzheimer's drugs. Both 5-chloro-7-iodo-8-hydroxyquinoline (CQ) and 5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline (PBT2) underwent unsuccessful clinical trials for the treatment of AD. To gain insight into the mechanism of action of 8HQs, we have investigated the potential interaction of CQ, PBT2, and 5,7-dibromo-8-hydroxyquinoline (B2Q) with Cu(II)-bound Aβ(1-42) using X-ray absorption spectroscopy (XAS), high energy resolution fluorescence detected (HERFD) XAS, and electron paramagnetic resonance (EPR). By XAS, we found CQ and B2Q sequestered ∼83% of the Cu(II) from Aβ(1-42), whereas PBT2 sequestered only ∼59% of the Cu(II) from Aβ(1-42), suggesting that CQ and B2Q have a higher relative Cu(II) affinity than PBT2. From our EPR, it became clear that PBT2 sequestered Cu(II) from a heterogeneous mixture of Cu(II)Aβ(1-42) species in solution, leaving a single Cu(II)Aβ(1-42) species. It follows that the Cu(II) site in this Cu(II)Aβ(1-42) species is inaccessible to PBT2 and may be less solvent-exposed than in other Cu(II)Aβ(1-42) species. We found no evidence to suggest that these 8HQs form ternary complexes with Cu(II)Aβ(1-42).

  View details for DOI 10.1021/acs.inorgchem.2c01694

  View details for PubMedID 36073854

• A new μ-high energy resolution fluorescence detection microprobe imaging spectrometer at the Stanford Synchrotron Radiation Lightsource beamline 6-2 REVIEW OF SCIENTIFIC INSTRUMENTS Edwards, N. P., Bargar, J. R., van Campen, D., van Veelen, A., Sokaras, D., Bergmann, U., Webb, S. M. 2022; 93 (8): 083101

  Abstract

  Here, we describe a new synchrotron X-ray Fluorescence (XRF) imaging instrument with an integrated High Energy Fluorescence Detection X-ray Absorption Spectroscopy (HERFD-XAS) spectrometer at the Stanford Synchrotron Radiation Lightsource at beamline 6-2. The X-ray beam size on the sample can be defined via a range of pinhole apertures or focusing optics. XRF imaging is performed using a continuous rapid scan system with sample stages covering a travel range of 250 × 200 mm2, allowing for multiple samples and/or large samples to be mounted. The HERFD spectrometer is a Johann-type with seven spherically bent 100 mm diameter crystals arranged on intersecting Rowland circles of 1 m diameter with a total solid angle of about 0.44% of 4π sr. A wide range of emission lines can be studied with the available Bragg angle range of ∼64.5°-82.6°. With this instrument, elements in a sample can be rapidly mapped via XRF and then selected features targeted for HERFD-XAS analysis. Furthermore, utilizing the higher spectral resolution of HERFD for XRF imaging provides better separation of interfering emission lines, and it can be used to select a much narrower emission bandwidth, resulting in increased image contrast for imaging specific element species, i.e., sparse excitation energy XAS imaging. This combination of features and characteristics provides a highly adaptable and valuable tool in the study of a wide range of materials.

  View details for DOI 10.1063/5.0095229

  View details for Web of Science ID 000843037400005

  View details for PubMedID 36050052

  View details for PubMedCentralID PMC9392580

• X-ray Raman Scattering: A Hard X-ray Probe of Complex Organic Systems CHEMICAL REVIEWS Georgiou, R., Sahle, C. J., Sokaras, D., Bernard, S., Bergmann, U., Rueff, J., Bertrand, L. 2022: 12977-13005

  Abstract

  This paper provides a review of the characterization of organic systems via X-ray Raman scattering (XRS) and a step-by-step guidance for its application. We present the fundamentals of XRS required to use the technique and discuss the main parameters of the experimental set-ups to optimize spectral and spatial resolution while maximizing signal-to-background ratio. We review applications that target the analysis of mixtures of organic compounds, the identification of minor spectral features, and the spatial discrimination in heterogeneous systems. We discuss the recent development of the direct tomography technique, which utilizes the XRS process as a contrast mechanism for assessing the three-dimensional spatially resolved carbon chemistry of complex organic materials. We conclude by exposing the current limitations and provide an outlook on how to overcome some of the existing challenges and advance future developments and applications of this powerful technique for complex organic systems.

  View details for DOI 10.1021/acs.chemrev.1c00953

  View details for Web of Science ID 000821478600001

  View details for PubMedID 35737888

• Effect of doping TiO₂ with Mn for electrocatalytic oxidation in acid and alkaline electrolytes ENERGY ADVANCES Vallez, L., Jimenez-Villegas, S., Garcia-Esparza, A. T., Jiang, Y., Park, S., Wu, Q., Gill, T., Sokaras, D., Siahrostami, S., Zheng, X. 2022; 1 (6): 357-366

  View details for DOI 10.1039/d2ya00027j

  View details for Web of Science ID 001105879200001

• Molecular Fates of Organometallic Mercury in Human Brain. ACS chemical neuroscience James, A. K., Dolgova, N. V., Nehzati, S., Korbas, M., Cotelesage, J. J., Sokaras, D., Kroll, T., O'Donoghue, J. L., Watson, G. E., Myers, G. J., Pickering, I. J., George, G. N. 2022; 13 (12): 1756-1768

  Abstract

  Mercury is ubiquitous in the environment, with rising levels due to pollution and climate change being a current global concern. Many mercury compounds are notorious for their toxicity, with the potential of organometallic mercury compounds for devastating effects on the structures and functions of the central nervous system being of particular concern. Chronic exposure of human populations to low levels of methylmercury compounds occurs through consumption of fish and other seafood, although the health consequences, if any, from this exposure remain controversial. We have used high energy resolution fluorescence detected X-ray absorption spectroscopy to determine the speciation of mercury and selenium in human brain tissue. We show that the molecular fate of mercury differs dramatically between individuals who suffered acute organometallic mercury exposure (poisoning) and individuals with chronic low-level exposure from a diet rich in marine fish. For long-term low-level methylmercury exposure from fish consumption, mercury speciation in brain tissue shows methylmercury coordinated to an aliphatic thiolate, resembling the coordination environment observed in marine fish. In marked contrast, for short-term high-level exposure, we observe the presence of biologically less available mercuric selenide deposits, confirmed by X-ray fluorescence imaging, as well as mercury(II)-bis-thiolate complexes, which may be signatures of severe poisoning in humans. These differences between low-level and high-level exposures challenge the relevance of studies involving acute exposure as a proxy for low-level chronic exposure.

  View details for DOI 10.1021/acschemneuro.2c00166

  View details for PubMedID 35543423

• Efficient and Stable Acidic Water Oxidation Enabled by Low-Concentration, High-Valence Iridium Sites ACS ENERGY LETTERS Shi, X., Peng, H., Hersbach, T. P., Jiang, Y., Zeng, Y., Baek, J., Winther, K. T., Sokaras, D., Zheng, X., Bajdich, M. 2022

  View details for DOI 10.1021/acsenergylett.2c00578

  View details for Web of Science ID 000821179900001

• Disentangling the chemistry of Australian plant exudates from a unique historical collection PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Georgiou, R., Popelka-Filcoff, R. S., Sokaras, D., Beltran, V., Bonaduce, I., Spangler, J., Cohen, S. X., Lehmann, R., Bernard, S., Rueff, J., Bergmann, U., Bertrand, L. 2022; 119 (22): e2116021119

  Abstract

  For thousands of years, the unique physicochemical properties of plant exudates have defined uses in material culture and practical applications. Native Australian plant exudates, including resins, kinos, and gums, have been used and continue to be used by Aboriginal Australians for numerous technical and cultural purposes. A historic collection of well-preserved native Australian plant exudates, assembled a century ago by plant naturalists, gives a rare window into the history and chemical composition of these materials. Here we report the full hierarchical characterization of four genera from this collection, Xanthorrhoea, Callitris, Eucalyptus, and Acacia, from the local elemental speciation, to functional groups and main molecular markers. We use high-resolution X-ray Raman spectroscopy (XRS) to achieve bulk-sensitive chemical speciation of these plant exudates, including insoluble, amorphous, and cross-linked fractions, without the limitation of invasive and/or surface specific methods. Combinatorial testing of the XRS data allows direct classification of these complex natural species as terpenoid, aromatic, phenolic, and polysaccharide materials. Differences in intragenera chemistry was evidenced by detailed interpretation of the XRS spectral features. We complement XRS with Fourier-transform infrared (FT-IR) spectroscopy, gas chromatography–mass spectrometry (GC-MS), and pyrolysis–GC-MS (Py-GC-MS). This multimodal approach provides a fundamental understanding of the chemistry of these natural materials long used by Aboriginal Australian peoples.

  View details for DOI 10.1073/pnas.2116021119

  View details for Web of Science ID 001042953600001

  View details for PubMedID 35617429

  View details for PubMedCentralID PMC9295781

• Thorium model and weak 5f delocalization JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Tobin, J. G., Nowak, S., Yu, S. W., Roussel, P., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T. C., Sokaras, D. 2022; 40 (3)

  View details for DOI 10.1116/6.0001754

  View details for Web of Science ID 000795077500001

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Corrigendum. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2022; 29 (Pt 3): 930

  Abstract

  A figure in the article by Huyke et al. [(2021), J. Synchrotron Rad. 28, 1100-1113] is corrected.

  View details for DOI 10.1107/S1600577522002806

  View details for PubMedID 35511027

• Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2. ACS nano Garcia-Esparza, A. T., Park, S., Abroshan, H., Paredes Mellone, O. A., Vinson, J., Abraham, B., Kim, T. R., Nordlund, D., Gallo, A., Alonso-Mori, R., Zheng, X., Sokaras, D. 2022

  Abstract

  The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS2. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS2 remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS2 under reaction conditions (H2 atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H2 atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS2 is herein elucidated for applications in optoelectronics and catalysis.

  View details for DOI 10.1021/acsnano.2c01388

  View details for PubMedID 35380038

• Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury. Inorganic chemistry Nehzati, S., Dolgova, N. V., Young, C. G., James, A. K., Cotelesage, J. J., Sokaras, D., Kroll, T., Qureshi, M., Pickering, I. J., George, G. N. 2022; 61 (13): 5201-5214

  Abstract

  Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD-XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.

  View details for DOI 10.1021/acs.inorgchem.1c03196

  View details for PubMedID 35073478

• Generation of intense phase-stable femtosecond hard X-ray pulse pairs. Proceedings of the National Academy of Sciences of the United States of America Zhang, Y., Kroll, T., Weninger, C., Michine, Y., Fuller, F. D., Zhu, D., Alonso-Mori, R., Sokaras, D., Lutman, A. A., Halavanau, A., Pellegrini, C., Benediktovitch, A., Yabashi, M., Inoue, I., Inubushi, Y., Osaka, T., Yamada, J., Babu, G., Salpekar, D., Sayed, F. N., Ajayan, P. M., Kern, J., Yano, J., Yachandra, V. K., Yoneda, H., Rohringer, N., Bergmann, U. 2022; 119 (12): e2119616119

  Abstract

  SignificanceThe generation of phase-stable femtosecond X-ray pulse pairs will advance nonlinear spectroscopies and imaging, providing direct insight into the coupled motions of electrons and nuclei with resolution on the electronic length scale and timescale. This paper presents the generation of such pulse pairs in the X-ray domain. The approach uses X-ray free-electron laser pulses to induce highly directional, intense, phase-stable pairs of superfluorescence and seeded stimulated emission at the 5.9 keV manganese Kalpha1 line. The finding is evidenced by strong interference fringes in the superfluorescence and stimulated emission signals.

  View details for DOI 10.1073/pnas.2119616119

  View details for PubMedID 35290124

• Unoccupied electronic structure of actinide dioxides PHYSICAL REVIEW B Tobin, J. G., Ramanantoanina, H., Daul, C., Roussel, P., Yu, S., Nowak, S., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2022; 105 (12)

  View details for DOI 10.1103/PhysRevB.105.125129

  View details for Web of Science ID 000800754600006

• Tunable metal hydroxide-organic frameworks for catalysing oxygen evolution NATURE MATERIALS Yuan, S., Peng, J., Cai, B., Huang, Z., Garcia-Esparza, A. T., Sokaras, D., Zhang, Y., Giordano, L., Akkiraju, K., Zhu, Y., Huebner, R., Zou, X., Roman-Leshkov, Y., Shao-Horn, Y. 2022; 21 (6): 673-+

  Abstract

  The oxygen evolution reaction is central to making chemicals and energy carriers using electrons. Combining the great tunability of enzymatic systems with known oxide-based catalysts can create breakthrough opportunities to achieve both high activity and stability. Here we report a series of metal hydroxide-organic frameworks (MHOFs) synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked using aromatic carboxylate linkers. MHOFs act as a tunable catalytic platform for the oxygen evolution reaction, where the π-π interactions between adjacent stacked linkers dictate stability, while the nature of transition metals in the hydroxides modulates catalytic activity. Substituting Ni-based MHOFs with acidic cations or electron-withdrawing linkers enhances oxygen evolution reaction activity by over three orders of magnitude per metal site, with Fe substitution achieving a mass activity of 80 A [Formula: see text] at 0.3 V overpotential for 20 h. Density functional theory calculations correlate the enhanced oxygen evolution reaction activity with the MHOF-based modulation of Ni redox and the optimized binding of oxygenated intermediates.

  View details for DOI 10.1038/s41563-022-01199-0

  View details for Web of Science ID 000760696700002

  View details for PubMedID 35210585

• Ultrafast structural response of shock-compressed plagioclase METEORITICS & PLANETARY SCIENCE Gleason, A. E., Park, S., Rittman, D. R., Ravasio, A., Langenhorst, F., Bolis, R. M., Granados, E., Hok, S., Kroll, T., Sikorski, M., Weng, T., Lee, H., Nagler, B., Sisson, T., Xing, Z., Zhu, D., Giuli, G., Mao, W. L., Glenzer, S. H., Sokaras, D., Alonso-Mori, R. 2022

  View details for DOI 10.1111/maps.13785

  View details for Web of Science ID 000755921800001

• Electronic structure studies reveal 4f/5d mixing and its effect on bonding characteristics in Ce-imido and -oxo complexes CHEMICAL SCIENCE Moreau, L. M., Lapsheva, E., Amaro-Estrada, J., Gau, M. R., Carroll, P. J., Manor, B. C., Qiao, Y., Yang, Q., Lukens, W. W., Sokaras, D., Schelter, E. J., Maron, L., Booth, C. H. 2022; 13 (6): 1759-1773

  Abstract

  This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2- t BuNO)C6H4CH2)3N]3- (TriNO x 3-) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce LIII absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce[double bond, length as m-dash]O vs. Ce[double bond, length as m-dash]N) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

  View details for DOI 10.1039/d1sc06623d

  View details for Web of Science ID 000746089000001

  View details for PubMedID 35282640

  View details for PubMedCentralID PMC8827158

• Characterization of a Dynamic Y2Ir2O7 Catalyst during the Oxygen Evolution Reaction in Acid JOURNAL OF PHYSICAL CHEMISTRY C Hubert, M. A., Gallo, A., Liu, Y., Valle, E., Sanchez, J., Sokaras, D., Sinclair, R., King, L. A., Jaramillo, T. F. 2022

  View details for DOI 10.1021/acs.jpcc.1c07760

  View details for Web of Science ID 000746684300001

• Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency. The journal of physical chemistry letters Liekhus-Schmaltz, C., Fox, Z. W., Andersen, A., Kjaer, K. S., Alonso-Mori, R., Biasin, E., Carlstad, J., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Poulter, B. I., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Cordones, A. A., Schoenlein, R. W., Govind, N., Khalil, M. 1800: 378-386

  Abstract

  Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)5FeIICNRuIII(NH3)5]-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an 60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

  View details for DOI 10.1021/acs.jpclett.1c03613

  View details for PubMedID 34985900

• Mechanistic and Electronic Insights into a Working NiAu Single-Atom Alloy Ethanol Dehydrogenation Catalyst. Journal of the American Chemical Society Giannakakis, G., Kress, P., Duanmu, K., Ngan, H. T., Yan, G., Hoffman, A. S., Qi, Z., Trimpalis, A., Annamalai, L., Ouyang, M., Liu, J., Eagan, N., Biener, J., Sokaras, D., Flytzani-Stephanopoulos, M., Bare, S. R., Sautet, P., Sykes, E. C. 1800

  Abstract

  Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multipronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective nonoxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge that increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogeneous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step toward developing structure-function relationships that inform the design of new catalysts.

  View details for DOI 10.1021/jacs.1c09274

  View details for PubMedID 34908398

• Comment on "Underlying simplicity of 5f unoccupied electronic structure" [J. Vac. Sci. Technol. A 39, 043205 (2021)] JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Tobin, J. G., Nowak, S., Yu, S., Roussel, P., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2021; 39 (6)

  View details for DOI 10.1116/6.0001315

  View details for Web of Science ID 000698374700001

• Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K₂WO₄/SiO₂ Catalyst Revealed by In Situ Characterization ACS CATALYSIS Li, D., Yoshida, S., Siritanaratkul, B., Garcia-Esparza, A. T., Sokaras, D., Ogasawara, H., Takanabe, K. 2021; 11 (22): 14237-14248

  View details for DOI 10.1021/acscatal.1c04206

  View details for Web of Science ID 000758014300004

• Effects of x-ray free-electron laser pulse intensity on the Mn K beta(1,3) x-ray emission spectrum in photosystem II-A case study for metalloprotein crystals and solutions STRUCTURAL DYNAMICS-US Fransson, T., Alonso-Mori, R., Chatterjee, R., Cheah, M., Ibrahim, M., Hussein, R., Zhang, M., Fuller, F., Gul, S., Kim, I., Simon, P. S., Bogacz, I., Makita, H., de Lichtenberg, C., Song, S., Batyuk, A., Sokaras, D., Massad, R., Doyle, M., Britz, A., Weninger, C., Zouni, A., Messinger, J., Yachandra, V. K., Yano, J., Kern, J., Bergmann, U. 2021; 8 (6): 064302

  Abstract

  In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.

  View details for DOI 10.1063/4.0000130

  View details for Web of Science ID 000723107700001

  View details for PubMedID 34849380

  View details for PubMedCentralID PMC8610604

• Manipulating electron redistribution to achieve electronic pyroelectricity in molecular [FeCo] crystals. Nature communications Sadhukhan, P., Wu, S., Long, J. I., Nakanishi, T., Kanegawa, S., Gao, K., Yamamoto, K., Okajima, H., Sakamoto, A., Baker, M. L., Kroll, T., Sokaras, D., Okazawa, A., Kojima, N., Shiota, Y., Yoshizawa, K., Sato, O. 2021; 12 (1): 4836

  Abstract

  Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

  View details for DOI 10.1038/s41467-021-25041-4

  View details for PubMedID 34376674

• Base-Accelerated Degradation of Nanosized Platinum Electrocatalysts ACS CATALYSIS Hersbach, T. P., Garcia, A. C., Kroll, T., Sokaras, D., Koper, M. M., Garcia-Esparza, A. T. 2021; 11 (15): 9904-9915

  View details for DOI 10.1021/acscatal.1c02468

  View details for Web of Science ID 000684035000092

• High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: An Analytical Method for Selenium Speciation. Analytical chemistry Nehzati, S., Dolgova, N. V., James, A. K., Cotelesage, J. J., Sokaras, D., Kroll, T., George, G. N., Pickering, I. J. 2021; 93 (26): 9235-9243

  Abstract

  Selenium is in many ways an enigmatic element. It is essential for health but toxic in excess, with the difference between the two doses being narrower than for any other element. Environmentally, selenium is of concern due to its toxicity. As the rarest of the essential elements, its low levels often provide challenges to the analytical chemist. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation but is severely limited by poor spectroscopic resolution arising from core-hole lifetime broadening. Here we explore selenium Kα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as a novel approach for chemical speciation of selenium, in comparison with conventional Se K-edge XAS. We present spectra of a range of selenium species relevant to environmental and life science studies, including spectra of seleno-amino acids, which show strong similarities with S K-edge XAS of their sulfur congeners. We discuss strengths and limitations of HERFD-XAS, showing improvements in both speciation performance and low concentration detection. We also develop a simple method to correct fluorescence self-absorption artifacts, which is generally applicable to any HERFD-XAS experiment.

  View details for DOI 10.1021/acs.analchem.1c01503

  View details for PubMedID 34164981

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2021; 28 (Pt 4): 1100-1113

  Abstract

  Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min-1) sheath stream with a low-flow-rate (5-90 l min-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in3D and sample widths as small as 5 m. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 m are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

  View details for DOI 10.1107/S1600577521003830

  View details for PubMedID 34212873

• Underlying simplicity of 5f unoccupied electronic structure JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Tobin, J. G., Nowak, S., Yu, S., Roussel, P., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2021; 39 (4)

  View details for DOI 10.1116/6.0001007

  View details for Web of Science ID 000659154600001

• Operando Elucidation on the Working State of Immobilized Fluorinated Iron Porphyrin for Selective Aqueous Electroreduction of CO2 to CO ACS CATALYSIS Lu, X., Ahsaine, H., Dereli, B., Garcia-Esparza, A. T., Reinhard, M., Shinagawa, T., Li, D., Adil, K., Tchalala, M., Kroll, T., Eddaoudi, M., Sokaras, D., Cavallo, L., Takanabe, K. 2021; 11 (11): 6499-6509

  View details for DOI 10.1021/acscatal.1c01157

  View details for Web of Science ID 000661125100017

• Hg(II) Binding to Thymine Bases in DNA. Inorganic chemistry Nehzati, S., Summers, A. O., Dolgova, N. V., Zhu, J., Sokaras, D., Kroll, T., Pickering, I. J., George, G. N. 2021; 60 (10): 7442-7452

  Abstract

  The compounds of mercury can be highly toxic and can interfere with a range of biological processes, although many aspects of the mechanism of toxicity are still obscure or unknown. One especially intriguing property of Hg(II) is its ability to bind DNA directly, making interstrand cross-links between thymine nucleobases in AT-rich sequences. We have used a combination of small molecule X-ray diffraction, X-ray spectroscopies, and computational chemistry to study the interactions of Hg(II) with thymine. We find that the energetically preferred mode of thymine binding in DNA is to the N3 and predict only minor distortions of the DNA structure on binding one Hg(II) to two cross-adjacent thymine nucleotides. The preferred geometry is predicted to be twisted away from coplanar through a torsion angle of between 32 and 43°. Using 1-methylthymine as a model, the bis-thymine coordination of Hg(II) is found to give a highly characteristic X-ray spectroscopic signature that is quite distinct from other previously described biological modes of binding of Hg(II). This work enlarges and deepens our view of significant biological targets of Hg(II) and demonstrates tools that can provide a characteristic signature for the binding of Hg(II) to DNA in more complex matrices including intact cells and tissues, laying the foundation for future studies of mechanisms of mercury toxicity.

