Sumana Raj

All Publications

• Multiconfigurational Electronic Structure of Nickel Cross-Coupling Catalysts Revealed by X-ray Absorption Spectroscopy. The journal of physical chemistry letters Nelson, K. J., Kazmierczak, N. P., Cagan, D. A., Follmer, A. H., Scott, T. R., Raj, S. L., Garratt, D., Powers-Riggs, N., Gaffney, K. J., Hadt, R. G., Cordones, A. A. 2024: 87-94

  Abstract

  NiII 2,2'-bipyridine complexes are commonly invoked intermediates in metallaphotoredox cross-coupling reactions. Despite their ubiquity, design principles targeting improved catalytic performance remain underdetermined. A series of Ni(Rbpy)(R'Ar)Cl (R = MeOOC, t-Bu, R' = CH3, CF3) complexes were proposed to have multiconfigurational electronic structures on the basis of multiconfigurational/multireference calculations, with significant mixing of Ni → bpy metal-to-ligand charge transfer (MLCT) configurations into the ground-state wave function. Here, Ni K-edge and L2,3-edge X-ray absorption spectroscopies provide experimental support for the highly covalent and multiconfigurational electronic structures of these complexes. The pre-edge intensity in the K-edge spectrum reflects highly covalent Ni-aryl bonding. The L3-edge spectral shape is dependent on ligand functionalization, and a feature reflecting the MLCT character is assigned using prior ab initio and new semiempirical calculations. The results suggest the push/pull effects of the aryl/bpy ligands moderate the changes in electron density on Ni during the multiredox cross-coupling reaction cycle.

  View details for DOI 10.1021/acs.jpclett.4c02917

  View details for PubMedID 39700059

• Time-Resolved X-ray Emission Spectroscopy and Resonant Inelastic X-ray Scattering Spectroscopy of Laser Irradiated Carbon. The journal of physical chemistry. B Riffe, E. J., Bernal, F., Kamal, C., Mizuno, H., Lindsey, R. K., Hamel, S., Raj, S. L., Hull, C. J., Kwon, S., Park, S. H., Cooper, J. K., Yang, F., Liu, Y., Guo, J., Nordlund, D., Drisdell, W. S., Zuerch, M. W., Whitley, H. D., Odelius, M., Schwartz, C. P., J Saykally, R. 2024

  Abstract

  The existence of liquid carbon as an intermediate phase preceding the formation of novel carbon materials has been a point of contention for several decades. Experimental observation of such a liquid state requires nonthermal melting of solid carbon materials at various laser fluences and pulse properties. Reflectivity experiments performed in the mid-1980s reached opposing conclusions regarding the metallic or insulating properties of the purported liquid state. Time-resolved X-ray absorption studies showed shortening of C-C bonds and increasing diffraction densities, thought to evidence a liquid or glassy carbon state, respectively. Nevertheless, none of these experiments provided information on the electronic structure of the proposed liquid state. Herein, we report the results of time-resolved resonant inelastic X-ray scattering (RIXS) and time-resolved X-ray emission spectroscopy (XES) studies on amorphous carbon (a-C) and ultrananocrystalline diamond (UNCD) as a function of delay time between the irradiating pulse and X-ray probe. For both a-C and UNCD, we attribute decreases in RIXS or XES signals to transition blocking, relaxation, and finally, ablation. Increased signal at 20 ps following the irradiation of the UNCD is attributed to the probable formation of nanoscale structures in the ablation plume. Differences in the amount of signal observed between a-C and UNCD are explained by the difference in sample thickness and, specifically, incomplete melting of the UNCD film. Comparisons to spectral simulations based on MD trajectories at extreme conditions indicate that the carbon state in our experiments is crystalline. Normal mode analysis confirmed that symmetrical bending or stretching of the C-C bonds in the diamond lattice results in XES spectra with small intensity differences. Overall, we observed no evidence of melting to a liquid state, as determined by the lack of changes in the spectral properties for up to 100 ps delays following the melting pulses.

  View details for DOI 10.1021/acs.jpcb.4c02862

  View details for PubMedID 38906826

• 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

• 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

• Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering. Journal of the American Chemical Society Powers-Riggs, N. E., Birgisson, B. O., Raj, S. L., Biasin, E., Lenzen, P., Zederkof, D. B., Haubro, M., Tveiten, D. K., Hartsock, R. W., van Driel, T. B., Kunnus, K., Chollet, M., Robinson, J. S., Nelson, S., Forbes, R., Haldrup, K., Pedersen, K. S., Levi, G., Ougaard Dohn, A., Jonsson, H., Mo Ller, K. B., Natan, A., Nielsen, M. M., Gaffney, K. J. 2024

  Abstract

  Dimeric complexes composed of d8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir2 (dimen)4]2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.

