Dr Razib Obaid

All Publications

• Design and performance of a magnetic bottle electron spectrometer for high-energy photoelectron spectroscopy. The Review of scientific instruments Borne, K., O'Neal, J. T., Wang, J., Isele, E., Obaid, R., Berrah, N., Cheng, X., Bucksbaum, P. H., James, J., Kamalov, A., Larsen, K. A., Li, X., Lin, M. F., Liu, Y., Marinelli, A., Summers, A. M., Thierstein, E., Wolf, T. J., Rolles, D., Walter, P., Cryan, J. P., Driver, T. 2024; 95 (12)

  Abstract

  We describe the design and performance of a magnetic bottle electron spectrometer (MBES) for high-energy electron spectroscopy. Our design features a 2 m long electron drift tube and electrostatic retardation lens, achieving sub-electronvolt (eV) electron kinetic energy resolution for high energy (several hundred eV) electrons with a close to 4π collection solid angle. A segmented anode electron detector enables the simultaneous collection of photoelectron spectra in high resolution and high collection efficiency modes. This versatile instrument is installed at the time-resolved molecular and optical sciences instrument at the Linac Coherent Light Source x-ray free-electron laser (XFEL). In this paper, we demonstrate its high resolution, collection efficiency, and spatial selectivity in measurements where it is coupled to an XFEL source. These combined characteristics are designed to enable high-resolution time-resolved measurements using x-ray photoelectron, absorption, and Auger-Meitner spectroscopy. We also describe the pervasive artifact in MBES time-of-flight spectra that arises from a periodic modulation in electron collection efficiency and present a robust analysis procedure for its removal.

  View details for DOI 10.1063/5.0223334

  View details for PubMedID 39704606

• "Beam à la carte": Laser heater shaping for attosecond pulses in a multiplexed x-ray free-electron laser APPLIED PHYSICS LETTERS Li, S., Zhang, Z., Alverson, S., Cesar, D., Driver, T., Franz, P., Isele, E., Duris, J. P., Larsen, K., Lin, M., Obaid, R., O'Neal, J. T., Robles, R., Sudar, N., Guo, Z., Vetter, S., Walter, P., Wang, A. L., Xu, J., Carbajo, S., Cryan, J. P., Marinelli, A. 2024; 125 (19)

  View details for DOI 10.1063/5.0233468

  View details for Web of Science ID 001349444700011

• Attosecond delays in X-ray molecular ionization. Nature Driver, T., Mountney, M., Wang, J., Ortmann, L., Al-Haddad, A., Berrah, N., Bostedt, C., Champenois, E. G., DiMauro, L. F., Duris, J., Garratt, D., Glownia, J. M., Guo, Z., Haxton, D., Isele, E., Ivanov, I., Ji, J., Kamalov, A., Li, S., Lin, M. F., Marangos, J. P., Obaid, R., O'Neal, J. T., Rosenberger, P., Shivaram, N. H., Wang, A. L., Walter, P., Wolf, T. J., Wörner, H. J., Zhang, Z., Bucksbaum, P. H., Kling, M. F., Landsman, A. S., Lucchese, R. R., Emmanouilidou, A., Marinelli, A., Cryan, J. P. 2024; 632 (8026): 762-767

  Abstract

  The photoelectric effect is not truly instantaneous but exhibits attosecond delays that can reveal complex molecular dynamics1-7. Sub-femtosecond-duration light pulses provide the requisite tools to resolve the dynamics of photoionization8-12. Accordingly, the past decade has produced a large volume of work on photoionization delays following single-photon absorption of an extreme ultraviolet photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required X-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. Here we report measurements of the X-ray photoemission delay of core-level electrons, with unexpectedly large delays, ranging up to 700 as in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft X-ray pulses from a free-electron laser to scan across the entire region near the K-shell threshold. Furthermore, we find that the delay spectrum is richly modulated, suggesting several contributions, including transient trapping of the photoelectron owing to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule and multi-electron scattering effects. The results demonstrate how X-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

  View details for DOI 10.1038/s41586-024-07771-9

  View details for PubMedID 39169246

  View details for PubMedCentralID 7399650

• Terawatt-scale attosecond X-ray pulses from a cascaded superradiant free-electron laser NATURE PHOTONICS Franz, P., Li, S., Driver, T., Robles, R. R., Cesar, D., Isele, E., Guo, Z., Wang, J., Duris, J. P., Larsen, K., Glownia, J. M., Cheng, X., Hoffmann, M. C., Li, X., Lin, M., Kamalov, A., Obaid, R., Summers, A., Sudar, N., Thierstein, E., Zhang, Z., Kling, M. F., Huang, Z., Cryan, J. P., Marinelli, A. 2024