  View details for DOI 10.1021/acs.inorgchem.1c00735

  View details for PubMedID 33938732

• Resonant Inelastic X-ray Scattering Calculations of Transition Metal Complexes Within a Simplified Time-Dependent Density Functional Theory Framework. Journal of chemical theory and computation Nascimento, D. R., Biasin, E., Poulter, B. I., Khalil, M., Sokaras, D., Govind, N. 2021

  Abstract

  We present a time-dependent density functional theory (TDDFT) approach to compute the light-matter couplings between two different manifolds of excited states relative to a common ground state in the context of 4d transition metal systems. These quantities are the necessary ingredients to solve the Kramers-Heisenberg (KH) equation for resonant inelastic X-ray scattering (RIXS) and several other types of two-photon spectroscopies. The procedure is based on the pseudo-wavefunction approach, where the solutions of a TDDFT calculation can be used to construct excited-state wavefunctions, and on the restricted energy window approach, where a manifold of excited states can be rigorously defined based on the energies of the occupied molecular orbitals involved in the excitation process. Thus, the present approach bypasses the need to solve the costly TDDFT quadratic-response equations. We illustrate the applicability of the method to 4d transition metal molecular complexes by calculating the 2p4d RIXS maps of three representative ruthenium complexes and comparing them to experimental results. The method can capture all the experimental features in all three complexes to allow the assignment of the experimental peaks, with relative energies correct to within 0.6 eV at the cost of two independent TDDFT calculations.

  View details for DOI 10.1021/acs.jctc.1c00144

  View details for PubMedID 33909424

• Effect of 3d/4p Mixing on 1s2p Resonant Inelastic X-ray Scattering: Electronic Structure of Oxo-Bridged Iron Dimers. Journal of the American Chemical Society Kroll, T., Baker, M. L., Wilson, S. A., Lundberg, M., Juhin, A., Arrio, M., Yan, J. J., Gee, L. B., Braun, A., Weng, T., Sokaras, D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2021

  Abstract

  1s2p resonant inelastic X-ray scattering (1s2p RIXS) has proven successful in the determination of the differential orbital covalency (DOC, the amount of metal vs ligand character in each d molecular orbital) of highly covalent centrosymmetric iron environments including heme models and enzymes. However, many reactive intermediates have noncentrosymmetric environments, e.g., the presence of strong metal-oxo bonds, which results in the mixing of metal 4p character into the 3d orbitals. This leads to significant intensity enhancement in the metal K-pre-edge and as shown here, the associated 1s2p RIXS features, which impact their insight into electronic structure. Binuclear oxo bridged high spin Fe(III) complexes are used to determine the effects of 4p mixing on 1s2p RIXS spectra. In addition to developing the analysis of 4p mixing on K-edge XAS and 1s2p RIXS data, this study explains the selective nature of the 4p mixing that also enhances the analysis of L-edge XAS intensity in terms of DOC. These 1s2p RIXS biferric model studies enable new structural insight from related data on peroxo bridged biferric enzyme intermediates. The dimeric nature of the oxo bridged Fe(III) complexes further results in ligand-to-ligand interactions between the Fe(III) sites and angle dependent features just above the pre-edge that reflect the superexchange pathway of the oxo bridge. Finally, we present a methodology that enables DOC to be obtained when L-edge XAS is inaccessible and only 1s2p RIXS experiments can be performed as in many metalloenzyme intermediates in solution.

  View details for DOI 10.1021/jacs.0c11193

  View details for PubMedID 33730507

• Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering CHEMICAL SCIENCE Biasin, E., Nascimento, D. R., Poulter, B. I., Abraham, B., Kunnus, K., Garcia-Esparza, A. T., Nowak, S. H., Kroll, T., Schoenlein, R. W., Alonso-Mori, R., Khalil, M., Govind, N., Sokaras, D. 2021; 12 (10): 3713–25

  View details for DOI 10.1039/d0sc06227h

  View details for Web of Science ID 000630166700031

• Operando Study of Thermal Oxidation of Monolayer MoS2 ADVANCED SCIENCE Park, S., Garcia-Esparza, A. T., Abroshan, H., Abraham, B., Vinson, J., Gallo, A., Nordlund, D., Park, J., Kim, T., Vallez, L., Alonso-Mori, R., Sokaras, D., Zheng, X. 2021

  View details for DOI 10.1002/advs.202002768

  View details for Web of Science ID 000623202400001

• Sulfur K beta X-ray emission spectroscopy: comparison with sulfur K-edge X-ray absorption spectroscopy for speciation of organosulfur compounds PHYSICAL CHEMISTRY CHEMICAL PHYSICS Qureshi, M., Nowak, S. H., Vogt, L. I., Cotelesage, J. H., Dolgova, N. V., Sharifi, S., Kroll, T., Nordlund, D., Alonso-Mori, R., Weng, T., Pickering, I. J., George, G. N., Sokaras, D. 2021; 23 (8): 4500-4508

  Abstract

  Until recently, sulfur was known as a "spectroscopically silent" element because of a paucity of convenient spectroscopic probes suitable for in situ chemical speciation. In recent years the technique of sulfur K-edge X-ray absorption spectroscopy (XAS) has been used extensively in sulfur speciation in a variety of different fields. With an initial focus on reduced forms of organic sulfur, we have explored a complementary X-ray based spectroscopy - sulfur Kβ X-ray emission spectroscopy (XES) - as a potential analytical tool for sulfur speciation in complex samples. We compare and contrast the sensitivity of sulfur Kβ XES with that of sulfur K-edge XAS, and find differing sensitivities for the two techniques. In some cases an approach involving both sulfur K-edge XAS and sulfur Kβ XES may be a powerful combination for deducing sulfur speciation in samples containing complex mixtures.

  View details for DOI 10.1039/d0cp05323f

  View details for Web of Science ID 000625306100057

  View details for PubMedID 33355326

• Author Correction: Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nature chemistry Biasin, E., Fox, Z. W., Andersen, A., Ledbetter, K., Kjar, K. S., Alonso-Mori, R., Carlstad, J. M., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Liekhus-Schmaltz, C., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Gaffney, K. J., Schoenlein, R. W., Govind, N., Cordones, A. A., Khalil, M. 2021

  View details for DOI 10.1038/s41557-021-00663-9

  View details for PubMedID 33627886

• Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nature chemistry Biasin, E., Fox, Z. W., Andersen, A., Ledbetter, K., Kjar, K. S., Alonso-Mori, R., Carlstad, J. M., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Liekhus-Schmaltz, C., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Gaffney, K. J., Schoenlein, R. W., Govind, N., Cordones, A. A., Khalil, M. 2021

  Abstract

  It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeiiRuiii) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of angstroms, 120-200cm-1 frequency and ~100fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

  View details for DOI 10.1038/s41557-020-00629-3

  View details for PubMedID 33589787

• Quantification of Ni-N-O Bond Angles and NO Activation by X-ray Emission Spectroscopy INORGANIC CHEMISTRY Phu, P. N., Gutierrez, C. E., Kundu, S., Sokaras, D., Kroll, T., Warren, T. H., Stieber, S. E. 2021; 60 (2): 737-745

  Abstract

  A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

  View details for DOI 10.1021/acs.inorgchem.0c02724

  View details for Web of Science ID 000643574200027

  View details for PubMedID 33373520

  View details for PubMedCentralID PMC9534352

• Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c. Nature communications Reinhard, M. E., Mara, M. W., Kroll, T., Lim, H., Hadt, R. G., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Driel, T. B., Hartsock, R. W., Kjaer, K. S., Weninger, C., Biasin, E., Gee, L. B., Hodgson, K. O., Hedman, B., Bergmann, U., Solomon, E. I., Gaffney, K. J. 2021; 12 (1): 1086

  Abstract

  The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kbeta) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.

  View details for DOI 10.1038/s41467-021-21423-w

  View details for PubMedID 33597529

• Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries ACS APPLIED MATERIALS & INTERFACES Yu, Y., Karayaylali, P., Giordano, L., Corchado-Garcia, J., Hwang, J., Sokaras, D., Maglia, F., Jung, R., Gittleson, F. S., Shao-Horn, Y. 2020; 12 (50): 55865-55875

  Abstract

  Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

  View details for DOI 10.1021/acsami.0c16285

  View details for Web of Science ID 000600202300030

  View details for PubMedID 33283495

• Kbeta X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase. Inorganic chemistry Lim, H., Baker, M. L., Cowley, R. E., Kim, S., Bhadra, M., Siegler, M. A., Kroll, T., Sokaras, D., Weng, T., Biswas, D. R., Dooley, D. M., Karlin, K. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2020

  Abstract

  Cu(I) active sites in metalloproteins are involved in O2 activation, but their O2 reactivity is difficult to study due to the Cu(I) d10 closed shell which precludes the use of conventional spectroscopic methods. Kbeta X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kbeta XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kbeta XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kbeta2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kbeta2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kbeta XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GOpre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kbeta2,5 emission feature in the Cu(I)-GOpre data has information about the MO containing mostly Cu 3dx2-y2 character that is the frontier molecular orbital (FMO) for O2 activation, which shows the potential of Kbeta XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

  View details for DOI 10.1021/acs.inorgchem.0c02495

  View details for PubMedID 33136386

• Acidic Oxygen Evolution Reaction Activity-Stability Relationships in Ru-Based Pyrochlores ACS CATALYSIS Hubert, M. A., Patel, A. M., Gallo, A., Liu, Y., Valle, E., Ben-Naim, M., Sanchez, J., Sokaras, D., Sinclair, R., Norskov, J. K., King, L. A., Bajdich, M., Jaramillo, T. F. 2020; 10 (20): 12182–96

  View details for DOI 10.1021/acscatal.0c02252

  View details for Web of Science ID 000614389200044

• Solution Chemistry of Copper(II) Binding to Substituted 8-Hydroxyquinolines. Inorganic chemistry Summers, K. L., Pushie, M. J., Sopasis, G. J., James, A. K., Dolgova, N. V., Sokaras, D., Kroll, T., Harris, H. H., Pickering, I. J., George, G. N. 2020; 59 (19): 13858-13874

  Abstract

  8-Hydroxyquinolines (8HQs) are a family of lipophilic metal ion chelators that have been used in a range of analytical and pharmaceutical applications over the last 100 years. More recently, CQ (clioquinol; 5-chloro-7-iodo-8-hydroxyquinoline) and PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) have undergone clinical trials for the treatment of Alzheimer's disease and Huntington's disease. Because CQ and PBT2 appear to redistribute metals into cells, these compounds have been redefined as copper and zinc ionophores. Despite the attention surrounding the clinical trials and the clear link between 8HQs and metals, the fundamental solution chemistry of how these compounds bind divalent metals such as copper and zinc, as well as their mechanism(s) of action in mammalian systems, remains poorly understood. In this study, we used a combination of X-ray absorption spectroscopy (XAS), high-energy resolution fluorescence detected (HERFD) XAS, electron paramagnetic resonance (EPR), and UV-visible absorption spectroscopies to investigate the aqueous solution chemistry of a range of 8HQ derivatives. To circumvent the known solubility issues with 8HQ compounds and their complexes with Cu(II), and to avoid the use of abiological organic solvents, we have devised a surfactant buffer system to investigate these Cu(II) complexes in aqueous solution. Our study comprises the first comprehensive investigation of the Cu(II) complexes formed with many 8HQs of interest in aqueous solution, and it provides the first structural information on some of these complexes. We find that halogen substitutions in 8HQ derivatives appear to have little effect on the Cu(II) coordination environment; 5,7-dihalogenated 8HQ conformers all have a pseudo square planar Cu(II) bound by two quinolin-8-olate anions, in agreement with previous studies. Conversely, substituents in the 2-position of the 8HQ moiety appear to cause significant distortions from the typical square-planar-like coordination of most Cu(II)-bis-8HQ complexes, such that the 8HQ moieties in the Cu(II)-bis-8HQ complex are rotated approximately 30-40° apart in a "propeller-like" arrangement.

  View details for DOI 10.1021/acs.inorgchem.0c01356

  View details for PubMedID 32936627

• Unveiling the critical role of the Mn dopant in a NiFe(OH)(2)catalyst for water oxidation JOURNAL OF MATERIALS CHEMISTRY A Zhang, Y., Cheng, C., Kuai, C., Sokaras, D., Zheng, X., Sainio, S., Lin, F., Dong, C., Nordlund, D., Du, X. 2020; 8 (34): 17471–76

  View details for DOI 10.1039/d0ta06353c

  View details for Web of Science ID 000566092600011

• Phase segregation reversibility in mixed-metal hydroxide water oxidation catalysts NATURE CATALYSIS Kuai, C., Xu, Z., Xi, C., Hu, A., Yang, Z., Zhang, Y., Sun, C., Li, L., Sokaras, D., Dong, C., Qiao, S., Du, X., Lin, F. 2020

  View details for DOI 10.1038/s41929-020-0496-z

  View details for Web of Science ID 000562347700002

• Probing Charge Ordering in Fractional Mixed-Valence Charge Density Wave Systems with Oriented HERFD-XANES Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY C Turner, N. A., Leicht, J. A., Sokaras, D., Kroll, T., Dexheimer, S. L. 2020; 124 (30): 16544-16552

  View details for DOI 10.1021/acs.jpcc.0c00685

  View details for Web of Science ID 000558662500032

• Observation of Seeded Mn K beta Stimulated X-Ray Emission Using Two-Color X-Ray Free-Electron Laser Pulses PHYSICAL REVIEW LETTERS Kroll, T., Weninger, C., Fuller, F. D., Guetg, M. W., Benediktovitch, A., Zhang, Y., Marinelli, A., Alonso-Mori, R., Aquila, A., Liang, M., Koglin, J. E., Koralek, J., Sokaras, D., Zhu, D., Kern, J., Yano, J., Yachandra, V. K., Rohringer, N., Lutman, A., Bergmann, U. 2020; 125 (3)

  View details for DOI 10.1103/PhysRevLett.125.037404

  View details for Web of Science ID 000549758200018

• Reply to Comments on "Rethinking the Minamata Tragedy: What Mercury Species Was Really Responsible?". Environmental science & technology James, A. K., Nehzati, S., Dolgova, N. V., Sokaras, D., Kroll, T., O'Donoghue, J. L., Watson, G. E., Myers, G. J., Krone, P. H., Pickering, I. J., George, G. N. 2020; 54 (13): 8484-8485

  View details for DOI 10.1021/acs.est.0c03061

  View details for PubMedID 32511907

• Reply to Comments on "Rethinking the Minamata Tragedy: What Mercury Species Was Really Responsible?". Environmental science & technology James, A. K., Nehzati, S., Dolgova, N. V., Sokaras, D., Kroll, T., O'Donoghue, J. L., Watson, G. E., Myers, G. J., Krone, P. H., Pickering, I. J., George, G. N. 2020; 54 (13): 8488-8490

  View details for DOI 10.1021/acs.est.0c02742

  View details for PubMedID 32559084

• Ni5Ga3 catalysts for CO2 reduction to methanol: Exploring the role of Ga surface oxidation/reduction on catalytic activity APPLIED CATALYSIS B-ENVIRONMENTAL Gallo, A., Snider, J. L., Sokaras, D., Nordlund, D., Kroll, T., Ogasawara, H., Kovarik, L., Duyar, M. S., Jaramillo, T. F. 2020; 267

  View details for DOI 10.1016/j.apcatb.2019.118369

  View details for Web of Science ID 000518865300047

• Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)(2)](2+) (vol 117, 013002, 2016) PHYSICAL REVIEW LETTERS Biasin, E., van Driel, T., Kjer, K. S., Dohn, A. O., Christensen, M., Harlang, T., Vester, P., Chabera, P., Liu, Y., Uhlig, J., Papai, M., Nemeth, Z., Hartsock, R., Liang, W., Zhang, J., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Assefa, T. A., Britz, A., Galler, A., Gawelda, W., Bressler, C., Gaffney, K. J., Lemke, H. T., Moller, K. B., Nielsen, M. M., Sundstrom, V., Vanko, G., Warnmark, K., Canton, S. E., Haldrup, K. 2020; 124 (19): 199902

  Abstract

  This corrects the article DOI: 10.1103/PhysRevLett.117.013002.

  View details for DOI 10.1103/PhysRevLett.124.199902

  View details for Web of Science ID 000533164500015

  View details for PubMedID 32469585

• Thermal stress-induced charge and structure heterogeneity in emerging cathode materials MATERIALS TODAY Alvarado, J., Wei, C., Nordlund, D., Kroll, T., Sokaras, D., Tian, Y., Liu, Y., Doeff, M. M. 2020; 35: 87-98

  View details for DOI 10.1016/j.mattod.2019.11.009

  View details for Web of Science ID 000537707100021

• Application of FEFF analyses to actinide 5f systems JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Tobin, J. G., Sokaras, D. 2020; 38 (3)

  View details for DOI 10.1116/6.0000215

  View details for Web of Science ID 000533893500001

• Rethinking the Minamata Tragedy: What Mercury Species Was Really Responsible? Environmental science & technology James, A. K., Nehzati, S., Dolgova, N. V., Sokaras, D., Kroll, T., Eto, K., O'Donoghue, J. L., Watson, G. E., Myers, G. J., Krone, P. H., Pickering, I. J., George, G. N. 2020; 54 (5): 2726-2733

  Abstract

  Industrial release of mercury into the local Minamata environment with consequent poisoning of local communities through contaminated fish and shellfish consumption is considered the classic case of environmental mercury poisoning. However, the mercury species in the factory effluent has proved controversial, originally suggested as inorganic, and more recently as methylmercury species. We used newly available methods to re-examine the cerebellum of historic Cat 717, which was fed factory effluent mixed with food to confirm the source. Synchrotron high-energy-resolution fluorescence detection-X-ray absorption spectroscopy revealed sulfur-bound organometallic mercury with a minor β-HgS phase. Density functional theory indicated energetic preference for α-mercuri-acetaldehyde as a waste product of aldehyde production. The consequences of this alternative species in the "classic" mercury poisoning should be re-evaluated.

  View details for DOI 10.1021/acs.est.9b06253

  View details for PubMedID 31951385

• Direct Observation of Methylmercury and Auranofin Binding to Selenocysteine in Thioredoxin Reductase. Inorganic chemistry Pickering, I. J., Cheng, Q., Rengifo, E. M., Nehzati, S., Dolgova, N. V., Kroll, T., Sokaras, D., George, G. N., Arnér, E. S. 2020; 59 (5): 2711-2718

  Abstract

  Selenoenzymes, containing a selenocysteine (Sec) residue, fulfill important roles in biology. The mammalian thioredoxin reductase selenoenzymes are key regulators of antioxidant defense and redox signaling and are inhibited by methylmercury species and by the gold-containing drug auranofin. It has been proposed that such inhibition is mediated by metal binding to Sec in the enzyme. However, direct structural observations of these classes of inhibitors binding to selenoenzymes have been few to date. Here we therefore have used extended X-ray absorption fine structure as a direct structural probe to investigate binding to the selenium site in recombinant rat thioredoxin reductase 1 (TrxR1). The results demonstrate for the first time the direct and complete binding of the metal atom of the inhibitors to the selenium atom in TrxR1 for both methylmercury and auranofin, indicating that TrxR1 inhibition indeed can be attributed to such direct metal-selenium binding.

  View details for DOI 10.1021/acs.inorgchem.9b03072

  View details for PubMedID 32049511

• A versatile Johansson-type tender x-ray emission spectrometer REVIEW OF SCIENTIFIC INSTRUMENTS Nowak, S. H., Armenta, R., Schwartz, C. P., Gallo, A., Abraham, B., Garcia-Esparza, A. T., Biasin, E., Prado, A., Maciel, A., Zhang, D., Day, D., Christensen, S., Kroll, T., Alonso-Mori, R., Nordlund, D., Weng, T., Sokaras, D. 2020; 91 (3): 033101

  Abstract

  We present a high energy resolution x-ray spectrometer for the tender x-ray regime (1.6-5.0 keV) that was designed and operated at Stanford Synchrotron Radiation Lightsource. The instrument is developed on a Rowland geometry (500 mm of radius) using cylindrically bent Johansson analyzers and a position sensitive detector. By placing the sample inside the Rowland circle, the spectrometer operates in an energy-dispersive mode with a subnatural line-width energy resolution (∼0.32 eV at 2400 eV), even when an extended incident x-ray beam is used across a wide range of diffraction angles (∼30° to 65°). The spectrometer is enclosed in a vacuum chamber, and a sample chamber with independent ambient conditions is introduced to enable a versatile and fast-access sample environment (e.g., solid/gas/liquid samples, in situ cells, and radioactive materials). The design, capabilities, and performance are presented and discussed.

  View details for DOI 10.1063/1.5121853

  View details for Web of Science ID 000519254000002

  View details for PubMedID 32259983

• Distinct Surface and Bulk Thermal Behaviors of LiNi0.6Mn0.2Co0.2O2 Cathode Materials as a Function of State of Charge. ACS applied materials & interfaces Tian, C., Xu, Y., Kan, W. H., Sokaras, D., Nordlund, D., Shen, H., Chen, K., Liu, Y., Doeff, M. 2020

  Abstract

  Understanding how structural and chemical transformations take place in particles under thermal conditions can inform designing thermally robust electrode materials. Such a study necessitates the use of diagnostic techniques that are capable of probing the transformations at multiple length scales and at different states of charge (SOC). In this study, the thermal behavior of LiNi0.6Mn0.2Co0.2O2 (NMC-622) was examined as a function of SOC, using an array of bulk and surface-sensitive techniques. In general, thermal stability decreases as lithium content is lowered and conversion in the bulk to progressively reduced metal oxides (spinels, rock salt) occurs as the temperature is raised. Hard X-ray absorption spectroscopy (XAS) and X-ray Raman spectroscopy (XRS) experiments, which probe the bulk, reveal that Ni and Co are eventually reduced when partially delithiated samples (regardless of the SOC) are heated, although Mn is not. Surface-sensitive synchrotron techniques, such as soft XAS and transmission X-ray microscopy (TXM), however, reveal that for 50% delithiated samples, apparent oxidation of nickel occurs at particle surfaces under some circumstances. This is partially compensated by reduction of cobalt but may also be a consequence of redistribution of lithium ions upon heating. TXM results indicate the movement of reduced nickel ions into particle interiors or oxidized nickel ions to the surface or both. These experiments illustrate the complexity of the thermal behavior of NMC cathode materials. The study also informs the importance of investigating the surface and bulk difference as a function of SOC when studying the thermal behaviors of battery materials.