  View details for DOI 10.1021/jacs.4c00817

  View details for PubMedID 38727611

• Site-specific electronic structure of covalently linked bimetallic dyads from nitrogen K-edge x-ray absorption spectroscopy. The Journal of chemical physics Ryland, E. S., Liu, X., Kumar, G., Raj, S. L., Xie, Z. L., Mengele, A. K., Fauth, S. S., Siewerth, K., Dietzek-Ivanšić, B., Rau, S., Mulfort, K. L., Li, X., Cordones, A. A. 2024; 160 (8)

  Abstract

  A nitrogen K-edge x-ray absorption near-edge structure (XANES) survey is presented for tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz)-bridged bimetallic assemblies that couple chromophore and catalyst transition metal complexes for light driven catalysis, as well as their individual molecular constituents. We demonstrate the high N site sensitivity of the N pre-edge XANES features, which are energetically well-separated for the phenazine bridge N atoms and for the individual metal-bound N atoms of the inner coordination sphere ligands. By comparison with the time-dependent density functional theory calculated spectra, we determine the origins of these distinguishable spectral features. We find that metal coordination generates large shifts toward higher energy for the metal-bound N atoms, with increasing shift for 3d < 4d < 5d metal bonding. This is attributed to increasing ligand-to-metal σ donation that increases the effective charge of the bound N atoms and stabilizes the N 1s core electrons. In contrast, the phenazine bridge N pre-edge peak is found at a lower energy due to stabilization of the low energy electron accepting orbital localized on the phenazine motif. While no sensitivity to ground state electronic coupling between the individual molecular subunits was observed, the spectra are sensitive to structural distortions of the tpphz bridge. These results demonstrate N K-edge XANES as a local probe of electronic structure in large bridging ligand motifs, able to distinctly investigate the ligand-centered orbitals involved in metal-to-ligand and ligand-to-ligand electron transfer following light absorption.

  View details for DOI 10.1063/5.0192809

  View details for PubMedID 38415835

• Optically Induced Anisotropy in Time-Resolved Scattering: Imaging Molecular-Scale Structure and Dynamics in Disordered Media with Experiment and Theory. Physical review letters Montoya-Castillo, A., Chen, M. S., Raj, S. L., Jung, K. A., Kjaer, K. S., Morawietz, T., Gaffney, K. J., van Driel, T. B., Markland, T. E. 2022; 129 (5): 056001

  Abstract

  Time-resolved scattering experiments enable imaging of materials at the molecular scale with femtosecond time resolution. However, in disordered media they provide access to just one radial dimension thus limiting the study of orientational structure and dynamics. Here we introduce a rigorous and practical theoretical framework for predicting and interpreting experiments combining optically induced anisotropy and time-resolved scattering. Using impulsive nuclear Raman and ultrafast x-ray scattering experiments of chloroform and simulations, we demonstrate that this framework can accurately predict and elucidate both the spatial and temporal features of these experiments.

  View details for DOI 10.1103/PhysRevLett.129.056001

  View details for PubMedID 35960558

• Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions PHYSICAL REVIEW LETTERS Schwartz, C. P., Raj, S. L., Jamnuch, S., Hull, C. J., Miotti, P., Lam, R. K., Nordlund, D., Uzundal, C. B., Das Pemmaraju, C., Mincigrucci, R., Foglia, L., Simoncig, A., Coreno, M., Masciovecchio, C., Giannessi, L., Poletto, L., Principi, E., Zuerch, M., Pascal, T. A., Drisdell, W. S., Saykally, R. J. 2021; 127 (9)

  View details for DOI 10.1103/PhysRevLett.127.096801

  View details for Web of Science ID 000688553600006

• The liquid state of carbon CHEMICAL PHYSICS LETTERS Hull, C. J., Raj, S. L., Saykally, R. J. 2020; 749

  View details for DOI 10.1016/j.cplett.2020.137341

  View details for Web of Science ID 000532697300008

• Free Electron Laser Measurement of Liquid Carbon Reflectivity in the Extreme Ultraviolet PHOTONICS Raj, S. L., Devlin, S. W., Mincigrucci, R., Schwartz, C. P., Principi, E., Bencivenga, F., Foglia, L., Gessini, A., Simoncig, A., Kurdi, G., Masciovecchio, C., Saykally, R. J. 2020; 7 (2)

  View details for DOI 10.3390/photonics7020035

  View details for Web of Science ID 000551229300013

• Early time dynamics of laser-ablated silicon using ultrafast grazing incidence X-ray scattering CHEMICAL PHYSICS LETTERS Hull, C., Raj, S., Lam, R., Katayama, T., Pascal, T., Drisdell, W. S., Saykally, R., Schwartz, C. P. 2019; 736

  View details for DOI 10.1016/j.cplett.2019.136811

  View details for Web of Science ID 000490975500002

• 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

• 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