  View details for DOI 10.1038/s41566-024-01427-w

  View details for Web of Science ID 001220935700001

• Experimental demonstration of attosecond pump-probe spectroscopy with an X-ray free-electron laser NATURE PHOTONICS Guo, Z., Driver, T., Beauvarlet, S., Cesar, D., Duris, J., Franz, P. L., Alexander, O., Bohler, D., Bostedt, C., Averbukh, V., Cheng, X., Dimauro, L. F., Doumy, G., Forbes, R., Gessner, O., Glownia, J. M., Isele, E., Kamalov, A., Larsen, K. A., Li, S., Li, X., Lin, M., Mccracken, G. A., Obaid, R., O'Neal, J. T., Robles, R. R., Rolles, D., Ruberti, M., Rudenko, A., Slaughter, D. S., Sudar, N. S., Thierstein, E., Tuthill, D., Ueda, K., Wang, E., Wang, A. L., Wang, J., Weber, T., Wolf, T. A., Young, L., Zhang, Z., Bucksbaum, P. H., Marangos, J. P., Kling, M. F., Huang, Z., Walter, P., Inhester, L., Berrah, N., Cryan, J. P., Marinelli, A. 2024

  View details for DOI 10.1038/s41566-024-01419-w

  View details for Web of Science ID 001200371400001

• Compact single-shot soft X-ray photon spectrometer for free-electron laser diagnostics OPTICS EXPRESS Larsen, K. A., Borne, K., Obaid, R., Kamalov, A., Liu, Y., Cheng, X., James, J., Driver, T., Li, K., Liu, Y., Sakdinawat, A., David, C., Wolf, T. A., Cryan, J. P., Walter, P., Lin, M. 2023; 31 (22): 35822-35834

  Abstract

  The photon spectrum from free-electron laser (FEL) light sources offers valuable information in time-resolved experiments and machine optimization in the spectral and temporal domains. We have developed a compact single-shot photon spectrometer to diagnose soft X-ray spectra. The spectrometer consists of an array of off-axis Fresnel zone plates (FZP) that act as transmission-imaging gratings, a Ce:YAG scintillator, and a microscope objective to image the scintillation target onto a two-dimensional imaging detector. This spectrometer operates in segmented energy ranges which covers tens of electronvolts for each absorption edge associated with several atomic constituents: carbon, nitrogen, oxygen, and neon. The spectrometer's performance is demonstrated at a repetition rate of 120 Hz, but our detection scheme can be easily extended to 200 kHz spectral collection by employing a fast complementary metal oxide semiconductor (CMOS) line-scan camera to detect the light from the scintillator. This compact photon spectrometer provides an opportunity for monitoring the spectrum downstream of an endstation in a limited space environment with sub-electronvolt energy resolution.

  View details for DOI 10.1364/OE.502105

  View details for Web of Science ID 001106418000001

  View details for PubMedID 38017746

• Sex and dose rate effects in automated cytogenetics. Radiation protection dosimetry Garty, G., Royba, E., Repin, M., Shuryak, I., Deoli, N., Obaid, R., Turner, H. C., Brenner, D. J. 2023; 199 (14): 1495-1500

  Abstract

  Testing and validation of biodosimetry assays is routinely performed using conventional dose rate irradiation platforms, at a dose rate of approximately 1 Gy/min. In contrast, the exposures from an improvised nuclear device will be delivered over a large range of dose rates with a prompt irradiation component, delivered in less than 1 μs, and a protracted component delivered over hours and days. We present preliminary data from a large demographic study we have undertaken for investigation of age, sex and dose rate effects on dicentric and micronucleus yields. Our data demonstrate reduced dicentric and micronucleus yields at very high dose rates. Additionally, we have seen small differences between males and females, with males having slightly fewer micronuclei and slightly more dicentrics than females, at high doses.

  View details for DOI 10.1093/rpd/ncac286

  View details for PubMedID 37721073

  View details for PubMedCentralID PMC10505938

• Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer. The journal of physical chemistry letters Wang, E., Kling, N. G., LaForge, A. C., Obaid, R., Pathak, S., Bhattacharyya, S., Meister, S., Trost, F., Lindenblatt, H., Schoch, P., Kübel, M., Pfeifer, T., Rudenko, A., Díaz-Tendero, S., Martín, F., Moshammer, R., Rolles, D., Berrah, N. 2023; 14 (18): 4372-4380

  Abstract

  Ultrafast H2+ and H3+ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H2 roaming that leads to H2+ and H3+ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H2+ to H3+ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H2 formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H2 undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H2 to C2H4O2+ and proton transfer from C2H4O2+ to H2.