  View details for DOI 10.1021/acsami.9b21288

  View details for PubMedID 32057227

• Probing U 5f Covalency in Uranium Compounds through Oxidant 2p Bonding JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Tobin, J. G., Yu, S., Shuh, D. K., Sokaras, D. 2020; 89 (2)

  View details for DOI 10.7566/JPSJ.89.024711

  View details for Web of Science ID 000512132000023

• Towards the Quantification of 5f Delocalization APPLIED SCIENCES-BASEL Tobin, J. G., Nowak, S., Yu, S., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2020; 10 (8)

  View details for DOI 10.3390/app10082918

  View details for Web of Science ID 000533352100282

• Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering. Nature communications Kunnus, K., Vacher, M., Harlang, T. C., Kjar, K. S., Haldrup, K., Biasin, E., van Driel, T. B., Papai, M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Kallman, E., Delcey, M., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Sandberg, L., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Moller, K. B., Nielsen, M. M., Vanko, G., Warnmark, K., Sundstrom, V., Persson, P., Lundberg, M., Uhlig, J., Gaffney, K. J. 2020; 11 (1): 634

  Abstract

  The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip=2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kalpha and Kbeta X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kalpha XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1s and 2p core-ionized states surfaces.

  View details for DOI 10.1038/s41467-020-14468-w

  View details for PubMedID 32005815

• Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy Journal of Chemical Physics Ledbetter, K., Reinhard, M. E., Kunnus, K., Gallo, A., Britz, A., Biasin, E., Glownia, J. M., Nelson, S., Van Driel, T. B., Weninger, C., Zederkof, D. B., Haldrup, K., Cordones, A. A., Gaffney, K. J., Sokaras, D., Alonso-Mori, R. 2020; 152

  View details for DOI 10.1063/1.5139441

• Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy. Scientific reports Alonso-Mori, R. n., Sokaras, D. n., Cammarata, M. n., Ding, Y. n., Feng, Y. n., Fritz, D. n., Gaffney, K. J., Hastings, J. n., Kao, C. C., Lemke, H. T., Maxwell, T. n., Robert, A. n., Schropp, A. n., Seiboth, F. n., Sikorski, M. n., Song, S. n., Weng, T. C., Zhang, W. n., Glenzer, S. n., Bergmann, U. n., Zhu, D. n. 2020; 10 (1): 16837

  Abstract

  We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted 'probe-before-destroy' measurement strategy can be adopted for electronic-structure related XFEL experiments.

  View details for DOI 10.1038/s41598-020-74003-1

  View details for PubMedID 33033373

• Observation of 5f intermediate coupling in uranium x-ray emission spectroscopy JOURNAL OF PHYSICS COMMUNICATIONS Tobin, J. G., Nowak, S., Yu, S., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2020; 4 (1)

  View details for DOI 10.1088/2399-6528/ab6940

  View details for Web of Science ID 000520894100001

• In situ X-ray diffraction of silicate liquids and glasses under dynamic and static compression to megabar pressures. Proceedings of the National Academy of Sciences of the United States of America Morard, G. n., Hernandez, J. A., Guarguaglini, M. n., Bolis, R. n., Benuzzi-Mounaix, A. n., Vinci, T. n., Fiquet, G. n., Baron, M. A., Shim, S. H., Ko, B. n., Gleason, A. E., Mao, W. L., Alonso-Mori, R. n., Lee, H. J., Nagler, B. n., Galtier, E. n., Sokaras, D. n., Glenzer, S. H., Andrault, D. n., Garbarino, G. n., Mezouar, M. n., Schuster, A. K., Ravasio, A. n. 2020

  Abstract

  Properties of liquid silicates under high-pressure and high-temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6,000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.

  View details for DOI 10.1073/pnas.1920470117

  View details for PubMedID 32414927

• Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite. Environmental science & technology Dewey, C. n., Sokaras, D. n., Kroll, T. n., Bargar, J. R., Fendorf, S. n. 2020

  Abstract

  Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO2(CO3)32- and Ca2UO2(CO3)3) by Fe(II)(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

  View details for DOI 10.1021/acs.est.9b05870

  View details for PubMedID 32315524

• QUANTIFYING THE APPLICATION OF FEFF TO F-DERIVED SPECTRAL STRUCTURE MRS ADVANCES Tobin, J. G., Sokaras, D. 2020; 5 (51): 2631-2638

  View details for DOI 10.1557/adv.2020.280

  View details for Web of Science ID 000581816200003

• Using N-Terminal Coordination of Cu(II) and Ni(II) to Isolate the Coordination Environment of Cu(I) and Cu(II) Bound to His13 and His14 in Amyloid-β(4-16) INORGANIC CHEMISTRY Pushie, M., Stefaniak, E., Sendzikr, M. R., Sokaras, D., Kro, T., Haas, K. L. 2019; 58 (22): 15138-15154

  Abstract

  The amyloid-β (Aβ) peptide is a cleavage product of the amyloid precursor protein and has been implicated as a central player in Alzheimer's disease. The N-terminal end of Aβ is variable, and different proportions of these variable-length Aβ peptides are present in healthy individuals and those with the disease. The N-terminally truncated form of Aβ starting at position 4 (Aβ4-x) has a His residue as the third amino acid (His6 using the formal Aβ numbering). The N-terminal sequence Xaa-Xaa-His is known as an amino terminal copper and nickel binding motif (ATCUN), which avidly binds Cu(II). This motif is not present in the commonly studied Aβ1-x peptides. In addition to the ATCUN site, Aβ4-x contains an additional metal binding site located at the tandem His residues (bis-His at His13 and 14) which is also found in other isoforms of Aβ. Using the ATCUN and bis-His motifs, the Aβ4-x peptide is capable of binding multiple metal ions simultaneously. We confirm that Cu(II) bound to this particular ATCUN site is redox silent, but the second Cu(II) site is redox active and can be readily reduced with ascorbate. We have employed surrogate metal ions to block copper coordination at the ATCUN or the tandem His site in order to isolate spectral features of the copper coordination environment for structural characterization using extended X-ray absorption fine structure (EXAFS) spectroscopy. This approach reveals that each copper coordination environment is independent in the Cu2Aβ4-x state. The identification of two functionally different copper binding environments within the Aβ4-x sequence may have important implications for this peptide in vivo.

  View details for DOI 10.1021/acs.inorgchem.9b01940

  View details for Web of Science ID 000498288300021

  View details for PubMedID 31657204

• Revealing Electronic Signatures of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-Ion Battery Material Designs CHEMISTRY OF MATERIALS Yu, Y., Karayaylali, P., Nowak, S. H., Giordano, L., Gauthier, M., Hong, W., Kou, R., Li, Q., Vinson, J., Kroll, T., Sokaras, D., Sun, C., Charles, N., Maglia, F., Jung, R., Shao-Horn, Y. 2019; 31 (19): 7864-7876

  Abstract

  Anion redox in lithium transition metal oxides such as Li2RuO3 and Li2MnO3, has catalyzed intensive research efforts to find transition metal oxides with anion redox that may boost the energy density of lithium-ion batteries. The physical origin of observed anion redox remains debated, and more direct experimental evidence is needed. In this work, we have shown electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-), in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. Experimental Ru L3-edge high-energy-resolution fluorescence detected X-ray absorption spectra (HERFD-XAS), supported by ab-initio simulations, revealed that the increased intensity in the high-energy shoulder upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ*-like states with π overlap with Ru d-manifolds, in agreement with O K-edge XAS spectra. Experimental and simulated O K-edge X-ray emission spectra (XES) further supported this observation with the broadening of the oxygen non-bonding feature upon charging, also originated from (O-O) σ* states. This lattice oxygen redox of Li2-xRuO3 was accompanied by a small amount of O2 evolution in the first charge from differential electrochemistry mass spectrometry (DEMS) but diminished in the subsequent cycles, in agreement with the more reduced states of Ru in later cycles from Ru L3-edge HERFD-XAS. These observations indicated that Ru redox contributed more to discharge capacities after the first cycle. This study has pinpointed the key spectral fingerprints related to lattice oxygen redox from a molecular level and constructed a transferrable framework to rationally interpret the spectroscopic features by combining advanced experiments and theoretical calculations to design materials for Li-ion batteries and electrocatalysis applications.

  View details for DOI 10.1021/acs.chemmater.9b01821

  View details for Web of Science ID 000489678800007

  View details for PubMedID 32210521

  View details for PubMedCentralID PMC7092754

• NpSe₂: a Binary Chalcogenide Containing Modulated Selenide Chains and Ambiguous-Valent Metal ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Jin, G., Malliakas, C. D., Hu, Y., Booth, C. H., Ibers, J. A., Sokaras, D., Weng, T., Skanthakumar, S., Soderholm, L. 2019; 58 (45): 16130-16133

  Abstract

  A new binary compound, NpSe2, possesses metal-chalcogen and chalcogen-chalcogen interactions different from those reported for other metal dichalcogenides. Its structure is incommensurately modulated and features linear Se chains and valence-ambiguous Np cations.

  View details for DOI 10.1002/anie.201910353

  View details for Web of Science ID 000487452700001

  View details for PubMedID 31549462

• Diagram, valence-to-core, and hypersatellite K beta X-ray transitions in metallic chromium Zeeshan, F., Hoszowska, J., Dousse, J., Sokaras, D., Weng, T., Alonso-Mori, R., Kavcic, M., Guerra, M., Sampaio, J., Parente, F., Indelicato, P., Marques, J., Santos, J. WILEY. 2019: 351-359

  View details for DOI 10.1002/xrs.3019

  View details for Web of Science ID 000483574400006

• Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Henthorn, J. T., Arias, R. J., Koroidov, S., Kroll, T., Sokaras, D., Bergmann, U., Rees, D. C., DeBeer, S. 2019; 141 (34): 13676-13688

  Abstract

  The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe-Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron-sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.

  View details for DOI 10.1021/jacs.9b06988

  View details for Web of Science ID 000484082700047

  View details for PubMedID 31356071

  View details for PubMedCentralID PMC6716209

• Fully Oxidized Ni-Fe Layered Double Hydroxide with 100% Exposed Active Sites for Catalyzing Oxygen Evolution Reaction ACS CATALYSIS Kuai, C., Zhang, Y., Wu, D., Sokaras, D., Mu, L., Spence, S., Nordlund, D., Lin, F., Du, X. 2019; 9 (7): 6027–32

  View details for DOI 10.1021/acscatal.9b01935

  View details for Web of Science ID 000474812400024

• Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy. Chemical science Kjar, K. S., Van Driel, T. B., Harlang, T. C., Kunnus, K., Biasin, E., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Dohn, A. O., Papai, M. I., Moller, K. B., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Jarenmark, M., Uhlig, J., Sundstom, V., Warnmark, K., Persson, P., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Canton, S. E., Lemke, H. T., Gaffney, K. J. 2019; 10 (22): 5749–60

  Abstract

  Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2'-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals.

  View details for DOI 10.1039/c8sc04023k

  View details for PubMedID 31293761

• Pheomelanin pigment remnants mapped in fossils of an extinct mammal NATURE COMMUNICATIONS Manning, P. L., Edwards, N. P., Bergmann, U., Anne, J., Sellers, W. I., van Veelen, A., Sokaras, D., Egerton, V. M., Alonso-Mori, R., Ignatyev, K., van Dongen, B. E., Wakamatsu, K., Ito, S., Knoll, F., Wogelius, R. A. 2019; 10: 2250

  Abstract

  Recent progress has been made in paleontology with respect to resolving pigmentation in fossil material. Morphological identification of fossilized melanosomes has been one approach, while a second methodology using chemical imaging and spectroscopy has also provided critical information particularly concerning eumelanin (black pigment) residue. In this work we develop the chemical imaging methodology to show that organosulfur-Zn complexes are indicators of pheomelanin (red pigment) in extant and fossil soft tissue and that the mapping of these residual biochemical compounds can be used to restore melanin pigment distribution in a 3 million year old extinct mammal species (Apodemus atavus). Synchotron Rapid Scanning X-ray Fluorescence imaging showed that the distributions of Zn and organic S are correlated within this fossil fur just as in pheomelanin-rich modern integument. Furthermore, Zn coordination chemistry within this fossil fur is closely comparable to that determined from pheomelanin-rich fur and hair standards. The non-destructive methods presented here provide a protocol for detecting residual pheomelanin in precious specimens.

  View details for DOI 10.1038/s41467-019-10087-2

  View details for Web of Science ID 000468448100007

  View details for PubMedID 31113945

  View details for PubMedCentralID PMC6529433

• Surface Characterization of Li-Substituted Compositionally Heterogeneous NaLi0.045Cu0.185Fe0.265Mn0.505O2 Sodium-Ion Cathode Material JOURNAL OF PHYSICAL CHEMISTRY C Rahman, M., Zhang, Y., Xia, S., Kan, W., Avdeev, M., Mu, L., Sokaras, D., Kroll, T., Du, X., Nordlund, D., Liu, Y., Lin, F. 2019; 123 (18): 11428–35

  View details for DOI 10.1021/acs.jpcc.9b01126

  View details for Web of Science ID 000467781000012

• Nature of cobalt species during the <i>in situ</i> sulfurization of Co(Ni)Mo/Al₂O₃ hydrodesulfurization catalysts JOURNAL OF SYNCHROTRON RADIATION al Samarai, M., van Oversteeg, C. M., Delgado-Jaime, M., Weng, T., Sokaras, D., Liu, B., van der Linden, M., van der Eerden, A. J., Vogt, E. C., Weckhuysen, B. M., de Groot, F. F. 2019; 26: 811-818

  Abstract

  The evolution in local structure and electronic properties of cobalt was investigated during in situ sulfurization. Using a combination of 1s X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (RIXS), the valence, coordination and symmetry of cobalt ions were tracked in two cobalt-promoted molybdenum oxide precursors of the hydrodesulfurization catalyst system, namely Co-Mo/Al2O3 and Co-Ni-Mo/Al2O3. Extended X-ray absorption fine structure shows that the Co-O bonds were replaced with Co-S bonds as a function of reaction temperature. The cobalt K pre-edge intensity shows that the symmetry of cobalt was modified from Co3+ Oh and Co2+ Oh to a Co2+ ion where the inversion symmetry is broken, in agreement with a square-pyramidal site. The 1s3p RIXS data revealed the presence of an intermediate cobalt oxy-sulfide species. This species was not detected from XAS and was determined from the increased information obtained from the 1s3p RIXS data. The cobalt XAS and RIXS data show that nickel has a significant influence on the formation of the cobalt oxy-sulfide intermediate species prior to achieving the fully sulfided state at T > 400°C.

  View details for DOI 10.1107/S1600577519002546

  View details for Web of Science ID 000467526100024

  View details for PubMedID 31074446

  View details for PubMedCentralID PMC6510205

• A high-throughput energy-dispersive tender X-ray spectrometer for shot-to-shot sulfur measurements. Journal of synchrotron radiation Abraham, B., Nowak, S., Weninger, C., Armenta, R., Defever, J., Day, D., Carini, G., Nakahara, K., Gallo, A., Nelson, S., Nordlund, D., Kroll, T., Hunter, M. S., van Driel, T., Zhu, D., Weng, T. C., Alonso-Mori, R., Sokaras, D. 2019; 26 (Pt 3): 629–34

  Abstract

  An X-ray emission spectrometer that can detect the sulfur Kalpha emission lines with large throughput and a high energy resolution is presented. The instrument is based on a large d-spacing perfect Bragg analyzer that diffracts the sulfur Kalpha emission at close to backscattering angles. This facilitates the application of efficient concepts routinely employed in hard X-ray spectrometers towards the tender X-ray regime. The instrument described in this work is based on an energy-dispersive von Hamos geometry that is well suited for photon-in photon-out spectroscopy at X-ray free-electron laser and synchrotron sources. Comparison of its performance with previously used instrumentation is presented through measurements using sulfur-containing species performed at the LCLS. It is shown that the overall signal intensity is increased by a factor of 15. Implementation of this approach in the design of a tender X-ray spectroscopy endstation for LCLS-II is also discussed.

  View details for DOI 10.1107/S1600577519002431

  View details for PubMedID 31074425

• X-ray Absorption Spectroscopy Investigations of Copper(II) Coordination in the Human Amyloid β Peptide. Inorganic chemistry Summers, K. L., Schilling, K. M., Roseman, G., Markham, K. A., Dolgova, N. V., Kroll, T., Sokaras, D., Millhauser, G. L., Pickering, I. J., George, G. N. 2019

  Abstract

  Alzheimer's disease (AD) is the main cause of age-related dementia and currently affects approximately 5.7 million Americans. Major brain changes associated with AD pathology include accumulation of amyloid beta (Aβ) protein fragments and formation of extracellular amyloid plaques. Redox-active metals mediate oligomerization of Aβ, and the resultant metal-bound oligomers have been implicated in the putative formation of harmful, reactive species that could contribute to observed oxidative damage. In isolated plaque cores, Cu(II) is bound to Aβ via histidine residues. Despite numerous structural studies of Cu(II) binding to synthetic Aβ in vitro, there is still uncertainty surrounding Cu(II) coordination in Aβ. In this study, we used X-ray absorption spectroscopy (XAS) and high energy resolution fluorescence detected (HERFD) XAS to investigate Cu(II) coordination in Aβ(1-42) under various solution conditions. We found that the average coordination environment in Cu(II)Aβ(1-42) is sensitive to X-ray photoreduction, changes in buffer composition, peptide concentration, and solution pH. Fitting of the extended X-ray absorption fine structure (EXAFS) suggests Cu(II) is bound in a mixture of coordination environments in monomeric Aβ(1-42) under all conditions studied. However, it was evident that on average only a single histidine residue coordinates Cu(II) in monomeric Aβ(1-42) at pH 6.1, in addition to 3 other oxygen or nitrogen ligands. Cu(II) coordination in Aβ(1-42) at pH 7.4 is similarly 4-coordinate with oxygen and nitrogen ligands, although an average of 2 histidine residues appear to coordinate at this pH. At pH 9.0, the average Cu(II) coordination environment in Aβ(1-42) appears to be 5-coordinate with oxygen and nitrogen ligands, including two histidine residues.

  View details for DOI 10.1021/acs.inorgchem.9b00507

  View details for PubMedID 31013069

• Visualizing sulfur with X-rays: From molecules to tissues PHOSPHORUS SULFUR AND SILICON AND THE RELATED ELEMENTS George, G. N., Pickering, I. J., Cotelesage, J. H., Vogt, L., Dolgova, N., Regnier, N., Sokaras, D., Kroll, T., Sneeden, E. Y., Hackett, M. J., Goto, K., Block, E. 2019; 194 (7): 618-623

  View details for DOI 10.1080/10426507.2019.1602618

  View details for Web of Science ID 000470385900001

• Separate measurement of the 5f_5/2 and 5f_7/2 unoccupied density of states of UO₂ JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Tobin, J. G., Nowak, S., Booth, C. H., Bauer, E. D., Yu, S., Alonso-Mori, R., Kroll, T., Nordlund, D., Weng, T., Sokaras, D. 2019; 232: 100-104

  View details for DOI 10.1016/j.elspec.2019.03.003

  View details for Web of Science ID 000463979300015

• Hybrid X-ray Spectroscopy-Based Approach To Acquire Chemical and Structural Information of Single-Walled Carbon Nanotubes with Superior Sensitivity JOURNAL OF PHYSICAL CHEMISTRY C Sainio, S., Wester, N., Titus, C. J., Liao, Y., Zhang, Q., Nordlund, D., Sokaras, D., Lee, S., Irwin, K. D., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J. N., Kauppinen, E., Laurila, T., Koskinen, J. 2019; 123 (10): 6114–20

  View details for DOI 10.1021/acs.jpcc.9b00714

  View details for Web of Science ID 000461537400032

• Electronic structure changes upon lithium intercalation into graphite - Insights from <i>ex situ</i> and <i>operando</i> x-ray Raman spectroscopy CARBON Boesenberg, U., Sokaras, D., Nordlund, D., Weng, T., Gorelov, E., Richardson, T. J., Kostecki, R., Cabana, J. 2019; 143: 371-377

  View details for DOI 10.1016/j.carbon.2018.11.031

  View details for Web of Science ID 000456710500042

• Electronic Structure of Naturally Occurring Aromatic Carbon ENERGY & FUELS Pomerantz, A. E., Bostrom, N. W., Kleinber, R. L., Crace, E., Weng, T., Sokaras, D., Nordlund, D. 2019; 33 (3): 2099–2105

  View details for DOI 10.1021/acs.energyfuels.8b04366

  View details for Web of Science ID 000462260600043

• Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yan, J. J., Kroll, T., Baker, M. L., Wilson, S. A., Decreau, R., Lundberg, M., Sokaras, D., Glatzel, P., Hedman, B., Hodgson, K. O., Solomon, E. I. 2019; 116 (8): 2854–59

  View details for DOI 10.1073/pnas.1815981116

  View details for Web of Science ID 000459074400018

• Identification of the active complex for CO oxidation over single-atom Ir-on-MgAl2O4 catalysts NATURE CATALYSIS Lu, Y., Wang, J., Yu, L., Kovarik, L., Zhang, X., Hoffman, A. S., Gallo, A., Bare, S. R., Sokaras, D., Kroll, T., Dagle, V., Xin, H., Karim, A. M. 2019; 2 (2): 149–56

  View details for DOI 10.1038/s41929-018-0192-4

  View details for Web of Science ID 000458554800010

• Operando Observation of Chemical Transformations of Iridium Oxide During Photoelectrochemical Water Oxidation ACS APPLIED ENERGY MATERIALS Li, L., Yang, J., Ali-Loytty, H., Weng, T., Toma, F. M., Sokaras, D., Sharp, I. D., Nilsson, A. 2019; 2 (2): 1371–79

  View details for DOI 10.1021/acsaem.8b01945

  View details for Web of Science ID 000459948900051

• Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy. Angewandte Chemie (International ed. in English) Tatsuno, H. n., Kjaer, K. S., Kunnus, K. n., Harlang, T. C., Timm, C. n., Guo, M. n., Chàbera, P. n., Fredin, L. A., Hartsock, R. W., Reinhard, M. E., Koroidov, S. n., Li, L. n., Cordones, A. A., Gordivska, O. n., Prakash, O. n., Liu, Y. n., Laursen, M. G., Biasin, E. n., Hansen, F. B., Vester, P. n., Christensen, M. n., Haldrup, K. n., Németh, Z. n., Sárosiné Szemes, D. n., Bajnóczi, É. n., Vankó, G. n., Van Driel, T. B., Alonso-Mori, R. n., Glownia, J. M., Nelson, S. n., Sikorski, M. n., Lemke, H. T., Sokaras, D. n., Canton, S. E., Dohn, A. O., Møller, K. B., Nielsen, M. M., Gaffney, K. J., Wärnmark, K. n., Sundström, V. n., Persson, P. n., Uhlig, J. n. 2019

  Abstract

  Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeII NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3 MLCT state, from the initially excited 1 MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3 MC state, in competition with vibrational relaxation and cooling to the relaxed 3 MLCT state. The relaxed 3 MLCT state then decays much more slowly (7.6 ps) to the 3 MC state. The 3 MC state is rapidly (2.2 ps) deactivated to the ground state. The 5 MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3 MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.

  View details for DOI 10.1002/anie.201908065

  View details for PubMedID 31602726

• Hammerhead, an Ultrahigh Resolution ePix Camera for Wavelength-Dispersive Spectrometers Blaj, G., Bhogadi, D., Chang, C., Doering, D., Kenney, C., Kroll, T., Segal, J., Sokaras, D., Haller, G., Gwo, S., Huang, D. J., Wei, D. H. AMER INST PHYSICS. 2019

  View details for DOI 10.1063/1.5084668

  View details for Web of Science ID 000472769500105

• Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS. Physical chemistry chemical physics : PCCP Britz, A. n., Abraham, B. n., Biasin, E. n., van Driel, T. B., Gallo, A. n., Garcia-Esparza, A. T., Glownia, J. n., Loukianov, A. n., Nelson, S. n., Reinhard, M. n., Sokaras, D. n., Alonso-Mori, R. n. 2019

  Abstract

  Femtosecond-resolved Extended X-ray Absorption Fine Structure (EXAFS) measurements of solvated transition metal complexes are successfully implemented at the X-ray Free Electron Laser LCLS. Benchmark experiments on [Fe(terpy)2]2+ in solution show a signal-to-noise ratio on the order of 500, comparable to typical 100 ps-resolution synchrotron measurements. In the few femtoseconds after photoexcitation, we observe the EXAFS fingerprints of a short-lived (∼100 fs) intermediate as well as those of a vibrationally hot long-lived (∼ns) excited state.

  View details for DOI 10.1039/c9cp03483h

  View details for PubMedID 31441480

• Soft X-ray spectroscopy with transition-edge sensors at Stanford Synchrotron Radiation Lightsource beamline 10-1. The Review of scientific instruments Lee, S. J., Titus, C. J., Alonso Mori, R. n., Baker, M. L., Bennett, D. A., Cho, H. M., Doriese, W. B., Fowler, J. W., Gaffney, K. J., Gallo, A. n., Gard, J. D., Hilton, G. C., Jang, H. n., Joe, Y. I., Kenney, C. J., Knight, J. n., Kroll, T. n., Lee, J. S., Li, D. n., Lu, D. n., Marks, R. n., Minitti, M. P., Morgan, K. M., Ogasawara, H. n., O'Neil, G. C., Reintsema, C. D., Schmidt, D. R., Sokaras, D. n., Ullom, J. N., Weng, T. C., Williams, C. n., Young, B. A., Swetz, D. S., Irwin, K. D., Nordlund, D. n. 2019; 90 (11): 113101

  Abstract

  We present results obtained with a new soft X-ray spectrometer based on transition-edge sensors (TESs) composed of Mo/Cu bilayers coupled to bismuth absorbers. This spectrometer simultaneously provides excellent energy resolution, high detection efficiency, and broadband spectral coverage. The new spectrometer is optimized for incident X-ray energies below 2 keV. Each pixel serves as both a highly sensitive calorimeter and an X-ray absorber with near unity quantum efficiency. We have commissioned this 240-pixel TES spectrometer at the Stanford Synchrotron Radiation Lightsource beamline 10-1 (BL 10-1) and used it to probe the local electronic structure of sample materials with unprecedented sensitivity in the soft X-ray regime. As mounted, the TES spectrometer has a maximum detection solid angle of 2 × 10-3 sr. The energy resolution of all pixels combined is 1.5 eV full width at half maximum at 500 eV. We describe the performance of the TES spectrometer in terms of its energy resolution and count-rate capability and demonstrate its utility as a high throughput detector for synchrotron-based X-ray spectroscopy. Results from initial X-ray emission spectroscopy and resonant inelastic X-ray scattering experiments obtained with the spectrometer are presented.

  View details for DOI 10.1063/1.5119155

  View details for PubMedID 31779391

• Morphological and chemical evidence for cyclic bone growth in a fossil hyaena JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Anne, J., Wogelius, R. A., Edwards, N. P., van Veelen, A., Buckley, M., Sellers, W. I., Bergmann, U., Sokaras, D., Alonso-Mori, R., Harvey, V. L., Egerton, V. M., Manning, P. L. 2018; 33 (12): 2062-2069

  View details for DOI 10.1039/c8ja00314a

  View details for Web of Science ID 000451536000002

• Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics COMMUNICATIONS PHYSICS Cheng, B., Li, T., Maity, P., Wei, P., Nordlund, D., Ho, K., Lien, D., Lin, C., Liang, R., Miao, X., Ajia, I. A., Yin, J., Sokaras, D., Javey, A., Roqan, I. S., Mohammed, O. F., He, J. 2018; 1

  View details for DOI 10.1038/s42005-018-0082-8

  View details for Web of Science ID 000452674600001

• Structures of the intermediates of Kok's photosynthetic water oxidation clock. Nature Kern, J., Chatterjee, R., Young, I. D., Fuller, F. D., Lassalle, L., Ibrahim, M., Gul, S., Fransson, T., Brewster, A. S., Alonso-Mori, R., Hussein, R., Zhang, M., Douthit, L., de Lichtenberg, C., Cheah, M. H., Shevela, D., Wersig, J., Seuffert, I., Sokaras, D., Pastor, E., Weninger, C., Kroll, T., Sierra, R. G., Aller, P., Butryn, A., Orville, A. M., Liang, M., Batyuk, A., Koglin, J. E., Carbajo, S., Boutet, S., Moriarty, N. W., Holton, J. M., Dobbek, H., Adams, P. D., Bergmann, U., Sauter, N. K., Zouni, A., Messinger, J., Yano, J., Yachandra, V. K. 2018

  Abstract

  Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II discovered by Joliot in 1969, Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok's S-state clock or cycle1,2. The model comprises four (meta)stable intermediates (S0, S1, S2 and S3) and one transient S4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn4CaO5) cluster in the oxygen-evolving complex3-7. This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone QB at the acceptor side of PSII. Here, using serial femtosecond X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation at room temperature, we visualize all (meta)stable states of Kok's cycle as high-resolution structures (2.04-2.08A). In addition, we report structures of two transient states at 150 and 400s, revealing notable structural changes including the binding of one additional 'water', Ox, during the S2S3 state transition. Our results suggest that one water ligand to calcium (W3) is directly involved in substrate delivery. The binding of the additional oxygen Ox in the S3 state between Ca and Mn1 supports O-O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O2 release. Thus, our results exclude peroxo-bond formation in the S3 state, and the nucleophilic attack of W3 onto W2 is unlikely.

  View details for DOI 10.1038/s41586-018-0681-2

  View details for PubMedID 30405241

• A Photochemically Generated Selenyl Free Radical Observed by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy. Inorganic chemistry Nehzati, S., Dolgova, N. V., Sokaras, D., Kroll, T., Cotelesage, J. J., Pickering, I. J., George, G. N. 2018; 57 (17): 10867-10872

  Abstract

  Selenium-based selenyl free radicals are chemical entities that may be involved in a range of biochemical processes. We report the first X-ray spectroscopic observation of a selenyl radical species generated photochemically by X-ray irradiation of low-temperature solutions of l-selenocysteine. We have employed high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and electron paramagnetic resonance (EPR) spectroscopy, coupled with density functional theory calculations, to characterize and understand the species. The HERFD-XAS spectrum of the selenyl radical is distinguished by a uniquely low-energy transition with a peak energy at 12 659.0 eV, which corresponds to a 1s → 4p transition to the singly occupied molecular orbital of the free radical. The EPR spectrum shows the broad features and highly anisotropic g-values that are expected for a selenium free radical species. The availability of spectroscopic probes for selenyl radicals may assist in understanding why life chooses selenium over sulfur in selected biochemical processes.

  View details for DOI 10.1021/acs.inorgchem.8b01522

  View details for PubMedID 30133265

• Empowering multicomponent cathode materials for sodium ion batteries by exploring three-dimensional compositional heterogeneities ENERGY & ENVIRONMENTAL SCIENCE Rahman, M., Xu, Y., Cheng, H., Shi, Q., Kou, R., Mu, L., Liu, Q., Xia, S., Xiao, X., Sun, C., Sokaras, D., Nordlund, D., Zheng, J., Liu, Y., Lin, F. 2018; 11 (9): 2496–2508

  View details for DOI 10.1039/c8ee00309b

  View details for Web of Science ID 000445215400016

• Exposed Equatorial Positions of Metal Centers via Sequential Ligand Elimination and Installation in MOFs JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yuan, S., Zhang, P., Zhang, L., Garcia-Esparza, A. T., Sokaras, D., Qin, J., Feng, L., Day, G. S., Chen, W., Drake, H. F., Elumalai, P., Madrahimov, S. T., Sun, D., Zhou, H. 2018; 140 (34): 10814–19

  Abstract

  Metal-organic frameworks (MOFs) provide highly designable platforms to construct complex coordination architectures for targeted applications. Herein, we demonstrate that trans-coordinated metal centers with exposed equatorial positions can be placed in a MOF matrix. A Zr-based MOF, namely, PCN-160, was initially synthesized as a scaffold structure. Postsynthetic linker labilization was subsequently implemented to partially remove the original dicarboxylate linkers and incorporate pyridinecarboxylates. A pair of neighboring pyridyl groups was arranged at proper proximity within the framework to form trans-binding sites that accommodate different metal cations including Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Pd2+. Furthermore, the trans-coordinated Ni2+ sites in porous frameworks can be readily accessed by substrates along the equatorial plane, facilitating the catalysis as manifested by the superior activity in ethylene dimerization over that observed for a cis-chelated catalyst.

  View details for PubMedID 30089362

• Targeted Ligand-Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wheeler, L. M., Sanehira, E. M., Marshall, A. R., Schulz, P., Suri, M., Anderson, N. C., Christians, J. A., Nordlund, D., Sokaras, D., Kroll, T., Harvey, S. P., Berry, J. J., Lin, L. Y., Luther, J. M. 2018; 140 (33): 10504-10513

  Abstract

  The ability to manipulate quantum dot (QD) surfaces is foundational to their technological deployment. Surface manipulation of metal halide perovskite (MHP) QDs has proven particularly challenging in comparison to that of more established inorganic materials due to dynamic surface species and low material formation energy; most conventional methods of chemical manipulation targeted at the MHP QD surface will result in transformation or dissolution of the MHP crystal. In previous work, we have demonstrated record-efficiency QD solar cells (QDSCs) based on ligand-exchange procedures that electronically couple MHP QDs yet maintain their nanocrystalline size, which stabilizes the corner-sharing structure of the constituent PbI64- octahedra with optoelectronic properties optimal for solar energy conversion. In this work, we employ a variety of spectroscopic techniques to develop a molecular-level understanding of the MHP QD surface chemistry in this system. We individually target both the anionic (oleate) and cationic (oleylammonium) ligands. We find that atmospheric moisture aids the process by hydrolysis of methyl acetate to generate acetic acid and methanol. Acetic acid then replaces native oleate ligands to yield QD surface-bound acetate and free oleic acid. The native oleylammonium ligands remain throughout this film deposition process and are exchanged during a final treatment step employing smaller cations-namely, formamidinium. This final treatment has a narrow processing window; initial treatment at this stage leads to a more strongly coupled QD regime followed by transformation into a bulk MHP film after longer treatment. These insights provide chemical understanding to the deposition of high-quality, electronically coupled MHP QD films that maintain both quantum confinement and their crystalline phase and attain high photovoltaic performance.

  View details for DOI 10.1021/jacs.8b04984

  View details for Web of Science ID 000442706400019

  View details for PubMedID 30044630

• Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons JOURNAL OF PHYSICAL CHEMISTRY A Pomerantz, A. E., Crace, E., Weng, T., Sokaras, D., Nordlun, D. 2018; 122 (26): 5730–34

  Abstract

  Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s-π* transitions can be broadened or even split in some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s-π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect.

  View details for PubMedID 29897245

• Ultrafast terahertz field control of electronic and structural interactions in vanadium dioxide PHYSICAL REVIEW B Gray, A. X., Hoffmann, M. C., Jeong, J., Aetukuri, N. P., Zhu, D., Hwang, H. Y., Brandt, N. C., Wen, H., Sternbach, A. J., Bonetti, S., Reid, A. H., Kukreja, R., Graves, C., Wang, T., Granitzka, P., Chen, Z., Higley, D. J., Chase, T., Jal, E., Abreu, E., Liu, M. K., Weng, T., Sokaras, D., Nordlund, D., Chollet, M., Alonso-Mori, R., Lemke, H., Glownia, J. M., Trigo, M., Zhu, Y., Ohldag, H., Freeland, J. W., Samant, M. G., Berakdar, J., Averitt, R. D., Nelson, K. A., Parkin, S. P., Durr, H. A. 2018; 98 (4)

  View details for DOI 10.1103/PhysRevB.98.045104

  View details for Web of Science ID 000436939100003

• Two-photon absorption of soft X-ray free electron laser radiation by graphite near the carbon K-absorption edge CHEMICAL PHYSICS LETTERS Lam, R. K., Raj, S. L., Pascal, T. A., Pemmaraju, C. D., Foglia, L., Simoncig, A., Fabris, N., Miotti, P., Hull, C. J., Rizzuto, A. M., Smith, J. W., Mincigrucci, R., Masciovecchio, C., Gessini, A., De Ninno, G., Diviacco, B., Roussel, E., Spampinati, S., Penco, G., Di Mitri, S., Trovo, M., Danailov, M. B., Christensen, S. T., Sokaras, D., Wengk, T., Coreno, M., Poletto, L., Drisdell, W. S., Prendergast, D., Giannessi, L., Principi, E., Nordlund, D., Saykally, R. J., Schwartz, C. P. 2018; 703: 112-116

  View details for DOI 10.1016/j.cplett.2018.05.021

  View details for Web of Science ID 000433247400018

• X-ray Emission Spectroscopy as an in Situ Diagnostic Tool for X-ray Crystallography of Metalloproteins Using an X-ray Free-Electron Laser. Biochemistry Fransson, T., Chatterjee, R., Fuller, F. D., Gul, S., Weninger, C., Sokaras, D., Kroll, T., Alonso-Mori, R., Bergmann, U., Kern, J., Yachandra, V. K., Yano, J. 2018

  Abstract

  Serial femtosecond crystallography (SFX) using the ultrashort X-ray pulses from a X-ray free-electron laser (XFEL) provides a new way of collecting structural data at room temperature that allows for following the reaction in real time after initiation. XFEL experiments are conducted in a shot-by-shot mode as the sample is destroyed and replenished after each X-ray pulse, and therefore, monitoring and controlling the data quality by using in situ diagnostic tools is critical. To study metalloenzymes, we developed the use of simultaneous collection of X-ray diffraction of crystals along with X-ray emission spectroscopy (XES) data that is used as a diagnostic tool for crystallography, by monitoring the chemical state of the metal catalytic center. We have optimized data analysis methods and sample delivery techniques for fast and active feedback to ensure the quality of each batch of samples and the turnover of the catalytic reaction caused by reaction triggering methods. Here, we describe this active in situ feedback system using Photosystem II as an example that catalyzes the oxidation of H2O to O2 at the Mn4CaO5 active site. We used the first moments of the Mn Kbeta1,3 emission spectra, which are sensitive to the oxidation state of Mn, as the primary diagnostics. This approach is applicable to different metalloproteins to determine the integrity of samples and follow changes in the chemical states of the reaction that can be initiated by light or activated by substrates and offers a metric for determining the diffraction images that are used for the final data sets.

  View details for DOI 10.1021/acs.biochem.8b00325

  View details for PubMedID 29906115

• Designing Boron Nitride Islands in Carbon Materials for Efficient Electrochemical Synthesis of Hydrogen Peroxide JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, S., Chen, Z., Siahrostami, S., Higgins, D., Nordlund, D., Sokaras, D., Kim, T., Liu, Y., Yan, X., Nilsson, E., Sinclair, R., Norskov, J. K., Jaramillo, T. F., Bao, Z. 2018; 140 (25): 7851-7859

  View details for DOI 10.1021/jacs.8b02798

  View details for Web of Science ID 000436910300018

• Ultrafast nonthermal heating of water initiated by an X-ray Free-Electron Laser PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Beyerlein, K. R., Jonsson, H., Alonso-Mori, R., Aquila, A., Barty, S., Barty, A., Bean, R., Koglin, J. E., Messerschmidt, M., Ragazzon, D., Sokaras, D., Williams, G. J., Hau-Riege, S., Boutet, S., Chapman, H. N., Imneanu, N. T., Caleman, C. 2018; 115 (22): 5652–57

  Abstract

  The bright ultrafast pulses of X-ray Free-Electron Lasers allow investigation into the structure of matter under extreme conditions. We have used single pulses to ionize and probe water as it undergoes a phase transition from liquid to plasma. We report changes in the structure of liquid water on a femtosecond time scale when irradiated by single 6.86 keV X-ray pulses of more than 106 J/cm2 These observations are supported by simulations based on molecular dynamics and plasma dynamics of a water system that is rapidly ionized and driven out of equilibrium. This exotic ionic and disordered state with the density of a liquid is suggested to be structurally different from a neutral thermally disordered state.

  View details for PubMedID 29760050

• Oxygen Release Induced Chemomechanical Breakdown of Layered Cathode Materials NANO LETTERS Mu, L., Lin, R., Xu, R., Han, L., Xia, S., Sokaras, D., Steiner, J. D., Weng, T., Nordlund, D., Doeff, M. M., Liu, Y., Zhao, K., Xin, H. L., Lin, F. 2018; 18 (5): 3241–49

  Abstract

  Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged Li1- xNi0.4Mn0.4Co0.2O2 materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice.

  View details for DOI 10.1021/acs.nanolett.8b01036

  View details for Web of Science ID 000432093200071

  View details for PubMedID 29667835

• Stimulated X-Ray Emission Spectroscopy in Transition Metal Complexes PHYSICAL REVIEW LETTERS Kroll, T., Weninger, C., Alonso-Mori, R., Sokaras, D., Zhu, D., Mercadier, L., Majety, V. P., Marinelli, A., Lutman, A., Guetg, M. W., Decker, F., Boutet, S., Aquila, A., Koglin, J., Koralek, J., DePonte, D. P., Kern, J., Fuller, F. D., Pastor, E., Fransson, T., Zhang, Y., Yano, J., Yachandra, V. K., Rohringer, N., Bergmann, U. 2018; 120 (13): 133203

  Abstract

  We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ∼1.7  eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ∼1  fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ∼10^{20}  W/cm^{2} for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous) x-ray emission spectroscopy are discussed. Our findings build a baseline for nonlinear x-ray spectroscopy for a wide range of transition metal complexes in inorganic chemistry, catalysis, and materials science.

  View details for DOI 10.1103/PhysRevLett.120.133203

  View details for Web of Science ID 000428394800008

  View details for PubMedID 29694162

• Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2 '-bipyridine)(CN)(4)](2-) PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kjaer, K. S., Kunnus, K., Harlang, T. B., Van Driel, T. B., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Biasin, E., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Uhlig, J., Sundstom, V., Nemeth, Z., Szemes, D., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Warnmark, K., Persson, P., Cordones, A. A., Gaffney, K. J. 2018; 20 (6): 4238–49

  Abstract

  The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.

  View details for PubMedID 29364300

• Soft X-Ray Second Harmonic Generation as an Interfacial Probe PHYSICAL REVIEW LETTERS Lam, R. K., Raj, S. L., Pascal, T. A., Pemmaraju, C. D., Foglia, L., Simoncig, A., Fabris, N., Miotti, P., Hull, C. J., Rizzuto, A. M., Smith, J. W., Mincigrucci, R., Masciovecchio, C., Gessini, A., Allaria, E., De Ninno, G., Diviacco, B., Roussel, E., Spampinati, S., Penco, G., Di Mitri, S., Trovo, M., Danailov, M., Christensen, S. T., Sokaras, D., Weng, T., Coreno, M., Poletto, L., Drisdell, W. S., Prendergast, D., Giannessi, L., Principi, E., Nordlund, D., Saykally, R. J., Schwartz, C. P. 2018; 120 (2): 023901

  Abstract

  Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284  eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.

  View details for DOI 10.1103/PhysRevLett.120.023901

  View details for Web of Science ID 000419478800006

  View details for PubMedID 29376703

• High Resolution, Seamless ePix10K Cameras for Wavelength-Dispersive Spectroscopy Chang, C., Blaj, G., Segal, J., Bhogadi, D., Kroll, T., Sokaras, D., Kenney, C., IEEE IEEE. 2018

  View details for Web of Science ID 000601256000125

• Depth-Dependent Redox Behavior of LiNi0.6Mn0.2Co0.2O2 JOURNAL OF THE ELECTROCHEMICAL SOCIETY Tian, C., Nordlund, D., Xin, H. L., Xu, Y., Liu, Y., Sokaras, D., Lin, F., Doeff, M. M. 2018; 165 (3): A696-A704

  View details for DOI 10.1149/2.1021803jes

  View details for Web of Science ID 000431790700033

• Defective Carbon-Based Materials for the Electrochemical Synthesis of Hydrogen Peroxide ACS SUSTAINABLE CHEMISTRY & ENGINEERING Chen, S., Chen, Z., Siahrostami, S., Kim, T., Nordlund, D., Sokaras, D., Nowak, S., To, J. F., Higgins, D., Sinclair, R., Norskov, J. K., Jaramillo, T. F., Bao, Z. 2018; 6 (1): 311–17

  View details for DOI 10.1021/acssuschemeng.7b02517

  View details for Web of Science ID 000419536800034

• L-edge spectroscopy of dilute, radiation-sensitive systems using a transition-edge-sensor array JOURNAL OF CHEMICAL PHYSICS Titus, C. J., Baker, M. L., Lee, S., Cho, H., Doriese, W. B., Fowler, J. W., Gaffney, K., Gard, J. D., Hilton, G. C., Kenney, C., Knight, J., Li, D., Marks, R., Minitti, M. P., Morgan, K. M., O'Neil, G. C., Reintsema, C. D., Schmidt, D. R., Sokaras, D., Swetz, D. S., Ullom, J. N., Weng, T., Williams, C., Young, B. A., Irwin, K. D., Solomon, E. I., Nordlund, D. 2017; 147 (21): 214201

  Abstract

  We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000 eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by grating spectrometers. These results show that soft-X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry, and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.

  View details for PubMedID 29221417

  View details for PubMedCentralID PMC5720893

• Systematic Structure Property Relationship Studies in Palladium Catalyzed Methane Complete Combustion ACS CATALYSIS Willis, J. J., Gallo, A., Sokaras, D., Aljama, H., Nowak, S. H., Goodman, E. D., Wu, L., Tassone, C. J., Jaramillo, T. F., Abild-Pedersen, F., Cargnello, M. 2017; 7 (11): 7810–21

  View details for DOI 10.1021/acscatal.7b02414

  View details for Web of Science ID 000414724700052

• High-Energy-Resolution X-ray Absorption Spectroscopy for Identification of Reactive Surface Species on Supported Single-Site Iridium Catalysts CHEMISTRY-A EUROPEAN JOURNAL Hoffman, A. S., Sokaras, D., Zhang, S., Debefve, L. M., Fang, C., Gallo, A., Kroll, T., Dixon, D. A., Bare, S. R., Gates, B. C. 2017; 23 (59): 14760–68

  Abstract

  We report high-energy-resolution X-ray absorption spectroscopy detection of ethylene and CO ligands adsorbed on catalytically active iridium centers isolated on zeolite HY and on MgO supports. The data are supported by density functional theory and FEFF X-ray absorption near-edge modelling, together with infrared (IR) spectra. The results demonstrate that high-energy-resolution X-ray absorption spectra near the iridium LIII (2p3/2 ) edge provide clearly ascribable, distinctive signatures of the ethylene and CO ligands and illustrate effects of supports and other ligands. This X-ray absorption technique is markedly more sensitive than conventional IR spectroscopy for characterizing surface intermediates, and it is applicable to samples having low metal loadings and in reactive atmospheres and is expected to have an increasing role in catalysis research by facilitating the determination of mechanisms of solid-catalyzed reactions through identification of reaction intermediates in working catalysts.