  View details for DOI 10.1021/acs.jpclett.2c03764

  View details for PubMedID 37140167

• Validation of a High-Throughput Dicentric Chromosome Assay Using Complex Radiation Exposures. Radiation research Royba, E., Repin, M., Balajee, A. S., Shuryak, I., Pampou, S., Karan, C., Wang, Y., Lemus, O. D., Obaid, R., Deoli, N., Wuu, C., Brenner, D. J., Garty, G. 2023; 199 (1): 1-16

  Abstract

  Validation of biodosimetry assays is routinely performed using primarily orthovoltage irradiators at a conventional dose rate of approximately 1 Gy/min. However, incidental/ accidental exposures caused by nuclear weapons can be more complex. The aim of this work was to simulate the DNA damage effects mimicking those caused by the detonation of a several kilotons improvised nuclear device (IND). For this, we modeled complex exposures to: 1. a mixed (photons + IND-neutrons) field and 2. different dose rates that may come from the blast, nuclear fallout, or ground deposition of radionuclides (ground shine). Additionally, we assessed whether myeloid cytokines affect the precision of radiation dose estimation by modulating the frequency of dicentric chromosomes. To mimic different exposure scenarios, several irradiation systems were used. In a mixed field study, human blood samples were exposed to a photon field enriched with neutrons (ranging from 10% to 37%) from a source that mimics Hiroshima's A-bomb's energy spectrum (0.2-9 MeV). Using statistical analysis, we assessed whether photons and neutrons act in an additive or synergistic way to form dicentrics. For the dose rates study, human blood was exposed to photons or electrons at dose rates ranging from low (where the dose was spread over 32 h) to extremely high (where the dose was delivered in a fraction of a microsecond). Potential effects of cytokine treatment on biodosimetry dose predictions were analyzed in irradiated blood subjected to Neupogen or Neulasta for 24 or 48 h at the concentration recommended to forestall manifestation of an acute radiation syndrome in bomb survivors. All measurements were performed using a robotic station, the Rapid Automated Biodosimetry Tool II, programmed to culture lymphocytes and score dicentrics in multiwell plates (the RABiT-II DCA). In agreement with classical concepts of radiation biology, the RABiT-II DCA calibration curves suggested that the frequency of dicentrics depends on the type of radiation and is modulated by changes in the dose rate. The resulting dose-response curves suggested an intermediate dicentric yields and additive effects of photons and IND-neutrons in the mixed field. At ultra-high dose rate (600 Gy/s), affected lymphocytes exhibited significantly fewer dicentrics (P < 0.004, t test). In contrast, we did not find the dose-response modification effects of radiomitigators on the yields of dicentrics (Bonferroni corrected P > 0.006, ANOVA test). This result suggests no bias in the dose predictions should be expected after emergency cytokine treatment initiated up to 48 h prior to blood collection for dicentric analysis.

  View details for DOI 10.1667/RADE-22-00007.1

  View details for PubMedID 35994701

• Ultra-high dose rate FLASH irradiator at the radiological research accelerator facility. Scientific reports Garty, G., Obaid, R., Deoli, N., Royba, E., Tan, Y., Harken, A. D., Brenner, D. J. 2022; 12 (1): 22149

  Abstract

  The Radiological Research Accelerator Facility has modified a decommissioned Varian Clinac to deliver ultra-high dose rates: operating in 9 MeV electron mode (FLASH mode), samples can be irradiated at a Source-Surface Distance (SSD) of 20 cm at average dose rates of up to 600 Gy/s (3.3 Gy per 0.13 µs pulse, 180 pulses per second). In this mode multiple pulses are required for most irradiations. By modulating pulse repetition rate and irradiating at SSD = 171 cm, dose rates below 1 Gy/min can be achieved, allowing comparison of FLASH and conventional irradiations with the same beam. Operating in 6 MV photon mode, with the conversion target removed (SuperFLASH mode), samples are irradiated at higher dose rates (0.2-150 Gy per 5 µs pulse, 360 pulses per second) and most irradiations can be performed with a single very high dose rate pulse. In both modes we have seen the expected inverse relation between dose rate and irradiated area, with the highest dose rates obtained for beams with a FWHM of about 2 cm and ± 10% uniformity over 1 cm diameter. As an example of operation of the ultra-high dose rate FLASH irradiator, we present dose rate dependence of dicentric chromosome yields.

  View details for DOI 10.1038/s41598-022-19211-7

  View details for PubMedID 36550150

  View details for PubMedCentralID PMC9780319

• The DREAM Endstation at the Linac Coherent Light Source APPLIED SCIENCES-BASEL Walter, P., Holmes, M., Obaid, R., Amores, L., Cheng, X., Cryan, J. P., Glownia, J. M., Li, X., Lin, M., Ng, M., Robinson, J., Shivaram, N., Yin, J., Fritz, D., James, J., Castagna, J., Osipov, T. 2022; 12 (20)