  View details for PubMedID 28749554

• Noninvasive Synchrotron-Based X-ray Raman Scattering Discriminates Carbonaceous Compounds in Ancient and Historical Materials ANALYTICAL CHEMISTRY Gueriau, P., Rueff, J., Bernard, S., Kaddissy, J. A., Goler, S., Sahle, C. J., Sokaras, D., Wogelius, R. A., Manning, P. L., Bergmann, U., Bertrand, L. 2017; 89 (20): 10819-10826

  Abstract

  Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems including ancient and historic materials ranging from cultural heritage to paleontology. Determining their speciation cannot only provide unique information on their origin but may also elucidate degradation processes. Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge (280-350 eV) is a very powerful method to probe carbon speciation. However, the short penetration depth of soft X-rays imposes stringent constraints on sample type, preparation, and analytical environment. A hard X-ray probe such as X-ray Raman scattering (XRS) can overcome many of these difficulties. Here we report the use of XRS at ∼6 keV incident energy to collect carbon K-edge XANES data and probe the speciation of organic carbon in several specimens relevant to cultural heritage and natural history. This methodology enables the measurement to be done in a nondestructive way, in air, and provides information that is not compromised by surface contamination by ensuring that the dominant signal contribution is from the bulk of the probed material. Using the backscattering geometry at large photon momentum transfer maximizes the XRS signal at the given X-ray energy and enhances nondipole contributions compared to conventional XANES, thereby augmenting the speciation sensitivity. The capabilities and limitations of the technique are discussed. We show that despite its small cross section, for a range of systems the XRS method can provide satisfactory signals at realistic experimental conditions. XRS constitutes a powerful complement to FT-IR, Raman, and conventional XANES spectroscopy, overcoming some of the limitations of these techniques.

  View details for DOI 10.1021/acs.analchem.7b02202

  View details for Web of Science ID 000413392500026

  View details for PubMedID 28902506

• Local vs Nonlocal States in FeTiO₃ Probed with 1s2pRIXS: Implications for Photochemistry INORGANIC CHEMISTRY Hunault, M. Y., Khan, W., Minar, J., Kroll, T., Sokaras, D., Zimmermann, P., Delgado-Jaime, M. U., de Groot, F. F. 2017; 56 (18): 10882-10892

  Abstract

  Metal-metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron-titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core-hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

  View details for DOI 10.1021/acs.inorgchem.7b00938

  View details for Web of Science ID 000411549400008

  View details for PubMedID 28872322

  View details for PubMedCentralID PMC5636175

• Insight into the electronic structure of transition metal ion complexes from resonant inelastic X-ray scattering Kroll, T., Hadt, R., Wilson, S., Baker, M., Lundberg, M., Yan, J., Weng, T., Sokaras, D., Alonso-Mori, R., Casa, D., Upton, M., Hedman, B., Hodgson, K., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000429556700220

• Janus monolayers of transition metal dichalcogenides NATURE NANOTECHNOLOGY Lu, A., Zhu, H., Xiao, J., Chuu, C., Han, Y., Chiu, M., Cheng, C., Yang, C., Wei, K., Yang, Y., Wang, Y., Sokaras, D., Nordlund, D., Yang, P., Muller, D. A., Chou, M., Zhang, X., Li, L. 2017; 12 (8): 744-+

  Abstract

  Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

  View details for DOI 10.1038/NNANO.2017.100

  View details for Web of Science ID 000406868800011

  View details for PubMedID 28507333

• Effects of Gold Substrates on the Intrinsic and Extrinsic Activity of High-Loading Nickel-Based Oxyhydroxide Oxygen Evolution Catalysts ACS CATALYSIS Chakthranont, P., Kibsgaard, J., Gallo, A., Park, J., Mitani, M., Sokaras, D., Kroll, T., Sinclair, R., Mogensen, M. B., Jaramillo, T. F. 2017; 7 (8): 5399–5409

  View details for DOI 10.1021/acscatal.7b01070

  View details for Web of Science ID 000407309100055

• Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2 '- bipyridine)(2)(CN)(2)] STRUCTURAL DYNAMICS Kjaer, K. S., Zhang, W., Alonso-Mori, R., Bergmann, U., Chollet, M., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Robinson, J. S., Solomon, E. I., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Persson, P., Warnmark, K., Sundstrom, V., Gaffney, K. J. 2017; 4 (4): 044030

  Abstract

  We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6-2N]2N-4, where N = 1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

  View details for PubMedID 28653021

• Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy SCIENCE Mara, M. W., Hadt, R. G., Reinhard, M., Kroll, T., Lim, H., Hartsock, R. W., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Hodgson, K. O., Hedman, B., Bergmann, U., Gaffney, K. J., Solomon, E. I. 2017; 356 (6344): 1276-+

  Abstract

  The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

  View details for PubMedID 28642436

  View details for PubMedCentralID PMC5706643

• An Oxygen-Insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-Based Layer for Overall Water Splitting ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Garcia-Esparza, A. T., Shinagawa, T., Ould-Chikh, S., Qureshi, M., Peng, X., Wei, N., Anjum, D. H., Clo, A., Weng, T., Nordlund, D., Sokaras, D., Kubota, J., Domen, K., Takanabe, K. 2017; 56 (21): 5780-5784

  Abstract

  For overall water-splitting systems, it is essential to establish O2-insensitive cathodes that allow cogeneration of H2and O2. An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H2evolution in the presence of O2. In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O2gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoOx/Pt/SrTiO3with inhibited water formation from H2and O2, which is the prevailing back reaction on the bare Pt/SrTiO3photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis.

  View details for DOI 10.1002/anie.201701861

  View details for Web of Science ID 000400755800021

  View details for PubMedID 28407339

• Determining Atomic-Scale Structure and Composition of Organo-Lead Halide Perovskites by Combining High-Resolution X-ray Absorption Spectroscopy and First-Principles Calculations ACS ENERGY LETTERS Drisdell, W. S., Leppert, L., Sutter-Fella, C. M., Liang, Y., Li, Y., Ngo, Q. P., Wan, L. F., Gul, S., Kroll, T., Sokaras, D., Javey, A., Yano, J., Neaton, J. B., Toma, F. M., Prendergast, D., Sharp, I. D. 2017; 2 (5): 1183-1189

  View details for DOI 10.1021/acsenergylett.7b00182

  View details for Web of Science ID 000401500200035

• Probing the local electronic structure of dilute bioinorganic active sites using ultra-sensitive soft X-ray detectors Titus, C., Alpert, B., Baker, M., Cho, H., Denison, E., Doriese, W., Fowler, J., Gaffney, K., Gao, B., Gard, J., Hilton, G., Irwin, K., Joe, Y., Kenney, C., Knight, J., Kroll, T., Lee, S., Li, D., Minitti, M., Morgan, K., Mori, R., Nordlund, D., O'Neil, G., Ogasawara, H., Reintsema, C., Schmidt, D., Sokaras, D., Song, Y., Swetz, D., Ullom, J., Weng, T., Williams, C., Young, B. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430568501597

• Fundamental understanding of defective carbon-based materials for electrochemical synthesis of hydrogen peroxide Chen, S., Chen, Z., Siahrostami, S., Kim, T., Nordlund, D., Sokaras, D., Nowak, S., To, J., Higgins, D., Sinclair, R., Studt, F., Norskov, J., Jaramillo, T., Bao, Z. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430568502299

• Ultra sensitive probing of the local electronic structure based on state-of-the-art Transition-Edge Sensor (TES) technology and soft x-ray spectroscopy Nordlund, D., Alpert, B., Baker, M., Cho, H., Denison, E., Doriese, W., Fowler, J., Gaffney, K., Gao, B., Gard, J., Hilton, G., Irwin, K., Joe, Y., Kenney, C., Knight, J., Kroll, T., Lee, S., Li, D., Marks, R., Minitti, M., Mori, R., Ogasawara, H., O'Neil, G., Schmidt, D., Sokaras, D., Swetz, D., Song, Y., Titus, C., Ullom, J., Weng, T., Williams, C., Young, B. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430568502277

• Ultrasensitive probing of the local electronic structure of nitrogen doped carbon and its applications to 2D electronics, catalysis and bio-physics Lee, S., Mori, R., Alpert, B., Baker, M., Berry, J., Cho, H., Denison, E., Doriese, W., Fowler, J., Gaffney, K., Gao, B., Gard, J., Hilton, G., Irwin, K., Joe, Y., Kaya, S., Kenney, C., Knight, J., Kroll, T., Li, D., Marks, R., Minitti, M., Morgan, K., Nordlund, D., O'Neil, G., Ogasawara, H., Pianetta, P., Reintsema, C., Schiros, T., Schmidt, D., Sokaras, D., Song, Y., Swetz, D., Titus, C., Ullom, J., Weng, T., Williams, C., Wolcott, A., Young, B. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430568506218

• Electrochemical generation of H2O2: Development of a reactor with carbon catalysts for portable low-cost water purification Chen, Z., Chen, S., Siahrostami, S., Chakthranont, P., Hahn, C., Nordlund, D., Sokaras, D., Norskov, J., Bao, Z., Jaramillo, T. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569100599

• Determination of differential orbital covalency of heme active sites by L-edge spectroscopy Baker, M., Alpert, B., Cho, H., Denison, E., Doriese, W., Fowler, J., Gaffney, K., Gard, J., Gao, B., Hilton, G., Irwin, K., Joe, Y., Kenney, C., Knight, J., Kroll, T., Lee, S., Li, D., Marks, R., Minitti, M., Morgan, K., Mori, R., Ogasawara, H., O'Neil, G., Schmidt, D., Sokaras, D., Swetz, D., Song, Y., Titus, C., Ullom, J., Weng, T., Williams, C., Yan, J., Young, B., Nordlund, D., Solomon, E. AMER CHEMICAL SOC. 2017

  View details for Web of Science ID 000430569103813

• Operando investigation of Au-MnOx thin films with improved activity for the oxygen evolution reaction ELECTROCHIMICA ACTA Frydendal, R., Seitz, L. C., Sokaras, D., Weng, T., Nordlund, D., Chorkendorff, I., Stephens, I. E., Jaramillo, T. F. 2017; 230: 22-28

  View details for DOI 10.1016/j.electacta.2017.01.085

  View details for Web of Science ID 000395599900003

• Drop-on-demand sample delivery for studying biocatalysts in action at X-ray free-electron lasers. Nature methods Fuller, F. D., Gul, S., Chatterjee, R., Burgie, E. S., Young, I. D., Lebrette, H., Srinivas, V., Brewster, A. S., Michels-Clark, T., Clinger, J. A., Andi, B., Ibrahim, M., Pastor, E., de Lichtenberg, C., Hussein, R., Pollock, C. J., Zhang, M., Stan, C. A., Kroll, T., Fransson, T., Weninger, C., Kubin, M., Aller, P., Lassalle, L., Bräuer, P., Miller, M. D., Amin, M., Koroidov, S., Roessler, C. G., Allaire, M., Sierra, R. G., Docker, P. T., Glownia, J. M., Nelson, S., Koglin, J. E., Zhu, D., Chollet, M., Song, S., Lemke, H., Liang, M., Sokaras, D., Alonso-Mori, R., Zouni, A., Messinger, J., Bergmann, U., Boal, A. K., Bollinger, J. M., Krebs, C., Högbom, M., Phillips, G. N., Vierstra, R. D., Sauter, N. K., Orville, A. M., Kern, J., Yachandra, V. K., Yano, J. 2017

  Abstract

  X-ray crystallography at X-ray free-electron laser sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy, both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing insights into the interplay between the protein structure and dynamics and the chemistry at an active site. The implementation of such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly affects the data quality. We present here a robust way of delivering controlled sample amounts on demand using acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. Studies with photosystem II, the phytochrome photoreceptor, and ribonucleotide reductase R2 illustrate the power and versatility of this method.

  View details for DOI 10.1038/nmeth.4195

  View details for PubMedID 28250468

  View details for PubMedCentralID PMC5376230

• Soft X-ray absorption spectroscopy investigation of the surface chemistry and treatments of copper indium gallium diselenide (CIGS) SOLAR ENERGY MATERIALS AND SOLAR CELLS Schwartz, C., Nordlund, D., Sokaras, D., Contreras, M., Weng, T., Mansfield, L., Hurst, K. E., Dameron, A., Ramanathan, K., Prendergast, D., Christensen, S. T. 2017; 160: 390-397

  View details for DOI 10.1016/j.solmat.2016.11.003

  View details for Web of Science ID 000390072700048

• Charge and Spin-State Characterization of Cobalt Bis(<i>o</i>-dioxolene) Valence Tautomers Using Co Kβ X-ray Emission and L-Edge X-ray Absorption Spectroscopies INORGANIC CHEMISTRY Liang, H., Kroll, T., Nordlund, D., Weng, T., Sokaras, D., Pierpont, C. G., Gaffney, K. J. 2017; 56 (2): 737-747

  Abstract

  The valence tautomeric states of Co(phen)(3,5-DBQ)2 and Co(tmeda)(3,5-DBQ)2, where 3,5-DBQ is either the semiquinone (SQ(-)) or catecholate (Cat(2-)) form of 3,5-di-tert-butyl-1,2-benzoquinone, have been examined by a series of cobalt-specific X-ray spectroscopies. In this work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy (Kβ XES) to the oxidation and spin states of 3d transition-metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their Kβ XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low-temperature valence tautomer can be described as a low-spin Co(III) configuration and the high-temperature valence tautomer as a high-spin Co(II) configuration. This conclusion is further supported by Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the Kβ XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ(-) in cobalt valence tautomers is discussed in view of the effective spin at the Co site from Kβ XES and the molecular spin moment from magnetic susceptibility measurements.

  View details for DOI 10.1021/acs.inorgchem.6b01666

  View details for Web of Science ID 000392262400011

  View details for PubMedID 28035824

• Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution CHEMICAL SCIENCE Zhang, W., Kjaer, K. S., Alonso-Mori, R., Bergmann, U., Chollet, M., Fredin, L. A., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Persson, P., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Warnmark, K., Sundstromb, V., Gaffney, K. J. 2017; 8 (1): 515-523

  Abstract

  Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)3]2+ by more than two orders of magnitude.

  View details for DOI 10.1039/c6sc03070j

  View details for Web of Science ID 000391454500060

  View details for PubMedCentralID PMC5341207

• Structure of photosystem II and substrate binding at room temperature NATURE Young, I. D., Ibrahim, M., Chatterjee, R., Gul, S., Fuller, F. D., Koroidov, S., Brewster, A. S., Tran, R., Alonso-Mori, R., Kroll, T., Michels-Clark, T., Laksmono, H., Sierra, R. G., Stan, C. A., Hussein, R., Zhang, M., Douthit, L., Kubin, M., de Lichtenberg, C., Long Vo Pham, L. V., Nilsson, H., Cheah, M. H., Shevela, D., Saracini, C., Bean, M. A., Seuffert, I., Sokaras, D., Weng, T., Pastor, E., Weninger, C., Fransson, T., Lassalle, L., Braeuer, P., Aller, P., Docker, P. T., Andi, B., Orville, A. M., Glownia, J. M., Nelson, S., Sikorski, M., Zhu, D., Hunter, M. S., Lane, T. J., Aquila, A., Koglin, J. E., Robinson, J., Liang, M., Boutet, S., Lyubimov, A. Y., Uervirojnangkoorn, M., Moriarty, N. W., Liebschner, D., Afonine, P. V., Waterman, D. G., Evans, G., Wernet, P., Dobbek, H., Weis, W. I., Brunger, A. T., Zwart, P. H., Adams, P. D., Zouni, A., Messinger, J., Bergmann, U., Sauter, N. K., Kern, J., Yachandra, V. K., Yano, J. 2016; 540 (7633): 453-?

  Abstract

  Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment protein complex, couples the one-electron photochemistry at the reaction centre with the four-electron redox chemistry of water oxidation at the Mn4CaO5 cluster in the oxygen-evolving complex (OEC). Under illumination, the OEC cycles through five intermediate S-states (S0 to S4), in which S1 is the dark-stable state and S3 is the last semi-stable state before O-O bond formation and O2 evolution. A detailed understanding of the O-O bond formation mechanism remains a challenge, and will require elucidation of both the structures of the OEC in the different S-states and the binding of the two substrate waters to the catalytic site. Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage-free, room temperature structures of dark-adapted (S1), two-flash illuminated (2F; S3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S3-enriched) PS II. Although the recent 1.95 Å resolution structure of PS II at cryogenic temperature using an XFEL provided a damage-free view of the S1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions, and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analogue, has been used as a marker, as it binds to the Mn4CaO5 cluster in the S2 and S3 states. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site. This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.

  View details for DOI 10.1038/nature20161

  View details for Web of Science ID 000389716800046

  View details for PubMedID 27871088

  View details for PubMedCentralID PMC5201176

• Elemental characterisation of melanin in feathers via synchrotron X-ray imaging and absorption spectroscopy SCIENTIFIC REPORTS Edwards, N. P., van Veelen, A., Anne, J., Manning, P. L., Bergmann, U., Sellers, W. I., Egerton, V. M., Sokaras, D., Alonso-Mori, R., Wakamatsu, K., Ito, S., Wogelius, R. A. 2016; 6: 34002

  Abstract

  Melanin is a critical component of biological systems, but the exact chemistry of melanin is still imprecisely known. This is partly due to melanin's complex heterogeneous nature and partly because many studies use synthetic analogues and/or pigments extracted from their natural biological setting, which may display important differences from endogenous pigments. Here we demonstrate how synchrotron X-ray analyses can non-destructively characterise the elements associated with melanin pigment in situ within extant feathers. Elemental imaging shows that the distributions of Ca, Cu and Zn are almost exclusively controlled by melanin pigment distribution. X-ray absorption spectroscopy demonstrates that the atomic coordination of zinc and sulfur is different within eumelanised regions compared to pheomelanised regions. This not only impacts our fundamental understanding of pigmentation in extant organisms but also provides a significant contribution to the evidence-based colour palette available for reconstructing the appearance of fossil organisms.

  View details for DOI 10.1038/srep34002

  View details for Web of Science ID 000384068300001

  View details for PubMedID 27658854

  View details for PubMedCentralID PMC5034265

• Probing 5 <i>f</i> -state configurations in URu₂Si₂ with U <i>L</i>_III-edge resonant x-ray emission spectroscopy PHYSICAL REVIEW B Booth, C. H., Medling, S. A., Tobin, J. G., Baumbach, R. E., Bauer, E. D., Sokaras, D., Nordlund, D., Weng, T. 2016; 94 (4)

  View details for DOI 10.1103/PhysRevB.94.045121

  View details for Web of Science ID 000379651600007

• Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)(2)](2+) PHYSICAL REVIEW LETTERS Biasin, E., van Driel, T., Kjaer, K. S., Dohn, A. O., Christensen, M., Harlang, T., Chabera, P., Liu, Y., Uhlig, J., Papai, M., Nemeth, Z., Hartsock, R., Liang, W., Zhang, J., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Assefa, T. A., Britz, A., Galler, A., Gawelda, W., Bressler, C., Gaffney, K. J., Lemke, H. T., Moller, K. B., Nielsen, M. M., Sundstrom, V., Vanko, G., Warnmark, K., Canton, S. E., Haldrup, K. 2016; 117 (1): 013002

  Abstract

  We study the structural dynamics of photoexcited [Co(terpy)_{2}]^{2+} in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7  ps.

  View details for DOI 10.1103/PhysRevLett.117.013002

  View details for Web of Science ID 000378881300004

  View details for PubMedID 27419566

• Electronic structure study of the CdS buffer layer in CIGS solar cells by X-ray absorption spectroscopy: Experiment and theory SOLAR ENERGY MATERIALS AND SOLAR CELLS Schwartz, C., Nordlund, D., Weng, T., Sokaras, D., Mansfield, L., Krishnapriyan, A. S., Ramanathan, K., Hurst, K. E., Prendergast, D., Christensen, S. T. 2016; 149: 275-283

  View details for DOI 10.1016/j.solmat.2016.01.043

  View details for Web of Science ID 000373539900036

• Geometry of electromechanically active structures in Gadolinium - doped Cerium oxides AIP ADVANCES Li, Y., Kraynis, O., Kas, J., Weng, T., Sokaras, D., Zacharowicz, R., Lubomirsky, I., Frenkel, A. I. 2016; 6 (5)

  View details for DOI 10.1063/1.4952645

  View details for Web of Science ID 000377962500091

• Kβ Valence to Core X-ray Emission Studies of Cu(I) Binding Proteins with Mixed Methionine - Histidine Coordination. Relevance to the Reactivity of the M- and H-sites of Peptidylglycine Monooxygenase INORGANIC CHEMISTRY Martin-Diaconescu, V., Chacon, K. N., Delgado-Jaime, M., Sokaras, D., Weng, T., DeBeer, S., Blackburn, N. J. 2016; 55 (7): 3431-3439

  Abstract

  Biological systems use copper as a redox center in many metalloproteins, where the role of the metal is to cycle between its +1 and +2 oxidation states. This chemistry requires the redox potential to be in a range that can stabilize both Cu(I) and Cu(II) states and often involves protein-derived ligand sets involving mixed histidine-methionine coordination that balance the preferences of both oxidation states. Transport proteins, on the other hand, utilize copper in the Cu(I) state and often contain sites comprised predominately of the cuprophilic residue methionine. The electronic factors that allow enzymes and transporters to balance their redox requirements are complex and are often elusive due to the dearth of spectroscopic probes of the Cu(I) state. Here we present the novel application of X-ray emission spectroscopy to copper proteins via a study of a series of mixed His-Met copper sites where the ligand set varies in a systematic way between the His3 and Met3 limits. The sites are derived from the wild-type peptidylglycine monooxygenase (PHM), two single-site variants which replicate each of its two copper sites (CuM-site and CuH-site), and the transporters CusF and CusB. Clear differences are observed in the Kβ2,5 region at the Met3 and His3 limits. CusB (Met3) has a distinct peak at 8978.4 eV with a broad shoulder at 8975.6 eV, whereas CuH (His3) has two well-resolved features: a more intense feature at 8974.8 eV and a second at 8977.2 eV. The mixed coordination sphere CusF (Met2His) and the PHM CuM variant (Met1His2) have very similar spectra consisting of two features at 8975.2 and 8977.8 eV. An analysis of DFT calculated spectra indicate that the intensity of the higher energy peak near 8978 eV is mediated by mixing of ligand-based orbitals into the Cu d(10) manifold, with S from Met providing more intensity by facilitating increased Cu p-d mixing. Furthermore, reaction of WT PHM with CO (an oxygen analogue) produced the M site CO complex, which showed a unique XES spectrum that could be computationally reproduced by including interactions between Cu(I) and the CO ligand. The study suggests that the valence-to-core (VtC) region can not only serve as a probe of ligand speciation but also offer insight into the coordination geometry, in a fashion similar to XAS pre-edges, and may be sufficiently sensitive to the coordination of exogenous ligands to be useful in the study of reaction mechanisms.