  View details for DOI 10.3390/app122010534

  View details for Web of Science ID 000874182200001

• The time-resolved atomic, molecular and optical science instrument at the Linac Coherent Light Source. Journal of synchrotron radiation Walter, P., Osipov, T., Lin, M. F., Cryan, J., Driver, T., Kamalov, A., Marinelli, A., Robinson, J., Seaberg, M. H., Wolf, T. J., Aldrich, J., Brown, N., Champenois, E. G., Cheng, X., Cocco, D., Conder, A., Curiel, I., Egger, A., Glownia, J. M., Heimann, P., Holmes, M., Johnson, T., Lee, L., Li, X., Moeller, S., Morton, D. S., Ng, M. L., Ninh, K., O'Neal, J. T., Obaid, R., Pai, A., Schlotter, W., Shepard, J., Shivaram, N., Stefan, P., Van, X., Wang, A. L., Wang, H., Yin, J., Yunus, S., Fritz, D., James, J., Castagna, J. C. 2022; 29 (Pt 4): 957-968

  Abstract

  The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

  View details for DOI 10.1107/S1600577522004283

  View details for PubMedID 35787561

• Attosecond coherent electron motion in Auger-Meitner decay. Science (New York, N.Y.) Li, S., Driver, T., Rosenberger, P., Champenois, E. G., Duris, J., Al-Haddad, A., Averbukh, V., Barnard, J. C., Berrah, N., Bostedt, C., Bucksbaum, P. H., Coffee, R. N., DiMauro, L. F., Fang, L., Garratt, D., Gatton, A., Guo, Z., Hartmann, G., Haxton, D., Helml, W., Huang, Z., LaForge, A. C., Kamalov, A., Knurr, J., Lin, M., Lutman, A. A., MacArthur, J. P., Marangos, J. P., Nantel, M., Natan, A., Obaid, R., O'Neal, J. T., Shivaram, N. H., Schori, A., Walter, P., Wang, A. L., Wolf, T. J., Zhang, Z., Kling, M. F., Marinelli, A., Cryan, J. P. 1800: eabj2096

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/science.abj2096

  View details for PubMedID 34990213

• At-the-Edge Data Processing for Low Latency High Throughput Machine Learning Algorithms Hirschman, J., Kamalov, A., Obaid, R., O'Shea, F. H., Coffee, R. N., Doug, K., Al, G., Pophale, S., Liu, H., Parete-Koon , S. SPRINGER INTERNATIONAL PUBLISHING AG. 2022: 101-119

  View details for DOI 10.1007/978-3-031-23606-8_7

  View details for Web of Science ID 000972629000007

• The X-ray Focusing System at the Time-Resolved AMO Instrument Seaberg, M., Mortin, D., Cheng, X., Cryan, J., Obaid, R., Ng, M. 2022: 20-28

  View details for DOI 10.1080/08940886.2022.2066416

• Peer Teaching as Bioinformatics Training Strategy: Incentives, Challenges, and Benefits. Medical reference services quarterly Rahman, N., Meyer, C., Thakral, D., Cai, W. L., Chen, A. T., Obaid, R., Garcia-Milian, R. 2022; 41 (1): 13-25

  Abstract

  Bioinformatics is essential for basic and clinical research. Peer-to-peer (P2P) teaching was used to respond to the bioinformatics training needs at a research-intensive institution. In addition to the data collected from the workshops, personal experiences of the teachers were used to understand incentives, challenges, and benefits of P2P teaching. Developing communication skills such as confidence in teaching, explaining complex concepts, and better understanding of topics benefited P2P teachers. Lack of time and classroom management were identified as major challenges. Hence, P2P teaching can be beneficial not only for bioinformatics trainees but also as a professional development opportunity for peer teachers.

  View details for DOI 10.1080/02763869.2022.2020568

  View details for PubMedID 35225737

• Ultrafast molecular dynamics in ionized 1- and 2-propanol: from simple fragmentation to complex isomerization and roaming mechanisms. Physical chemistry chemical physics : PCCP Mishra, D., Reino-González, J., Obaid, R., LaForge, A. C., Díaz-Tendero, S., Martín, F., Berrah, N. 2021; 24 (1): 433-443

  Abstract

  Upon photoexcitation, molecules can undergo numerous complex processes, such as isomerization and roaming, leading to changes in the molecular and electronic structure. Here, we report on the time-resolved ultrafast nuclear dynamics, initiated by laser ionization, in the two structural isomers, 1- and 2-propanol, using a combination of pump-probe spectroscopy and coincident Coulomb explosion imaging. Our measurements, paired with quantum chemistry calculations, identify the mechanisms for the observed two- and three-body dissociation channels for both isomers. In particular, the fragmentation channel of 2-propanol associated with the loss of CH3 shows possible evidence of methyl roaming. Moreover, the electronic structure of this roaming methyl fragment could be responsible for the enhanced ionization also observed for this channel. Finally, comparison with similar studies done on ethanol and acetonitrile helps establish a correlation between the length of the alkyl chain and the likelihood of hydrogen migration.