  View details for DOI 10.1021/acs.inorgchem.5b02842

  View details for Web of Science ID 000373550700029

  View details for PubMedID 26965786

  View details for PubMedCentralID PMC4878823

• On the valence fluctuation in the early actinide metals JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Soederlind, P., Landa, A., Tobin, J. G., Allen, P., Medling, S., Booth, C. H., Bauer, E. D., Cooley, J. C., Sokaras, D., Weng, T., Nordlund, D. 2016; 207: 14-18

  View details for DOI 10.1016/j.elspec.2015.11.014

  View details for Web of Science ID 000371940400003

• Towards characterization of photo-excited electron transfer and catalysis in natural and artificial systems using XFELs FARADAY DISCUSSIONS Alonso-Mori, R., Asa, K., Bergmann, U., Brewster, A. S., Chatterjee, R., Cooper, J. K., Frei, H. M., Fuller, F. D., Goggins, E., Gul, S., Fukuzawa, H., Iablonskyi, D., Ibrahim, M., Katayama, T., Kroll, T., Kumagai, Y., McClure, B. A., Messinger, J., Motomura, K., Nagaya, K., Nishiyama, T., Saracini, C., Sato, Y., Sauter, N. K., Sokaras, D., Takanashi, T., Togashi, T., Ueda, K., Weare, W. W., Weng, T., Yabashi, M., Yachandra, V. K., Young, I. D., Zouni, A., Kern, J. F., Yano, J. 2016; 194: 621-638

  Abstract

  The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn-Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.

  View details for DOI 10.1039/c6fd00084c

  View details for Web of Science ID 000392422200028

  View details for PubMedID 27711803

  View details for PubMedCentralID PMC5177497

• [Ni<SUP>III</SUP>(OMe)]-mediated reductive activation of CO₂ affording a Ni(κ<SUP>1</SUP>-OCO) complex CHEMICAL SCIENCE Chiou, T., Tseng, Y., Lu, T., Weng, T., Sokaras, D., Ho, W., Kuo, T., Jang, L., Lee, J., Liaw, W. 2016; 7 (6): 3640-3644

  Abstract

  Carbon dioxide is expected to be employed as an inexpensive and potential feedstock of C1 sources for the mass production of valuable chemicals and fuel. Versatile chemical transformations of CO2, i.e. insertion of CO2 producing bicarbonate/acetate/formate, cleavage of CO2 yielding μ-CO/μ-oxo transition-metal complexes, and electrocatalytic reduction of CO2 affording CO/HCOOH/CH3OH/CH4/C2H4/oxalate were well documented. Herein, we report a novel pathway for the reductive activation of CO2 by the [NiIII(OMe)(P(C6H3-3-SiMe3-2-S)3)]- complex, yielding the [NiIII(κ1-OCO˙-)(P(C6H3-3-SiMe3-2-S)3)]- complex. The formation of this unusual NiIII(κ1-OCO˙-) complex was characterized by single-crystal X-ray diffraction, EPR, IR, SQUID, Ni/S K-edge X-ray absorption spectroscopy, and Ni valence-to-core X-ray emission spectroscopy. The inertness of the analogous complexes [NiIII(SPh)], [NiII(CO)], and [NiII(N2H4)] toward CO2, in contrast, demonstrates that the ionic [NiIII(OMe)] core attracts the binding of weak σ-donor CO2 and triggers the subsequent reduction of CO2 by the nucleophilic [OMe]- in the immediate vicinity. This metal-ligand cooperative activation of CO2 may open a novel pathway promoting the subsequent incorporation of CO2 in the buildup of functionalized products.

  View details for DOI 10.1039/c5sc04652a

  View details for Web of Science ID 000377262200019

  View details for PubMedID 30008996

  View details for PubMedCentralID PMC6008733

• Structural changes correlated with magnetic spin state isomorphism in the S₂ state of the Mn₄CaO₅ cluster in the oxygen-evolving complex of photosystem II CHEMICAL SCIENCE Chatterjee, R., Han, G., Kern, J., Gul, S., Fuller, F. D., Garachtchenko, A., Young, I. D., Weng, T., Nordlund, D., Alonso-Mori, R., Bergmann, U., Sokaras, D., Hatakeyama, M., Yachandra, V. K., Yano, J. 2016; 7 (8): 5236-5248

  Abstract

  The Mn4CaO5 cluster in Photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (Si, i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2 state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2 isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2 states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. Such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.

  View details for DOI 10.1039/c6sc00512h

  View details for Web of Science ID 000380893900054

  View details for PubMedID 28044099

  View details for PubMedCentralID PMC5201215

• To Transfer or Not to Transfer? Development of a Dinitrosyl Iron Complex as a Nitroxyl Donor for the Nitroxylation of an Fe<SUP>III</SUP>-Porphyrin Center CHEMISTRY-A EUROPEAN JOURNAL Tseng, Y., Chen, C., Lin, J., Li, B., Lu, Y., Lin, C., Chen, H., Weng, T., Sokaras, D., Chen, H., Soo, Y., Lu, T. 2015; 21 (49): 17570-17573

  Abstract

  A positive myocardial inotropic effect achieved using HNO/NO(-) , compared with NO⋅, triggered attempts to explore novel nitroxyl donors for use in clinical applications in vascular and myocardial pharmacology. To develop M-NO complexes for nitroxyl chemistry and biology, modulation of direct nitroxyl-transfer reactivity of dinitrosyl iron complexes (DNICs) is investigated in this study using a Fe(III) -porphyrin complex and proteins as a specific probe. Stable dinuclear {Fe(NO)2 }(9) DNIC [Fe(μ-(Me) Pyr)(NO)2 ]2 was discovered as a potent nitroxyl donor for nitroxylation of Fe(III) -heme centers through an associative mechanism. Beyond the efficient nitroxyl transfer, transformation of DNICs into a chemical biology probe for nitroxyl and for pharmaceutical applications demands further efforts using in vitro/in vivo studies.

  View details for DOI 10.1002/chem.201503176

  View details for Web of Science ID 000367185700004

  View details for PubMedID 26437878

• Bioturbating animals control the mobility of redox-sensitive trace elements in organic-rich mudstone GEOLOGY Harazim, D., McIlroy, D., Edwards, N. P., Wogelius, R. A., Manning, P. L., Poduska, K. M., Layne, G. D., Sokaras, D., Alonso-Mori, R., Bergmann, U. 2015; 43 (11): 1007-1010

  View details for DOI 10.1130/G37025.1

  View details for Web of Science ID 000364057700020

• Revealing and suppressing surface Mn(II) formation of Na_0.44MnO₂ electrodes for Na-ion batteries NANO ENERGY Qiao, R., Dai, K., Mao, J., Weng, T., Sokaras, D., Nordlund, D., Song, X., Battaglia, V. S., Hussain, Z., Liu, G., Yang, W. 2015; 16: 186-195

  View details for DOI 10.1016/j.nanoen.2015.06.024

  View details for Web of Science ID 000364579300020

• Covalency in oxidized uranium PHYSICAL REVIEW B Tobin, J. G., Yu, S., Qiao, R., Yang, W. L., Booth, C. H., Shuh, D. K., Duffin, A. M., Sokaras, D., Nordlund, D., Weng, T. 2015; 92 (4)

  View details for DOI 10.1103/PhysRevB.92.045130

  View details for Web of Science ID 000358845700006

• Oxidation and crystal field effects in uranium PHYSICAL REVIEW B Tobin, J. G., Yu, S., Booth, C. H., Tyliszczak, T., Shuh, D. K., Van der Laan, G., Sokaras, D., Nordlund, D., Weng, T., Bagus, P. S. 2015; 92 (3)

  View details for DOI 10.1103/PhysRevB.92.035111

  View details for Web of Science ID 000357486100005

• Accurate macromolecular structures using minimal measurements from X-ray free-electron lasers (vol 11, pg 545, 2014) NATURE METHODS Hattne, J., Echols, N., Tran, R., Kern, J., Gildea, R. J., Brewster, A. S., Alonso-Mori, R., Gloeckner, C., Hellmich, J., Laksmono, H., Sierra, R. G., Lassalle-Kaiser, B., Lampe, A., Han, G., Gul, S., DiFiore, D., Milathianaki, D., Fry, A. R., Miahnahri, A., White, W. E., Schafer, D. W., Seibert, M., Koglin, J. E., Sokaras, D., Weng, T., Sellberg, J., Latimer, M. J., Glatzel, P., Zwart, P. H., Grosse-Kunstleve, R. W., Bogan, M. J., Messerschmidt, M., Williams, G. J., Boutet, S., Messinger, J., Zouni, A., Yano, J., Bergmann, U., Yachandra, V. K., Adams, P. D., Sauter, N. K. 2015; 12 (7): 692

  View details for DOI 10.1038/nmeth0715-692d

  View details for Web of Science ID 000357405700033

• Focus characterization at an X-ray free-electron laser by coherent scattering and speckle analysis JOURNAL OF SYNCHROTRON RADIATION Sikorski, M., Song, S., Schropp, A., Seiboth, F., Feng, Y., Alonso-Mori, R., Chollet, M., Lemke, H. T., Sokaras, D., Weng, T., Zhang, W., Robert, A., Zhu, D. 2015; 22: 599-605

  Abstract

  X-ray focus optimization and characterization based on coherent scattering and quantitative speckle size measurements was demonstrated at the Linac Coherent Light Source. Its performance as a single-pulse free-electron laser beam diagnostic was tested for two typical focusing configurations. The results derived from the speckle size/shape analysis show the effectiveness of this technique in finding the focus' location, size and shape. In addition, its single-pulse compatibility enables users to capture pulse-to-pulse fluctuations in focus properties compared with other techniques that require scanning and averaging.

  View details for DOI 10.1107/S1600577515004361

  View details for Web of Science ID 000353920300021

  View details for PubMedID 25931074

  View details for PubMedCentralID PMC4416675

• Photon-in photon-out hard X-ray spectroscopy at the Linac Coherent Light Source JOURNAL OF SYNCHROTRON RADIATION Alonso-Mori, R., Sokaras, D., Zhu, D., Kroll, T., Chollet, M., Feng, Y., Glownia, J. M., Kern, J., Lemke, H. T., Nordlund, D., Robert, A., Sikorski, M., Song, S., Weng, T., Bergmann, U. 2015; 22: 612-620

  Abstract

  X-ray free-electron lasers (FELs) have opened unprecedented possibilities to study the structure and dynamics of matter at an atomic level and ultra-fast timescale. Many of the techniques routinely used at storage ring facilities are being adapted for experiments conducted at FELs. In order to take full advantage of these new sources several challenges have to be overcome. They are related to the very different source characteristics and its resulting impact on sample delivery, X-ray optics, X-ray detection and data acquisition. Here it is described how photon-in photon-out hard X-ray spectroscopy techniques can be applied to study the electronic structure and its dynamics of transition metal systems with ultra-bright and ultra-short FEL X-ray pulses. In particular, some of the experimental details that are different compared with synchrotron-based setups are discussed and illustrated by recent measurements performed at the Linac Coherent Light Source.

  View details for DOI 10.1107/S1600577515004488

  View details for Web of Science ID 000353920300023

  View details for PubMedID 25931076

  View details for PubMedCentralID PMC4416677

• Hard X-rays in-soft X-rays out: An operando piggyback view deep into a charging lithium ion battery with X-ray Raman spectroscopy JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Braun, A., Nordlund, D., Song, S., Huang, T., Sokaras, D., Liu, X., Yang, W., Weng, T., Liu, Z. 2015; 200: 257-263

  View details for DOI 10.1016/j.elspec.2015.03.005

  View details for Web of Science ID 000362607600023

• Electronic structure study of CIGS solar cells by X-ray absorption spectroscopy: Experiment and theory Schwartz, C., Nordlund, D., Weng, T., Ramanathan, K., Sokaras, D., Prendergast, D., Christensen, S. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186503471

• Experimental and theoretical determinations of covalency in d- and f-block metal oxides Minasian, S., Batista, E., Booth, C., Keith, J., Lukens, W., Kozimor, S., Martin, R., Nordlund, D., Shuh, D., Sokaras, D., Tyliszczak, T., Wen, X., Weng, T. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186502692

• Investigation of valence tautomerism in cobalt-dioxolene vomplexes using K X-ray emission spectroscopy Liang, W., Nordlund, D., Weng, T., Sokaras, D., Pierpont, C., Gaffney, K. AMER CHEMICAL SOC. 2015

  View details for Web of Science ID 000411186502135

• Indications of radiation damage in ferredoxin microcrystals using high-intensity X-FEL beams JOURNAL OF SYNCHROTRON RADIATION Nass, K., Foucar, L., Barends, T. M., Hartmann, E., Botha, S., Shoeman, R. L., Doak, R., Alonso-Mori, R., Aquila, A., Bajt, S., Barty, A., Bean, R., Beyerlein, K. R., Bublitz, M., Drachmann, N., Gregersen, J., Joensson, H., Kabsch, W., Kassemeyer, S., Koglin, J. E., Krumrey, M., Mattle, D., Messerschmidt, M., Nissen, P., Reinhard, L., Sitsel, O., Sokaras, D., Williams, G. J., Hau-Riege, S., Timneanu, N., Caleman, C., Chapman, H. N., Boutet, S., Schlichting, I. 2015; 22: 225–38

  Abstract

  Proteins that contain metal cofactors are expected to be highly radiation sensitive since the degree of X-ray absorption correlates with the presence of high-atomic-number elements and X-ray energy. To explore the effects of local damage in serial femtosecond crystallography (SFX), Clostridium ferredoxin was used as a model system. The protein contains two [4Fe-4S] clusters that serve as sensitive probes for radiation-induced electronic and structural changes. High-dose room-temperature SFX datasets were collected at the Linac Coherent Light Source of ferredoxin microcrystals. Difference electron density maps calculated from high-dose SFX and synchrotron data show peaks at the iron positions of the clusters, indicative of decrease of atomic scattering factors due to ionization. The electron density of the two [4Fe-4S] clusters differs in the FEL data, but not in the synchrotron data. Since the clusters differ in their detailed architecture, this observation is suggestive of an influence of the molecular bonding and geometry on the atomic displacement dynamics following initial photoionization. The experiments are complemented by plasma code calculations.

  View details for PubMedID 25723924

• The mapping and differentiation of biological and environmental elemental signatures in the fossil remains of a 50 million year old bird JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Egerton, V. M., Wogelius, R. A., Norell, M. A., Edwards, N. P., Sellers, W. I., Bergmann, U., Sokaras, D., Alonso-Mori, R., Ignatyev, K., van Veelen, A., Anne, J., van Dongen, B., Knoll, F., Manning, P. L. 2015; 30 (3): 627-634

  View details for DOI 10.1039/c4ja00395k

  View details for Web of Science ID 000350650800007

• Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Friebel, D., Louie, M. W., Bajdich, M., Sanwald, K. E., Cai, Y., Wise, A. M., Cheng, M., Sokaras, D., Weng, T., Alonso-Mori, R., Davis, R. C., Bargar, J. R., Norskov, J. K., Nilsson, A., Bell, A. T. 2015; 137 (3): 1305-1313

  Abstract

  Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

  View details for DOI 10.1021/ja511559d

  View details for Web of Science ID 000348690100042

  View details for PubMedID 25562406

• Chemically directing d-block heterometallics to nanocrystal surfaces as molecular beacons of surface structure CHEMICAL SCIENCE Rosen, E. L., Gilmore, K., Sawvel, A. M., Hammack, A. T., Doris, S. E., Aloni, S., Altoe, V., Nordlund, D., Weng, T., Sokaras, D., Cohen, B. E., Urban, J. J., Ogletree, D., Milliron, D. J., Prendergasta, D., Helms, B. A. 2015; 6 (11): 6295-6304

  Abstract

  Our understanding of structure and bonding in nanoscale materials is incomplete without knowledge of their surface structure. Needed are better surveying capabilities responsive not only to different atoms at the surface, but also their respective coordination environments. We report here that d-block organometallics, when placed at nanocrystal surfaces through heterometallic bonds, serve as molecular beacons broadcasting local surface structure in atomic detail. This unique ability stems from their elemental specificity and the sensitivity of their d-orbital level alignment to local coordination environment, which can be assessed spectroscopically. Re-surfacing cadmium and lead chalcogenide nanocrystals with iron- or ruthenium-based molecular beacons is readily accomplished with trimethylsilylated cyclopentadienyl metal carbonyls. For PbSe nanocrystals with iron-based beacons, we show how core-level X-ray spectroscopies and DFT calculations enrich our understanding of both charge and atomic reorganization at the surface when beacons are bound.

  View details for DOI 10.1039/c5sc01474c

  View details for Web of Science ID 000362977000032

  View details for PubMedID 30090247

  View details for PubMedCentralID PMC6054122

• Why LiFePO4 is a safe battery electrode: Coulomb repulsion induced electron-state reshuffling upon lithiation PHYSICAL CHEMISTRY CHEMICAL PHYSICS Liu, X., Wang, Y. J., Barbiellini, B., Hafiz, H., Basak, S., Liu, J., Richardson, T., Shu, G., Chou, F., Weng, T., Nordlund, D., Sokaras, D., Moritz, B., Devereaux, T. P., Qiao, R., Chuang, Y., Bansil, A., Hussain, Z., Yang, W. 2015; 17 (39): 26369-26377

  Abstract

  LiFePO4 is a battery cathode material with high safety standards due to its unique electronic structure. We performed systematic experimental and theoretical studies based on soft X-ray emission, absorption, and hard X-ray Raman spectroscopy of LixFePO4 nanoparticles and single crystals. The results clearly show a non-rigid electron-state reconfiguration of both the occupied and unoccupied Fe-3d and O-2p states during the (de)lithiation process. We focus on the energy configurations of the occupied states of LiFePO4 and the unoccupied states of FePO4, which are the critical states where electrons are removed and injected during the charge and discharge process, respectively. In LiFePO4, the soft X-ray emission spectroscopy shows that, due to the Coulomb repulsion effect, the occupied Fe-3d states with the minority spin sit close to the Fermi level. In FePO4, the soft X-ray absorption and hard X-ray Raman spectroscopy show that the unoccupied Fe-3d states again sit close to the Fermi level. These critical 3d electron state configurations are consistent with the calculations based on modified Becke and Johnson potentials GGA+U (MBJGGA+U) framework, which improves the overall lineshape prediction compared with the conventionally used GGA+U method. The combined experimental and theoretical studies show that the non-rigid electron state reshuffling guarantees the stability of oxygen during the redox reaction throughout the charge and discharge process of LiFePO4 electrodes, leading to the intrinsic safe performance of the electrodes.

  View details for DOI 10.1039/c5cp04739k

  View details for Web of Science ID 000362291300078

  View details for PubMedID 26388021

• <i>In situ</i> scanning micro-XRF analyses of gilded bronze figurines at the National Museum of Damascus JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Kantarelou, V., Karydas, A. G., Sokaras, D., Mahfouz, L., Qurdab, A., Al-Saadi, M., Giannoulaki, M., Argyropoulos, V. 2015; 30 (8): 1787-1798

  View details for DOI 10.1039/c5ja00079c

  View details for Web of Science ID 000358500800011

• Simultaneous detection of electronic structure changes from two elements of a bifunctional catalyst using wavelength-dispersive X-ray emission spectroscopy and in situ electrochemistry PHYSICAL CHEMISTRY CHEMICAL PHYSICS Gul, S., Ng, J. W., Alonso-Mori, R., Kern, J., Sokaras, D., Anzenberg, E., Lassalle-Kaiser, B., Gorlin, Y., Weng, T., Zwart, P. H., Zhang, J. Z., Bergmann, U., Yachandra, V. K., Jaramillo, T. F., Yano, J. 2015; 17 (14): 8901-8912

  Abstract

  Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. Kβ X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse Kβ signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions.

  View details for DOI 10.1039/c5cp01023c

  View details for Web of Science ID 000351933600043

  View details for PubMedID 25747045

• Resonant Inelastic X-ray Scattering on Ferrous and Ferric Bis-imidazole Porphyrin and Cytochrome c: Nature and Role of the Axial Methionine-Fe Bond. Journal of the American Chemical Society Kroll, T., Hadt, R. G., Wilson, S. A., Lundberg, M., Yan, J. J., Weng, T., Sokaras, D., Alonso-Mori, R., Casa, D., Upton, M. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2014; 136 (52): 18087-18099

  Abstract

  Axial Cu-S(Met) bonds in electron transfer (ET) active sites are generally found to lower their reduction potentials. An axial S(Met) bond is also present in cytochrome c (cyt c) and is generally thought to increase the reduction potential. The highly covalent nature of the porphyrin environment in heme proteins precludes using many spectroscopic approaches to directly study the Fe site to experimentally quantify this bond. Alternatively, L-edge X-ray absorption spectroscopy (XAS) enables one to directly focus on the 3d-orbitals in a highly covalent environment and has previously been successfully applied to porphyrin model complexes. However, this technique cannot be extended to metalloproteins in solution. Here, we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole porphyrin model complex and cyt c. The RIXS data on the model complex are directly correlated to L-edge XAS data to develop the complementary nature of these two spectroscopic methods. Comparison between the bis-imidazole model complex and cyt c in ferrous and ferric oxidation states show quantitative differences that reflect differences in axial ligand covalency. The data reveal an increased covalency for the S(Met) relative to N(His) axial ligand and a higher degree of covalency for the ferric states relative to the ferrous states. These results are reproduced by DFT calculations, which are used to evaluate the thermodynamics of the Fe-S(Met) bond and its dependence on redox state. These results provide insight into a number of previous chemical and physical results on cyt c.

  View details for DOI 10.1021/ja5100367

  View details for PubMedID 25475739

  View details for PubMedCentralID PMC4291809

• High-resolution x-ray-emission study of 1<i>s</i>4<i>p</i> and 1<i>s</i>3<i>d</i> two-electron photoexcitations in Kr PHYSICAL REVIEW A Kavcic, M., Zitnik, M., Sokaras, D., Weng, T., Alonso-Mori, R., Nordlund, D., Dousse, J., Hoszowska, J. 2014; 90 (2)

  View details for DOI 10.1103/PhysRevA.90.022513

  View details for Web of Science ID 000341160500001

• Synchrotron imaging reveals bone healing and remodelling strategies in extinct and extant vertebrates JOURNAL OF THE ROYAL SOCIETY INTERFACE Anne, J., Edwards, N. P., Wogelius, R. A., Tumarkin-Deratzian, A. R., Sellers, W. I., van Veelen, A., Bergmann, U., Sokaras, D., Alonso-Mori, R., Ignatyev, K., Egerton, V. M., Manning, P. L. 2014; 11 (96): 20140277

  Abstract

  Current understanding of bone healing and remodelling strategies in vertebrates has traditionally relied on morphological observations through the histological analysis of thin sections. However, chemical analysis may also be used in such interpretations, as different elements are known to be absorbed and used by bone for different physiological purposes such as growth and healing. These chemical signatures are beyond the detection limit of most laboratory-based analytical techniques (e.g. scanning electron microscopy). However, synchrotron rapid scanning-X-ray fluorescence (SRS-XRF) is an elemental mapping technique that uniquely combines high sensitivity (ppm), excellent sample resolution (20-100 µm) and the ability to scan large specimens (decimetre scale) approximately 3000 times faster than other mapping techniques. Here, we use SRS-XRF combined with microfocus elemental mapping (2-20 µm) to determine the distribution and concentration of trace elements within pathological and normal bone of both extant and extinct archosaurs (Cathartes aura and Allosaurus fragilis). Results reveal discrete chemical inventories within different bone tissue types and preservation modes. Chemical inventories also revealed detail of histological features not observable in thin section, including fine structures within the interface between pathological and normal bone as well as woven texture within pathological tissue.

  View details for DOI 10.1098/rsif.2014.0277

  View details for Web of Science ID 000336159200018

  View details for PubMedID 24806709

  View details for PubMedCentralID PMC4032541

• Taking snapshots of photosynthetic water oxidation using femtosecond X-ray diffraction and spectroscopy NATURE COMMUNICATIONS Kern, J., Tran, R., Alonso-Mori, R., Koroidov, S., Echols, N., Hattne, J., Ibrahim, M., Gul, S., Laksmono, H., Sierra, R. G., Gildea, R. J., Han, G., Hellmich, J., Lassalle-Kaiser, B., Chatterjee, R., Brewster, A. S., Stan, C. A., Gloeckner, C., Lampe, A., DiFiore, D., Milathianaki, D., Fry, A. R., Seibert, M. M., Koglin, J. E., Gallo, E., Uhlig, J., Sokaras, D., Weng, T., Zwart, P. H., Skinner, D. E., Bogan, M. J., Messerschmidt, M., Glatzel, P., Williams, G. J., Boutet, S., Adams, P. D., Zouni, A., Messinger, J., Sauter, N. K., Bergmann, U., Yano, J., Yachandra, V. K. 2014; 5

  Abstract

  The dioxygen we breathe is formed by light-induced oxidation of water in photosystem II. O2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction centre is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2-flash (2F) and 3-flash (3F) photosystem II samples, and of a transient 3F' state (250 μs after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O-O bond formation, coupled to Mn reduction, does not yet occur within 250 μs after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 Å. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.

  View details for DOI 10.1038/ncomms5371

  View details for Web of Science ID 000340615500062

  View details for PubMedID 25006873

  View details for PubMedCentralID PMC4151126

• Delocalization and occupancy effects of 5f orbitals in plutonium intermetallics using L-3-edge resonant X-ray emission spectroscopy JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Booth, C. H., Medling, S. A., Jiang, Y., Bauer, E. D., Tobash, P. H., Mitchell, J. N., Veirs, D. K., Wall, M. A., Allen, P. G., Kas, J. J., Sokaras, D., Nordlund, D., Weng, T. 2014; 194: 57–65

  View details for DOI 10.1016/j.elspec.2014.03.004

  View details for Web of Science ID 000339040000009

• Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature Zhang, W., Alonso-Mori, R., Bergmann, U., Bressler, C., Chollet, M., Galler, A., Gawelda, W., Hadt, R. G., Hartsock, R. W., Kroll, T., Kjær, K. S., Kubicek, K., Lemke, H. T., Liang, H. W., Meyer, D. A., Nielsen, M. M., Purser, C., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Vankó, G., Weng, T., Zhu, D., Gaffney, K. J. 2014; 509 (7500): 345-348

  Abstract

  Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

  View details for DOI 10.1038/nature13252

  View details for PubMedID 24805234

• Accurate macromolecular structures using minimal measurements from X-ray free-electron lasers NATURE METHODS Hattne, J., Echols, N., Rosalie Tran, R., Kern, J., Gildea, R. J., Brewster, A. S., Alonso-Mori, R., Gloeckner, C., Hellmich, J., Laksmono, H., Sierra, R. G., Lassalle-Kaiser, B., Lampe, A., Han, G., Gul, S., DiFiore, D., Milathianaki, D., Fry, A. R., Miahnahri, A., White, W. E., Schafer, D. W., Seibert, M. M., Koglin, J. E., Sokaras, D., Weng, T., Sellberg, J., Latimers, M. J., Glatzel, P., Zwart, P. H., Grosse-Kunstleve, R. W., Bogan, M. J., Messerschmidt, M., Williams, G. J., Boutet, S., Messinger, J., Zouni, A., Yano, J., Bergmann, U., Yachandra, V. K., Adams, P. D., Sauter, N. K. 2014; 11 (5): 545-548

  Abstract

  X-ray free-electron laser (XFEL) sources enable the use of crystallography to solve three-dimensional macromolecular structures under native conditions and without radiation damage. Results to date, however, have been limited by the challenge of deriving accurate Bragg intensities from a heterogeneous population of microcrystals, while at the same time modeling the X-ray spectrum and detector geometry. Here we present a computational approach designed to extract meaningful high-resolution signals from fewer diffraction measurements.

  View details for DOI 10.1038/NMETH.2887

  View details for Web of Science ID 000335873400020

  View details for PubMedID 24633409

  View details for PubMedCentralID PMC4008696

• Structure, Redox Chemistry, and Interfacial Alloy Formation in Monolayer and Multilayer Cu/Au(111) Model Catalysts for CO2 Electroreduction JOURNAL OF PHYSICAL CHEMISTRY C Friebel, D., Mbuga, F., Rajasekaran, S., Miller, D. J., Ogasawara, H., Alonso-Mori, R., Sokaras, D., Nordlund, D., Weng, T., Nilsson, A. 2014; 118 (15): 7954-7961

  View details for DOI 10.1021/jp412000j

  View details for Web of Science ID 000334730300023

• Understanding Interactions between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation. Journal of the American Chemical Society Gorlin, Y., Chung, C., Benck, J. D., Nordlund, D., Seitz, L., Weng, T., Sokaras, D., Clemens, B. M., Jaramillo, T. F. 2014; 136 (13): 4920-4926

  Abstract

  To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnOx, a promising OER catalyst. We conclusively demonstrate that adding Au to MnOx significantly enhances OER activity relative to MnOx in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnOx catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnOx that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnOx.

  View details for DOI 10.1021/ja407581w

  View details for PubMedID 24661269

• Self-Doping and Electrical Conductivity in Spinel Oxides: Experimental Validation of Doping Rules CHEMISTRY OF MATERIALS Shi, Y., Ndione, P. F., Lim, L. Y., Sokaras, D., Weng, T., Nagaraja, A. R., Karydas, A. G., Perkins, J. D., Mason, T. O., Ginley, D. S., Zunger, A., Toney, M. F. 2014; 26 (5): 1867-1873

  View details for DOI 10.1021/cm404031k

  View details for Web of Science ID 000332913400014

• Finite temperature effects on the X-ray absorption spectra of lithium compounds: First-principles interpretation of X-ray Raman measurements JOURNAL OF CHEMICAL PHYSICS Pascal, T. A., Boesenberg, U., Kostecki, R., Richardson, T. J., Weng, T., Sokaras, D., Nordlund, D., McDermott, E., Moewes, A., Cabana, J., Prendergast, D. 2014; 140 (3): 034107

  Abstract

  We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.

  View details for DOI 10.1063/1.4856835

  View details for Web of Science ID 000330614400009

  View details for PubMedID 25669363

• <i>In situ</i> X-ray Raman spectroscopy study of the hydrogen sorption properties of lithium borohydride nanocomposites PHYSICAL CHEMISTRY CHEMICAL PHYSICS Miedema, P. S., Ngene, P., van der Eerden, A. J., Sokaras, D., Weng, T., Nordlund, D., Au, Y. S., de Groot, F. F. 2014; 16 (41): 22651-22658

  Abstract

  Nanoconfined alkali metal borohydrides are promising materials for reversible hydrogen storage applications, but the characterization of hydrogen sorption in these materials is difficult. Here we show that with in situ X-ray Raman spectroscopy (XRS) we can track the relative amounts of intermediates and final products formed during de- and re-hydrogenation of nanoconfined lithium borohydride (LiBH4) and therefore we can possibly identify the de- and re-hydrogenation pathways. In the XRS of nanoconfined LiBH4 at different points in the de- and re-hydrogenation, we identified phases that lead to the conclusion that de- and re-hydrogenation pathways in nanoconfined LiBH4 are different from bulk LiBH4: intercalated lithium (LiCx), boron and lithium hydride were formed during de-hydrogenation, but as well Li2B12H12 was observed indicating that there is possibly some bulk LiBH4 present in the nanoconfined sample LiBH4-C as prepared. Surprisingly, XRS revealed that the de-hydrogenated products of the LiBH4-C nanocomposites can be partially rehydrogenated to about 90% of Li2B12H12 and 2-5% of LiBH4 at a mild condition of 1 bar H2 and 350 °C. This suggests that re-hydrogenation occurs via the formation of Li2B12H12. Our results show that XRS is an elegant technique that can be used for in and ex situ study of the hydrogen sorption properties of nanoconfined and bulk light-weight metal hydrides in energy storage applications.

  View details for DOI 10.1039/c4cp02918f

  View details for Web of Science ID 000342766500026

  View details for PubMedID 25231357

• Leaf metallome preserved over 50 million years METALLOMICS Edwards, N. P., Manning, P. L., Bergmann, U., Larson, P. L., van Dongen, B. E., Sellers, W. I., Webb, S. M., Sokaras, D., Alonso-Mori, R., Ignatyev, K., Barden, H. E., van Veelen, A., Anne, J., Egerton, V. M., Wogelius, R. A. 2014; 6 (4): 774-782

  Abstract

  Large-scale Synchrotron Rapid Scanning X-ray Fluorescence (SRS-XRF) elemental mapping and X-ray absorption spectroscopy are applied here to fossil leaf material from the 50 Mya Green River Formation (USA) in order to improve our understanding of the chemistry of fossilized plant remains. SRS-XRF of fossilized animals has previously shown that bioaccumulated trace metals and sulfur compounds may be preserved in their original distributions and these elements can also act as biomarkers for specific biosynthetic pathways. Similar spatially resolved chemical data for fossilized plants is sparsely represented in the literature despite the multitude of other chemical studies performed. Here, synchrotron data from multiple specimens consistently show that fossil leaves possess chemical inventories consisting of organometallic and organosulfur compounds that: (1) map discretely within the fossils, (2) resolve fine scale biological structures, and (3) are distinct from embedding sedimentary matrices. Additionally, the chemical distributions in fossil leaves are directly comparable to those of extant leaves. This evidence strongly suggests that a significant fraction of the chemical inventory of the examined fossil leaf material is derived from the living organisms and that original bioaccumulated elements have been preserved in situ for 50 million years. Chemical information of this kind has so far been unknown for fossilized plants and could for the first time allow the metallome of extinct flora to be studied.

  View details for DOI 10.1039/c3mt00242j

  View details for Web of Science ID 000333566300004

  View details for PubMedID 24804302

• Experimental and Computational X-ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged Mn<SUP>IV</SUP> Dimers Relevant to Redox-Active Metalloproteins INORGANIC CHEMISTRY Lassalle-Kaiser, B., Boron, T. T., Krewald, V., Kern, J., Beckwith, M. A., Delgado-Jaime, M. U., Schroeder, H., Alonso-Mori, R., Nordlund, D., Weng, T., Sokaras, D., Neese, F., Bergmann, U., Yachandra, V. K., DeBeer, S., Pecoraro, V. L., Yano, J. 2013; 52 (22): 12915-12922

  Abstract

  The protonation state of oxo bridges in nature is of profound importance for a variety of enzymes, including the Mn4CaO5 cluster of photosystem II and the Mn2O2 cluster in Mn catalase. A set of dinuclear bis-μ-oxo-bridged Mn(IV) complexes in different protonation states was studied by Kβ emission spectroscopy to form the foundation for unraveling the protonation states in the native complex. The valence-to-core regions (valence-to-core XES) of the spectra show significant changes in intensity and peak position upon protonation. DFT calculations were performed to simulate the valence-to-core XES spectra and to assign the spectral features to specific transitions. The Kβ(2,5) peaks arise primarily from the ligand 2p to Mn 1s transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite Kβ" peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated μ-oxo bridges dominate this spectral region. The energies of the Kβ" features differ by ~3 eV and thus are well resolved in the experimental spectra. Additionally, our work explores the chemical resolution limits of the method, namely, whether a mixed (μ-O)(μ-OH2) motif can be distinguished from a symmetric (μ-OH)2 one. The results reported here highlight the sensitivity of Kβ valence-to-core XES to single protonation state changes of bridging ligands, and form the basis for further studies of oxo-bridged polymetallic complexes and metalloenzyme active sites. In a complementary paper, the results from X-ray absorption spectroscopy of the same Mn(IV) dimer series are discussed.

  View details for DOI 10.1021/ic400821g

  View details for Web of Science ID 000327225900012

  View details for PubMedID 24161081

  View details for PubMedCentralID PMC3867288

• On the chemical state of Co oxide electrocatalysts during alkaline water splitting. Physical chemistry chemical physics Friebel, D., Bajdich, M., Yeo, B. S., Louie, M. W., Miller, D. J., Sanchez Casalongue, H., Mbuga, F., Weng, T., Nordlund, D., Sokaras, D., Alonso-Mori, R., Bell, A. T., Nilsson, A. 2013; 15 (40): 17460-17467

  Abstract

  Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

  View details for DOI 10.1039/c3cp52981a

  View details for PubMedID 24026021

• Origin of Electrochromism in High-Performing Nanocomposite Nickel Oxide ACS APPLIED MATERIALS & INTERFACES Lin, F., Nordlund, D., Weng, T., Sokaras, D., Jones, K. M., Reed, R. B., Gillaspie, D. T., Weir, D. G., Moore, R. G., Dillon, A. C., Richards, R. M., Engtrakul, C. 2013; 5 (9): 3643-3649

  Abstract

  Electrochromic effects of transition metal oxides provide a great platform for studying lithium intercalation chemistry in solids. Herein, we report on an electronically modified nanocomposite nickel oxide (i.e., Li2.34NiZr0.28Ox) that exhibits significantly improved electrochromic performance relative to the state-of-the-art inorganic electrochromic metal oxides in terms of charge/discharge kinetics, bleached-state transparency, and optical modulation. The knowledge obtained from O K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that the internally grown lithium peroxide (i.e., Li2O2) species plays a major role in facilitating charge transfer thus enabling optimal electrochromic performance. This understanding is relevant to recent theoretical studies concerning conductivity in Li2O2 for lithium-air batteries (as cited in the main text). Furthermore, we elucidate the electrochromism in modified nickel oxide in lithium ion electrolyte with the aid of Ni K-edge XAS and Ni L-edge XAS studies. The electrochromism in the nickel oxide materials arises from the reversible formation of hole states on the NiO6 units, which then impacts the Ni oxidation state through the Ni3d-O2p hybridization states. This study sheds light on the lithium intercalation chemistry for general energy storage and semiconductor applications.

  View details for DOI 10.1021/am400105y

  View details for Web of Science ID 000318839100023

  View details for PubMedID 23547738

• A seven-crystal Johann-type hard x-ray spectrometer at the Stanford Synchrotron Radiation Lightsource REVIEW OF SCIENTIFIC INSTRUMENTS Sokaras, D., Weng, T., Nordlund, D., Alonso-Mori, R., Velikov, P., Wenger, D., Garachtchenko, A., George, M., Borzenets, V., Johnson, B., Rabedeau, T., Bergmann, U. 2013; 84 (5): 053102

  Abstract

  We present a multicrystal Johann-type hard x-ray spectrometer (~5-18 keV) recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The instrument is set at the wiggler beamline 6-2 equipped with two liquid nitrogen cooled monochromators--Si(111) and Si(311)--as well as collimating and focusing optics. The spectrometer consists of seven spherically bent crystal analyzers placed on intersecting vertical Rowland circles of 1 m of diameter. The spectrometer is scanned vertically capturing an extended backscattering Bragg angular range (88°-74°) while maintaining all crystals on the Rowland circle trace. The instrument operates in atmospheric pressure by means of a helium bag and when all the seven crystals are used (100 mm of projected diameter each), has a solid angle of about 0.45% of 4π sr. The typical resolving power is in the order of E/ΔE ~ 10,000. The spectrometer's high detection efficiency combined with the beamline 6-2 characteristics permits routine studies of x-ray emission, high energy resolution fluorescence detected x-ray absorption and resonant inelastic x-ray scattering of very diluted samples as well as implementation of demanding in situ environments.

  View details for DOI 10.1063/1.4803669

  View details for Web of Science ID 000319999300003

  View details for PubMedID 23742527

  View details for PubMedCentralID PMC4108715

• Simultaneous Femtosecond X-ray Spectroscopy and Diffraction of Photosystem II at Room Temperature SCIENCE Kern, J., Alonso-Mori, R., Tran, R., Hattne, J., Gildea, R. J., Echols, N., Gloeckner, C., Hellmich, J., Laksmono, H., Sierra, R. G., Lassalle-Kaiser, B., Koroidov, S., Lampe, A., Han, G., Gul, S., DiFiore, D., Milathianaki, D., Fry, A. R., Miahnahri, A., Schafer, D. W., Messerschmidt, M., Seibert, M. M., Koglin, J. E., Sokaras, D., Weng, T., Sellberg, J., Latimer, M. J., Grosse-Kunstleve, R. W., Zwart, P. H., White, W. E., Glatzel, P., Adams, P. D., Bogan, M. J., Williams, G. J., Boutet, S., Messinger, J., Zouni, A., Sauter, N. K., Yachandra, V. K., Bergmann, U., Yano, J. 2013; 340 (6131): 491-495

  Abstract

  Intense femtosecond x-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) of microcrystals of photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. Our simultaneous XRD-XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation-sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.

  View details for DOI 10.1126/science.1234273

  View details for Web of Science ID 000318016700046

  View details for PubMedID 23413188

• Solvation structures of protons and hydroxide ions in water. journal of chemical physics Chen, C., Huang, C., Waluyo, I., Nordlund, D., Weng, T., Sokaras, D., Weiss, T., Bergmann, U., Pettersson, L. G., Nilsson, A. 2013; 138 (15): 154506-?

  View details for DOI 10.1063/1.4801512

  View details for PubMedID 23614429

• Multiconfigurational nature of 5f orbitals in uranium and plutonium and some of their intermetallic compounds Booth, C. H., Jiang, Y., Wang, D. L., Mitchell, J. N., Tobash, P. H., Bauer, E. D., Wall, M. A., Allen, P. G., Sokaras, D., Nordlund, D., Weng, T., Nordlund, D., Torrez, M. A., Sarrao, J. L. AMER CHEMICAL SOC. 2013

  View details for Web of Science ID 000324303602700

• Covalency in Metal-Oxygen Multiple Bonds Evaluated Using Oxygen K-edge Spectroscopy and Electronic Structure Theory JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Minasian, S. G., Keith, J. M., Batista, E. R., Boland, K. S., Bradley, J. A., Daly, S. R., Kozimor, S. A., Lukens, W. W., Martin, R. L., Nordlund, D., Seidler, G. T., Shuh, D. K., Sokaras, D., Tyliszczak, T., Wagner, G. L., Weng, T., Yang, P. 2013; 135 (5): 1864-1871

  Abstract

  Advancing theories of how metal-oxygen bonding influences metal oxo properties can expose new avenues for innovation in materials science, catalysis, and biochemistry. Historically, spectroscopic analyses of the transition metal MO(4)(x-) anions have formed the basis for new M-O bonding theories. Herein, relative changes in M-O orbital mixing in MO(4)(2-) (M = Cr, Mo, W) and MO(4)(-) (M = Mn, Tc, Re) are evaluated for the first time by nonresonant inelastic X-ray scattering, X-ray absorption spectroscopy using fluorescence and transmission (via a scanning transmission X-ray microscope), and time-dependent density functional theory. The results suggest that moving from Group 6 to Group 7 or down the triads increases M-O e* (π*) mixing; for example, it more than doubles in ReO(4)(-) relative to CrO(4)(2-). Mixing in the t(2)* orbitals (σ* + π*) remains relatively constant within the same Group, but increases on moving from Group 6 to Group 7. These unexpected changes in orbital energy and composition for formally isoelectronic tetraoxometalates are evaluated in terms of periodic trends in d orbital energy and radial extension.

  View details for DOI 10.1021/ja310223b

  View details for Web of Science ID 000314794400040

  View details for PubMedID 23351138

• Synchrotron-based chemical imaging reveals plumage patterns in a 150 million year old early bird JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Manning, P. L., Edwards, N. P., Wogelius, R. A., Bergmann, U., Barden, H. E., Larson, P. L., Schwarz-Wings, D., Egerton, V. M., Sokaras, D., Mori, R. A., Sellers, W. I. 2013; 28 (7): 1024-1030

  View details for DOI 10.1039/c3ja50077b

  View details for Web of Science ID 000320270000007

• New insights into the chemical and isotopic composition of human-body biominerals. I: Cholesterol gallstones from England and Greece JOURNAL OF TRACE ELEMENTS IN MEDICINE AND BIOLOGY Athanasiadou, D., Godelitsas, A., Sokaras, D., Karydas, A., Dotsika, E., Potamitis, C., Zervou, M., Xanthos, S., Chatzitheodoridis, E., Gooi, H., Becker, U. 2013; 27 (2): 79-84

  Abstract

  We have analyzed gallstones from four patients of Europe and particularly from England (including samples from a mother and a daughter) and Greece. According to the XRD, FTIR, NMR and laser micro-Raman results the studied materials correspond to typical cholesterol monohydrate (ChM). The micro-morphology of cholesterol microcrystals was investigated by means of SEM-EDS. The XRF results revealed that Ca is the dominant non-organic metal in all gallstones (up to ∼1.95wt.%) together with Fe, Cu, Pb and Ni (up to ~19ppm for each metal). Gallstones from England contain additional Mn (up to ~87ppm) and Zn (up to ∼6ppm) while the sample of the mother contains negligible Zn and Mn, compared to that of her daughter, but significant As (~4.5ppm). All cholesterol gallstones examined are well enriched in potentially toxic metals (Pb, as well as Ni in one case) and metalloids (As also in one case) as compared to the global average. The position of Zn, which is a characteristic biometal, in the structure of cholesterol, was investigated by molecular simulation using the Accelrys Materials Studio(®) software. On the basis of IRMS results, all gallstones examined exhibit a very light δ(13)C signature (average δ(13)C ~-24‰ PDB). Gamma-ray spectrometry measurements indicate the presence of (214)Pb and (214)Bi natural radionuclides due to the (238)U series as well as an additional amount of (40)K.