  View details for DOI 10.1039/d1cp04011a

  View details for PubMedID 34897321

• Multi-resolution electron spectrometer array for future free-electron laser experiments. Journal of synchrotron radiation Walter, P., Kamalov, A., Gatton, A., Driver, T., Bhogadi, D., Castagna, J. C., Cheng, X., Shi, H., Obaid, R., Cryan, J., Helml, W., Ilchen, M., Coffee, R. N. 2021; 28 (Pt 5): 1364-1376

  Abstract

  The design of an angular array of electron time-of-flight (eToF) spectrometers is reported, intended for non-invasive spectral, temporal, and polarization characterization of single shots of high-repetition rate, quasi-continuous, short-wavelength free-electron lasers (FELs) such as the LCLS II at SLAC. This array also enables angle-resolved, high-resolution eToF spectroscopy to address a variety of scientific questions on ultrafast and nonlinear light-matter interactions at FELs. The presented device is specifically designed for the time-resolved atomic, molecular and optical science endstation (TMO) at LCLS II. In its final version, the spectrometer comprises up to 20 eToF spectrometers aligned to collect electrons from the interaction point, which is defined by the intersection of the incoming FEL radiation and a gaseous target. The full composition involves 16 spectrometers forming a circular equiangular array in the plane normal to the X-ray propagation and four spectrometers at 54.7° angle relative to the principle linear X-ray polarization axis with orientations in the forward and backward direction of the light propagation. The spectrometers are capable of independent and minimally chromatic electrostatic lensing and retardation, in order to enable simultaneous angle-resolved photo- and Auger-Meitner electron spectroscopy with high energy resolution. They are designed to ensure an energy resolution of 0.25 eV across an energy window of up to 75 eV, which can be individually centered via the adjustable retardation to cover the full range of electron kinetic energies relevant to soft X-ray methods, 0-2 keV. The full spectrometer array will enable non-invasive and online spectral-polarimetry measurements, polarization-sensitive attoclock spectroscopy for characterizing the full time-energy structure of SASE or seeded LCLS II pulses, and support emerging trends in molecular-frame spectroscopy measurements.

  View details for DOI 10.1107/S1600577521007700

  View details for PubMedID 34475285

• Transient resonant Auger-Meitner spectra of photoexcited thymine. Faraday discussions Wolf, T. J., Paul, A. C., Folkestad, S. D., Myhre, R. H., Cryan, J. P., Berrah, N., Bucksbaum, P. H., Coriani, S., Coslovich, G., Feifel, R., Martinez, T. J., Moeller, S. P., Mucke, M., Obaid, R., Plekan, O., Squibb, R. J., Koch, H., Guhr, M. 2021

  Abstract

  We present the first investigation of excited state dynamics by resonant Auger-Meitner spectroscopy (also known as resonant Auger spectroscopy) using the nucleobase thymine as an example. Thymine is photoexcited in the UV and probed with X-ray photon energies at and below the oxygen K-edge. After initial photoexcitation to a pipi* excited state, thymine is known to undergo internal conversion to an npi* excited state with a strong resonance at the oxygen K-edge, red-shifted from the ground state pi* resonances of thymine (see our previous study Wolf, et al., Nat. Commun., 2017, 8, 29). We resolve and compare the Auger-Meitner electron spectra associated both with the excited state and ground state resonances, and distinguish participator and spectator decay contributions. Furthermore, we observe simultaneously with the decay of the npi* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the npi* state and the ground state resonances. We assign these contributions to population transfer from the npi* state to a pipi* triplet state via intersystem crossing on the picosecond timescale based on simulations of the X-ray absorption spectra in the vibrationally hot triplet state. Moreover, we identify signatures from the initially excited pipi* singlet state which we have not observed in our previous study.

  View details for DOI 10.1039/d0fd00112k

  View details for PubMedID 33566045

• Differentiating and Quantifying Gas-Phase Conformational Isomers Using Coulomb Explosion Imaging. The journal of physical chemistry letters Pathak, S., Obaid, R., Bhattacharyya, S., Bürger, J., Li, X., Tross, J., Severt, T., Davis, B., Bilodeau, R. C., Trallero-Herrero, C. A., Rudenko, A., Berrah, N., Rolles, D. 2020; 11 (23): 10205-10211

  Abstract

  Conformational isomerism plays a crucial role in defining the physical and chemical properties and biological activity of molecules ranging from simple organic compounds to complex biopolymers. However, it is often a significant challenge to differentiate and separate these isomers experimentally as they can easily interconvert due to their low rotational energy barrier. Here, we use the momentum correlation of fragment ions produced after inner-shell photoionization to distinguish conformational isomers of 1,2-dibromoethane (C2H4Br2). We demonstrate that the three-body breakup channel, C2H4+ + Br+ + Br+, contains signatures of both sequential and concerted breakup, which are decoupled to distinguish the geometries of two conformational isomers and to quantify their relative abundance. The sensitivity of our method to quantify these yields is established by measuring the relative abundance change with sample temperature, which agrees well with calculations. Our study paves the way for using Coulomb explosion imaging to track subtle molecular structural changes.

  View details for DOI 10.1021/acs.jpclett.0c02959

  View details for PubMedID 33206545

• Ultrafast Laser-Induced Isomerization Dynamics in Acetonitrile. The journal of physical chemistry letters McDonnell, M., LaForge, A. C., Reino-González, J., Disla, M., Kling, N. G., Mishra, D., Obaid, R., Sundberg, M., Svoboda, V., Díaz-Tendero, S., Martín, F., Berrah, N. 2020; 11 (16): 6724-6729

  Abstract

  Isomerization induced by laser ionization in acetonitrile (CH3CN) was investigated using pump-probe spectroscopy in combination with ion-ion coincident Coulomb explosion imaging. We deduced five primary channels indicating direct C-C breakup, single and double hydrogen migration, and H and H2 dissociation in the acetonitrile cation. Surprisingly, the hydrogen-migration channels dominate over direct fragmentation. This observation is supported by quantum chemistry calculations showing that isomerization through single and double hydrogen migration leads to very stable linear and ring isomers, with most of them more stable than the original linear structure following ionization of the parent molecule. This is unlike most molecules investigated previously using similar schemes. By varying the delay between the pump and probe pulses, we have also determined the time scales of the corresponding dynamical processes. Isomerization typically occurs in a few hundred femtoseconds, a time scale that is comparable to that found for H and H2 dissociation and direct molecular fragmentation.

  View details for DOI 10.1021/acs.jpclett.0c01344

  View details for PubMedID 32614185

• Electronic Population Transfer via Impulsive Stimulated X-Ray Raman Scattering with Attosecond Soft-X-Ray Pulses. Physical review letters O'Neal, J. T., Champenois, E. G., Oberli, S., Obaid, R., Al-Haddad, A., Barnard, J., Berrah, N., Coffee, R., Duris, J., Galinis, G., Garratt, D., Glownia, J. M., Haxton, D., Ho, P., Li, S., Li, X., MacArthur, J., Marangos, J. P., Natan, A., Shivaram, N., Slaughter, D. S., Walter, P., Wandel, S., Young, L., Bostedt, C., Bucksbaum, P. H., Picón, A., Marinelli, A., Cryan, J. P. 2020; 125 (7): 073203

  Abstract

  Free-electron lasers provide a source of x-ray pulses short enough and intense enough to drive nonlinearities in molecular systems. Impulsive interactions driven by these x-ray pulses provide a way to create and probe valence electron motions with high temporal and spatial resolution. Observing these electronic motions is crucial to understand the role of electronic coherence in chemical processes. A simple nonlinear technique for probing electronic motion, impulsive stimulated x-ray Raman scattering (ISXRS), involves a single impulsive interaction to produce a coherent superposition of electronic states. We demonstrate electronic population transfer via ISXRS using broad bandwidth (5.5 eV full width at half maximum) attosecond x-ray pulses produced by the Linac Coherent Light Source. The impulsive excitation is resonantly enhanced by the oxygen 1s→2π^{*} resonance of nitric oxide (NO), and excited state neutral molecules are probed with a time-delayed UV laser pulse.

  View details for DOI 10.1103/PhysRevLett.125.073203

  View details for PubMedID 32857563

• Electronic Population Transfer via Impulsive Stimulated X-Ray Raman Scattering with Attosecond Soft-X-Ray Pulses PHYSICAL REVIEW LETTERS O'Neal, J. T., Champenois, E. G., Oberli, S., Obaid, R., Al-Haddad, A., Barnard, J., Berrah, N., Coffee, R., Duris, J., Galinis, G., Garratt, D., Glownia, J. M., Haxton, D., Ho, P., Li, S., Li, X., MacArthur, J., Marangos, J. P., Natan, A., Shivaram, N., Slaughter, D. S., Walter, P., Wandel, S., Young, L., Bostedt, C., Bucksbaum, P. H., Picon, A., Marinelli, A., Cryan, J. P. 2020; 125 (7)

  View details for DOI 10.1103/PhysRevLett.125.073203

  View details for Web of Science ID 000558086800003

• Intermolecular Coulombic Decay in Endohedral Fullerene at the 4d→4f Resonance. Physical review letters Obaid, R. n., Xiong, H. n., Augustin, S. n., Schnorr, K. n., Ablikim, U. n., Battistoni, A. n., Wolf, T. J., Bilodeau, R. C., Osipov, T. n., Gokhberg, K. n., Rolles, D. n., LaForge, A. C., Berrah, N. n. 2020; 124 (11): 113002

  Abstract

  Intermolecular processes offer unique decay mechanisms for complex systems to internally relax. Here, we report the observation of an intermolecular Coulombic decay channel in an endohedral fullerene, a holmium nitride complex (Ho_{3}N) embedded within a C_{80} fullerene, between neighboring holmium ions, and between the holmium complex and the carbon cage. By measuring the ions and the electrons in coincidence after XUV photoabsorption, we can isolate the different decay channels, which are found to be more prevalent relative to intra-atomic Auger decay.