  View details for DOI 10.1016/j.jtemb.2012.08.004

  View details for Web of Science ID 000318321500001

  View details for PubMedID 23117070

• Proton induced quasi-monochromatic x-ray beams for soft x-ray spectroscopy studies and selective x-ray fluorescence analysis REVIEW OF SCIENTIFIC INSTRUMENTS Sokaras, D., Zarkadas, C., Fliegauf, R., Beckhoff, B., Karydas, A. G. 2012; 83 (12): 123102

  Abstract

  We present the analytical features and performance of an x-ray spectroscopy end station of moderate energy resolution operating with proton-induced quasi-monochromatic x-ray beams. The apparatus was designed, installed and operated at the 5.5 MV Tandem VdG Accelerator Laboratory of the Institute of Nuclear Physics, N.C.S.R. "Demokritos," Athens. The setup includes a two-level ultrahigh vacuum chamber that hosts in the lower level up to six primary targets in a rotatable holder; there, the irradiation of pure element materials-used as primary targets-with few-MeV high current (~μA) proton beams produces intense quasi-monochromatic x-ray beams of selectable energy. In the chamber's upper level, a six-position rotatable sample holder hosts the targets considered for x-ray spectroscopy studies. The proton-induced x-ray beam, after proper collimation, is guided to the sample position whereas various filters can be also inserted along the beam's path to eliminate the backscattered protons or/and to absorb selectively components of the x-ray beam. The apparatus incorporates an ultrathin window Si(Li) spectrometer (FWHM 136 eV at 5.89 keV) coupled with low-noise electronics capable of efficiently detecting photons down to carbon Kα. Exemplary soft x-ray spectroscopy studies and results of selective x-ray fluorescence analysis are presented.

  View details for DOI 10.1063/1.4768735

  View details for Web of Science ID 000312834300003

  View details for PubMedID 23277967

• Energy-dispersive X-ray emission spectroscopy using an X-ray free-electron laser in a shot-by-shot mode PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Alonso-Mori, R., Kern, J., Gildea, R. J., Sokaras, D., Weng, T., Lassalle-Kaiser, B., Rosalie Tran, R., Hattne, J., Laksmono, H., Hellmich, J., Gloeckner, C., Echols, N., Sierra, R. G., Schafer, D. W., Sellberg, J., Kenney, C., Herbst, R., Pines, J., Hart, P., Herrmann, S., Grosse-Kunstleve, R. W., Latimer, M. J., Fry, A. R., Messerschmidt, M. M., Miahnahri, A., Seibert, M. M., Zwart, P. H., White, W. E., Adams, P. D., Bogan, M. J., Boutet, S., Williams, G. J., Zouni, A., Messinger, J., Glatzel, P., Sauter, N. K., Yachandra, V. K., Yano, J., Bergmann, U. 2012; 109 (47): 19103-19107

  Abstract

  The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this "probe-before-destroy" approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ(1,3) XES spectra of Mn(II) and Mn(2)(III,IV) complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to >100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.

  View details for DOI 10.1073/pnas.1211384109

  View details for Web of Science ID 000311997200025

  View details for PubMedID 23129631

  View details for PubMedCentralID PMC3511075

• Nanoflow electrospinning serial femtosecond crystallography ACTA CRYSTALLOGRAPHICA SECTION D-BIOLOGICAL CRYSTALLOGRAPHY Sierra, R. G., Laksmono, H., Kern, J., Rosalie Tran, R., Hattne, J., Alonso-Mori, R., Lassalle-Kaiser, B., Gloeckner, C., Hellmich, J., Schafer, D. W., Echols, N., Gildea, R. J., Grosse-Kunstleve, R. W., Sellberg, J., McQueen, T. A., Fry, A. R., Messerschmidt, M. M., Miahnahri, A., Seibert, M. M., Hampton, C. Y., Starodub, D., Loh, N. D., Sokaras, D., Weng, T., Zwart, P. H., Glatzel, P., Milathianaki, D., White, W. E., Adams, P. D., Williams, G. J., Boutet, S., Zouni, A., Messinger, J., Sauter, N. K., Bergmann, U., Yano, J., Yachandra, V. K., Bogan, M. J. 2012; 68: 1584-1587

  Abstract

  An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14-3.1 µl min(-1) to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min(-1) and diffracted to beyond 4 Å resolution, producing 14,000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.

  View details for DOI 10.1107/S0907444912038152

  View details for Web of Science ID 000310069500017

  View details for PubMedID 23090408

  View details for PubMedCentralID PMC3478121

• Electrochemical Oxidation of Size-Selected Pt Nanoparticles Studied Using in Situ High-Energy-Resolution X-ray Absorption Spectroscopy ACS CATALYSIS Merte, L. R., Behafarid, F., Miller, D. J., Friebel, D., Cho, S., Mbuga, F., Sokaras, D., Alonso-Mori, R., Weng, T., Nordlund, D., Nilsson, A., Cuenya, B. R. 2012; 2 (11): 2371-2376

  View details for DOI 10.1021/cs300494f

  View details for Web of Science ID 000310723900018

• Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, X., Liu, J., Qiao, R., Yu, Y., Li, H., Suo, L., Hu, Y., Chuang, Y., Shu, G., Chou, F., Weng, T., Nordlund, D., Sokaras, D., Wang, Y. J., Lin, H., Barbiellini, B., Bansil, A., Song, X., Liu, Z., Yan, S., Liu, G., Qjao, S., Richardson, T. J., Prendergast, D., Hussain, Z., de Groot, F. M., Yang, W. 2012; 134 (33): 13708-13715

  Abstract

  Through soft X-ray absorption spectroscopy, hard X-ray Raman scattering, and theoretical simulations, we provide the most in-depth and systematic study of the phase transformation and (de)lithiation effect on electronic structure in Li(x)FePO(4) nanoparticles and single crystals. Soft X-ray reveals directly the valence states of Fe 3d electrons in the vicinity of Fermi level, which is sensitive to the local lattice distortion, but more importantly offers detailed information on the evolution of electronic states at different electrochemical stages. The soft X-ray spectra of Li(x)FePO(4) nanoparticles evolve vividly with the (de)lithiation level. The spectra fingerprint the (de)lithiation process with rich information on Li distribution, valency, spin states, and crystal field. The high-resolution spectra reveal a subtle but critical deviation from two-phase transformation in our electrochemically prepared samples. In addition, we performed both first-principles calculations and multiplet simulations of the spectra and quantitatively determined the 3d valence states that are completely redistributed through (de)lithiation. This electronic reconfiguration was further verified by the polarization-dependent spectra collected on LiFePO(4) single crystals, especially along the lithium diffusion direction. The evolution of the 3d states is overall consistent with the local lattice distortion and provides a fundamental picture of the (de)lithiation effects on electronic structure in the Li(x)FePO(4) system.

  View details for DOI 10.1021/ja303225e

  View details for Web of Science ID 000307699000030

  View details for PubMedID 22835006

• A multi-crystal wavelength dispersive x-ray spectrometer REVIEW OF SCIENTIFIC INSTRUMENTS Alonso-Mori, R., Kern, J., Sokaras, D., Weng, T., Nordlund, D., Tran, R., Montanez, P., Delor, J., Yachandra, V. K., Yano, J., Bergmann, U. 2012; 83 (7): 073114

  Abstract

  A multi-crystal wavelength dispersive hard x-ray spectrometer with high-energy resolution and large solid angle collection is described. The instrument is specifically designed for time-resolved applications of x-ray emission spectroscopy (XES) and x-ray Raman scattering (XRS) at X-ray Free Electron Lasers (XFEL) and synchrotron radiation facilities. It also simplifies resonant inelastic x-ray scattering (RIXS) studies of the whole 2d RIXS plane. The spectrometer is based on the Von Hamos geometry. This dispersive setup enables an XES or XRS spectrum to be measured in a single-shot mode, overcoming the scanning needs of the Rowland circle spectrometers. In conjunction with the XFEL temporal profile and high-flux, it is a powerful tool for studying the dynamics of time-dependent systems. Photo-induced processes and fast catalytic reaction kinetics, ranging from femtoseconds to milliseconds, will be resolvable in a wide array of systems circumventing radiation damage.

  View details for DOI 10.1063/1.4737630

  View details for Web of Science ID 000307527900015

  View details for PubMedID 22852678

  View details for PubMedCentralID PMC3422323

• Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Booth, C. H., Jiang, Y., Wang, D. L., Mitchell, J. N., Tobash, P. H., Bauer, E. D., Wall, M. A., Allen, P. G., Sokaras, D., Nordlund, D., Weng, T., Torrez, M. A., Sarrao, J. L. 2012; 109 (26): 10205–9

  Abstract

  Uranium and plutonium's 5f electrons are tenuously poised between strongly bonding with ligand spd-states and residing close to the nucleus. The unusual properties of these elements and their compounds (e.g., the six different allotropes of elemental plutonium) are widely believed to depend on the related attributes of f-orbital occupancy and delocalization for which a quantitative measure is lacking. By employing resonant X-ray emission spectroscopy (RXES) and X-ray absorption near-edge structure (XANES) spectroscopy and making comparisons to specific heat measurements, we demonstrate the presence of multiconfigurational f-orbital states in the actinide elements U and Pu and in a wide range of uranium and plutonium intermetallic compounds. These results provide a robust experimental basis for a new framework toward understanding the strongly-correlated behavior of actinide materials.

  View details for DOI 10.1073/pnas.1200725109

  View details for Web of Science ID 000306291400030

  View details for PubMedID 22706643

  View details for PubMedCentralID PMC3387120

• Room temperature femtosecond X-ray diffraction of photosystem II microcrystals PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Kern, J., Alonso-Mori, R., Hellmich, J., Rosalie Tran, R., Hattne, J., Laksmono, H., Gloeckner, C., Echols, N., Sierra, R. G., Sellberg, J., Lassalle-Kaiser, B., Gildea, R. J., Glatzel, P., Grosse-Kunstleve, R. W., Latimer, M. J., McQueen, T. A., DiFiore, D., Fry, A. R., Messerschmidt, M., Miahnahri, A., Schafer, D. W., Seibert, M. M., Sokaras, D., Weng, T., Zwart, P. H., White, W. E., Adams, P. D., Bogan, M. J., Boutet, S., Williams, G. J., Messinger, J., Sauter, N. K., Zouni, A., Bergmann, U., Yano, J., Yachandra, V. K. 2012; 109 (25): 9721-9726

  Abstract

  Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn(4)CaO(5) cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O-O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort (< 50 fs) 9 keV X-ray pulses from a hard X-ray free electron laser, namely the Linac Coherent Light Source. The results presented here demonstrate that the "probe before destroy" approach using an X-ray free electron laser works even for the highly-sensitive Mn(4)CaO(5) cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn(4)CaO(5) cluster without any damage at room temperature, and of the reaction intermediates of PS II during O-O bond formation.

  View details for DOI 10.1073/pnas.1204598109

  View details for Web of Science ID 000306061400021

  View details for PubMedID 22665786

  View details for PubMedCentralID PMC3382516

• A high resolution and large solid angle x-ray Raman spectroscopy end-station at the Stanford Synchrotron Radiation Lightsource REVIEW OF SCIENTIFIC INSTRUMENTS Sokaras, D., Nordlund, D., Weng, T., Mori, R., Velikov, P., Wenger, D., Garachtchenko, A., George, M., Borzenets, V., Johnson, B., Qian, Q., Rabedeau, T., Bergmann, U. 2012; 83 (4): 043112

  Abstract

  We present a new x-ray Raman spectroscopy end-station recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The end-station is located at wiggler beamline 6-2 equipped with two monochromators-Si(111) and Si(311) as well as collimating and focusing optics. It consists of two multi-crystal Johann type spectrometers arranged on intersecting Rowland circles of 1 m diameter. The first one, positioned at the forward scattering angles (low-q), consists of 40 spherically bent and diced Si(110) crystals with 100 mm diameters providing about 1.9% of 4π sr solid angle of detection. When operated in the (440) order in combination with the Si (311) monochromator, an overall energy resolution of 270 meV is obtained at 6462.20 eV. The second spectrometer, consisting of 14 spherically bent Si(110) crystal analyzers (not diced), is positioned at the backward scattering angles (high-q) enabling the study of non-dipole transitions. The solid angle of this spectrometer is about 0.9% of 4π sr, with a combined energy resolution of 600 meV using the Si (311) monochromator. These features exceed the specifications of currently existing relevant instrumentation, opening new opportunities for the routine application of this photon-in/photon-out hard x-ray technique to emerging research in multidisciplinary scientific fields, such as energy-related sciences, material sciences, physical chemistry, etc.

  View details for DOI 10.1063/1.4704458

  View details for Web of Science ID 000303415300013

  View details for PubMedID 22559520

  View details for PubMedCentralID PMC4108631

• A deep view in cultural heritage-confocal micro X-ray spectroscopy for depth resolved elemental analysis APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Kanngiesser, B., Malzer, W., Mantouvalou, I., Sokaras, D., Karydas, A. G. 2012; 106 (2): 325-338

  View details for DOI 10.1007/s00339-011-6698-0

  View details for Web of Science ID 000299749000006

• In situ X-ray Raman spectroscopy of LiBH4 PHYSICAL CHEMISTRY CHEMICAL PHYSICS Miedema, P. S., Ngene, P., van der Eerden, A. J., Weng, T., Nordlund, D., Sokaras, D., Alonso-Mori, R., Juhin, A., de Jongh, P. E., de Groot, F. F. 2012; 14 (16): 5581-5587

  Abstract

  X-Ray Raman Spectroscopy (XRS) is used to study the electronic properties of bulk lithium borohydride (LiBH(4)) and LiBH(4) in porous carbon nano-composites (LiBH(4)/C) during dehydrogenation. The lithium (Li), boron (B) and carbon (C) K-edges are studied and compared with calculations of the starting material and intermediate compounds. Comparison of the B and C K-edge XRS spectra of the as-prepared samples with rehydrogenated samples shows that the B and C electronic structure is largely regained after rehydrogenation. Both Li and C K-edge spectra show that during dehydrogenation, part of the Li intercalates into the porous carbon. This study shows that XRS in combination with calculations is a promising tool to study the electronic properties of nano-crystalline light-weight materials for energy storage.

  View details for DOI 10.1039/c2cp24025d

  View details for Web of Science ID 000302062200027

  View details for PubMedID 22428166

• Construction of a confocal PIXE set-up at the Jozef Stefan Institute and first results NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Grlj, N., Pelicon, P., Zitnik, M., Vavpetic, P., Sokaras, D., Karydas, A. G., Kanngiesser, B. 2011; 269 (20): 2237-2243

  View details for DOI 10.1016/j.nimb.2011.02.072

  View details for Web of Science ID 000296544900017

• X-ray Fluorescence analytical criteria to assess the fineness of ancient silver coins: Application on Ptolemaic coinage SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY Kantarelou, V., Ager, F., Eugenidou, D., Chaves, F., Andreou, A., Kontou, E., Katsikosta, N., Respaldiza, M., Serafin, P., Sokaras, D., Zarkadas, C., Polikreti, K., Karydas, A. 2011; 66 (9-10): 681-690

  View details for DOI 10.1016/j.sab.2011.08.001

  View details for Web of Science ID 000298724200003

• Cascade <i>L</i>-shell soft-x-ray emission as incident x-ray photons are tuned across the 1<i>s</i> ionization threshold PHYSICAL REVIEW A Sokaras, D., Kochur, A. G., Mueller, M., Kolbe, M., Beckhoff, B., Mantler, M., Zarkadas, C., Andrianis, M., Lagoyannis, A., Karydas, A. G. 2011; 83 (5)

  View details for DOI 10.1103/PhysRevA.83.052511

  View details for Web of Science ID 000290758100006

• The new external ion beam analysis setup at the Demokritos Tandem accelerator and first applications in cultural heritage NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Sokaras, D., Bistekos, E., Georgiou, L., Salomon, J., Bogovac, M., Aloupi-Siotis, E., Paschalis, V., Aslani, I., Karabagia, S., Lagoyannis, A., Harissopulos, S., Kantarelou, V., Karydas, A. 2011; 269 (5): 519-527

  View details for DOI 10.1016/j.nimb.2011.01.002

  View details for Web of Science ID 000288735200002

• Determination of differential cross-sections for the <SUP>nat</SUP>K(<i>p</i>, <i>p</i>₀) and <SUP>39</SUP>K(<i>p</i>, α₀) reactions in the backscattering geometry Kokkoris, M., Tsaris, A., Misaelides, P., Sokaras, D., Lagoyannis, A., Harissopulos, S., Vlastou, R., Papadopoulos, C. T. ELSEVIER SCIENCE BV. 2010: 1797-1801

  View details for DOI 10.1016/j.nimb.2010.02.077

  View details for Web of Science ID 000278702300023

• Resonant Raman scattering of polarized and unpolarized x-ray radiation from Mg, Al, and Si PHYSICAL REVIEW A Sokaras, D., Mueller, M., Kolbe, M., Beckhoff, B., Zarkadas, C., Karydas, A. G. 2010; 81 (1)

  View details for DOI 10.1103/PhysRevA.81.012703

  View details for Web of Science ID 000274001500072

• 3D-reconstruction of an object by means of a confocal micro-PIXE JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Zitnik, M., Grlj, N., Vaupetic, P., Pelicon, P., Bucar, K., Sokaras, D., Karydas, A., Kanngiesser, B. 2010; 25 (1): 28-33

  View details for DOI 10.1039/b912058k

  View details for Web of Science ID 000272799600003

• Combined elemental analysis of ancient glass beads by means of ion beam, portable XRF, and EPMA techniques ANALYTICAL AND BIOANALYTICAL CHEMISTRY Sokaras, D., Karydas, A. G., Oikonomou, A., Zacharias, N., Beltsios, K., Kantarelou, V. 2009; 395 (7): 2199-2209

  Abstract

  Ion beam analysis (IBA)- and X-ray fluorescence (XRF)-based techniques have been well adopted in cultural-heritage-related analytical studies covering a wide range of diagnostic role, i.e., from screening purposes up to full quantitative characterization. In this work, a systematic research was carried out towards the identification and evaluation of the advantages and the limitations of laboratory-based (IBA, electron probe microanalyzer) and portable (milli-XRF and micro-XRF) techniques. The study focused on the analysis of an Archaic glass bead collection recently excavated from the city of Thebes (mainland, Greece), in order to suggest an optimized and synergistic analytical methodology for similar studies and to assess the reliability of the quantification procedure of analyses conducted in particular by portable XRF spectrometers. All the employed analytical techniques and methodologies proved efficient to provide in a consistent way characterization of the glass bead composition, with analytical range and sensitivity depending on the particular technique. The obtained compositional data suggest a solid basis for the understanding of the main technological features related to the raw major and minor materials utilized for the manufacture of the Thebian ancient glass bead collection.

  View details for DOI 10.1007/s00216-009-3156-3

  View details for Web of Science ID 000272017000027

  View details for PubMedID 19821114

• Element-selective three-dimensional imaging of microparticles with a confocal micro-PIXE arrangement X-RAY SPECTROMETRY Zitnik, M., Pelicon, P., Bucar, K., Grlj, N., Karydas, A. G., Sokaras, D., Schuetz, R., Kanngiesser, B. 2009; 38 (6): 526-539

  View details for DOI 10.1002/xrs.1210

  View details for Web of Science ID 000271519900010

• Secondary Fluorescence Enhancement in Confocal X-ray Microscopy Analysis ANALYTICAL CHEMISTRY Sokaras, D., Karydas, A. 2009; 81 (12): 4946-4954

  Abstract

  In the present work, the influence of the secondary fluorescence enhancement in confocal X-ray microscopy analysis is studied when stratified type of materials are examined. Through a proper mathematical formalism, an exact global theoretical model is presented which accounts for the secondary fluorescence enhancement when either particle (3D-Micro particle induced X-ray emission) or photon (3D-Micro X-ray fluorescence) microbeams are used in the excitation channel. The contribution of the secondary fluorescence effect to the confocal X-ray intensity profiles was calculated for some typical representative cases. In addition, the influence of several experimental parameters was examined in terms of their influence in the absolute intensity and shape of the secondary fluorescence intensity profile.

  View details for DOI 10.1021/ac900688n

  View details for Web of Science ID 000266969700035

  View details for PubMedID 19462969

• Quantitative analysis in confocal micro-PIXE-general concept and layered materials JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Sokaras, D., Karydas, A., Malzer, W., Schuetz, R., Kanngiesser, B., Grlj, N., Pelicon, P., Zitnik, M. 2009; 24 (5): 611-621

  View details for DOI 10.1039/b817100a

  View details for Web of Science ID 000266597800004

• Performance of a polycapillary halflens as focussing and collecting optic-a comparison JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Wolff, T., Mantouvalou, I., Malzer, W., Nissen, J., Berger, D., Zizak, I., Sokaras, D., Karydas, A., Grlj, N., Pelicon, P., Schuetz, R., Zitnik, M., Kanngiesser, B. 2009; 24 (5): 669-675

  View details for DOI 10.1039/b817828c

  View details for Web of Science ID 000266597800011

• Three-dimensional imaging of aerosol particles with scanning proton microprobe in a confocal arrangement APPLIED PHYSICS LETTERS Zitnik, M., Pelicon, P., Grlj, N., Karydas, A. G., Sokaras, D., Schuetz, R., Kanngiesser, B. 2008; 93 (9)

  View details for DOI 10.1063/1.2976163

  View details for Web of Science ID 000258975800088

• Reference-free x-ray fluorescence analysis of an ancient Chinese ceramic X-RAY SPECTROMETRY Beckhoff, B., Kolbe, M., Hahn, O., Karydas, A. G., Zarkadas, C., Sokaras, D., Mantler, M. 2008; 37 (4): 462-465

  View details for DOI 10.1002/xrs.1073

  View details for Web of Science ID 000258670600026

• 3D Micro-PIXE at atmospheric pressure:: A new tool for the investigation of art and archaeological objects NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Kanngiesser, B., Karydas, A., Schuetz, R., Sokaras, D., Reiche, I., Roehrs, S., Pichon, L., Salomon, J. 2007; 264 (2): 383-388

  View details for DOI 10.1016/j.nimb.2007.09.019

  View details for Web of Science ID 000251873600026

• 3D micro PIXE -: a new technique for depth-resolved elemental analysis JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY Karydas, A., Sokaras, D., Zarkadas, C., Grlj, N., Pelicon, P., Zitnik, M., Schuetz, R., Malzer, W., Kanngiesser, B. 2007; 22 (10): 1260-1265

  View details for DOI 10.1039/b700851c

  View details for Web of Science ID 000249925600014