  View details for DOI 10.1103/PhysRevLett.124.113002

  View details for PubMedID 32242685

• Attosecond transient absorption spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy. Physical chemistry chemical physics : PCCP Driver, T., Li, S., Champenois, E. G., Duris, J., Ratner, D., Lane, T. J., Rosenberger, P., Al-Haddad, A., Averbukh, V., Barnard, T., Berrah, N., Bostedt, C., Bucksbaum, P. H., Coffee, R., DiMauro, L. F., Fang, L., Garratt, D., Gatton, A., Guo, Z., Hartmann, G., Haxton, D., Helml, W., Huang, Z., LaForge, A., Kamalov, A., Kling, M. F., Knurr, J., Lin, M., Lutman, A. A., MacArthur, J. P., Marangos, J. P., Nantel, M., Natan, A., Obaid, R., O'Neal, J. T., Shivaram, N. H., Schori, A., Walter, P., Li Wang, A., Wolf, T. J., Marinelli, A., Cryan, J. P. 2019

  Abstract

  The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime. In this paper we propose and demonstrate a new technique to conduct transient absorption spectroscopy with broad bandwidth attosecond pulses with the aid of ghost imaging, recovering sub-bandwidth resolution in photoproduct-based absorption measurements.

  View details for DOI 10.1039/c9cp03951a

  View details for PubMedID 31793561

• Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization NATURE PHYSICS Berrah, N., Sanchez-Gonzalez, A., Jurek, Z., Obaid, R., Xiong, H., Squibb, R. J., Osipov, T., Lutman, A., Fang, L., Barillot, T., Bozek, J. D., Cryan, J., Wolf, T. A., Rolles, D., Coffee, R., Schnorr, K., Augustin, S., Fukuzawa, H., Motomura, K., Niebuhr, N., Frasinski, L. J., Feifel, R., Schulz, C. P., Toyota, K., Son, S., Ueda, K., Pfeifer, T., Marangos, J. P., Santra, R. 2019; 15 (12): 1279-+

  View details for DOI 10.1038/s41567-019-0665-7

  View details for Web of Science ID 000500574300023

• A coincidence velocity map imaging spectrometer for ions and high-energy electrons to study inner-shell photoionization of gas-phase molecules REVIEW OF SCIENTIFIC INSTRUMENTS Ablikim, U., Bomme, C., Osipov, T., Xiong, H., Obaid, R., Bilodeau, R. C., Kling, N. G., Dumitriu, I., Augustin, S., Pathak, S., Schnorr, K., Kilcoyne, D., Berrah, N., Rolles, D. 2019; 90 (5): 055103

  Abstract

  We report on the design and performance of a double-sided coincidence velocity map imaging spectrometer optimized for electron-ion and ion-ion coincidence experiments studying inner-shell photoionization of gas-phase molecules with soft X-ray synchrotron radiation. The apparatus employs two microchannel plate detectors equipped with delay-line anodes for coincident, time- and position-resolved detection of photoelectrons and Auger electrons with kinetic energies up to 300 eV on one side of the spectrometer and photoions up to 25 eV per unit charge on the opposite side. We demonstrate its capabilities by measuring valence photoelectrons and ion spectra of neon and nitrogen and by studying channel-resolved photoelectron and Auger spectra along with fragment-ion momentum correlations for chlorine 2p inner-shell ionization of cis- and trans-1,2-dichloroethene.

  View details for DOI 10.1063/1.5093420

  View details for Web of Science ID 000483942900056

  View details for PubMedID 31153288

• Photo-ionization and fragmentation of Sc3N@C80 following excitation above the Sc K-edge. The Journal of chemical physics Obaid, R. n., Schnorr, K. n., Wolf, T. J., Takanashi, T. n., Kling, N. G., Kooser, K. n., Nagaya, K. n., Wada, S. I., Fang, L. n., Augustin, S. n., You, D. n., Campbell, E. E., Fukuzawa, H. n., Schulz, C. P., Ueda, K. n., Lablanquie, P. n., Pfeifer, T. n., Kukk, E. n., Berrah, N. n. 2019; 151 (10): 104308

  Abstract

  We have investigated the ionization and fragmentation of a metallo-endohedral fullerene, Sc3N@C80, using ultrashort (10 fs) x-ray pulses. Following selective ionization of a Sc (1s) electron (hν = 4.55 keV), an Auger cascade leads predominantly to either a vibrationally cold multiply charged parent molecule or multifragmentation of the carbon cage following a phase transition. In contrast to previous studies, no intermediate regime of C2 evaporation from the carbon cage is observed. A time-delayed, hard x-ray pulse (hν = 5.0 keV) was used to attempt to probe the electron transfer dynamics between the encapsulated Sc species and the carbon cage. A small but significant change in the intensity of Sc-containing fragment ions and coincidence counts for a delay of 100 fs compared to 0 fs, as well as an increase in the yield of small carbon fragment ions, may be indicative of incomplete charge transfer from the carbon cage on the sub-100 fs time scale.

  View details for DOI 10.1063/1.5110297

  View details for PubMedID 31521092

• Neon in ultrashort and intense x-rays from free electron lasers JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Buth, C., Beerwerth, R., Obaid, R., Berrah, N., Cederbaum, L. S., Fritzsche, S. 2018; 51 (5)

  View details for DOI 10.1088/1361-6455/aaa39a

  View details for Web of Science ID 000424508200001

• The LAMP instrument at the Linac Coherent Light Source free-electron laser REVIEW OF SCIENTIFIC INSTRUMENTS Osipov, T., Bostedt, C., Castagna, J., Ferguson, K. R., Bucher, M., Montero, S. C., Swiggers, M. L., Obaid, R., Rolles, D., Rudenko, A., Bozek, J. D., Berrah, N. 2018; 89 (3): 035112

  Abstract

  The Laser Applications in Materials Processing (LAMP) instrument is a new end-station for soft X-ray imaging, high-field physics, and ultrafast X-ray science experiments that is available to users at the Linac Coherent Light Source (LCLS) free-electron laser. While the instrument resides in the Atomic, Molecular and Optical science hutch, its components can be used at any LCLS beamline. The end-station has a modular design that provides high flexibility in order to meet user-defined experimental requirements and specifications. The ultra-high-vacuum environment supports different sample delivery systems, including pulsed and continuous atomic, molecular, and cluster jets; liquid and aerosols jets; and effusive metal vapor beams. It also houses movable, large-format, high-speed pnCCD X-ray detectors for detecting scattered and fluorescent photons. Multiple charged-particle spectrometer options are compatible with the LAMP chamber, including a double-sided spectrometer for simultaneous and even coincident measurements of electrons, ions, and photons produced by the interaction of the high-intensity X-ray beam with the various samples. Here we describe the design and capabilities of the spectrometers along with some general aspects of the LAMP chamber and show some results from the initial instrument commissioning.

  View details for DOI 10.1063/1.5017727

  View details for Web of Science ID 000428988300065

  View details for PubMedID 29604777

• Fragmentation of endohedral fullerene Ho3N@C-80 in an intense femtosecond near-infrared laser field PHYSICAL REVIEW A Xiong, H., Fang, L., Osipov, T., Kling, N. G., Wolf, T. A., Sistrunk, E., Obaid, R., Guehr, M., Berrah, N. 2018; 97 (2)

  View details for DOI 10.1103/PhysRevA.97.023419

  View details for Web of Science ID 000425729200007

• LCLS in-photon out: fluorescence measurement of neon using soft x-rays JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Obaid, R., Buth, C., Dakovski, G. L., Beerwerth, R., Holmes, M., Aldrich, J., Lin, M., Minitti, M., Osipov, T., Schlotter, W., Cederbaum, L. S., Fritzsche, S., Berrah, N. 2018; 51 (3)

  View details for DOI 10.1088/1361-6455/aaa189

  View details for Web of Science ID 000419798400001

• Soft-x-ray-induced ionization and fragmentation dynamics of Sc3N@C-80 investigated using an ion-ion-coincidence momentum-imaging technique PHYSICAL REVIEW A Xiong, H., Obaid, R., Fang, L., Bomme, C., Kling, N. G., Ablikim, U., Petrovic, V., Liekhus-Schmaltz, C. E., Li, H., Bilodeau, R. C., Wolf, T., Osipov, T., Rolles, D., Berrah, N. 2017; 96 (3)

  View details for DOI 10.1103/PhysRevA.96.033408

  View details for Web of Science ID 000410589500005

• Identification of absolute geometries of <i>cis</i> and <i>trans</i> molecular isomers by Coulomb Explosion Imaging SCIENTIFIC REPORTS Ablikim, U., Bomme, C., Xiong, H., Savelyev, E., Obaid, R., Kaderiya, B., Augustin, S., Schnorr, K., Dumitriu, I., Osipov, T., Bilodeau, R., Kilcoyne, D., Kumarappan, V., Rudenko, A., Berrah, N., Rolles, D. 2016; 6: 38202

  Abstract

  An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C2H2Br2). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

  View details for DOI 10.1038/srep38202

  View details for Web of Science ID 000389185800001

  View details for PubMedID 27910943

  View details for PubMedCentralID PMC5133590

• Towards a microchannel-based X-ray detector with two-dimensional spatial and time resolution and high dynamic range JOURNAL OF SYNCHROTRON RADIATION Adams, B. W., Mane, A. U., Elam, J. W., Obaid, R., Wetstein, M., Chollet, M. 2015; 22: 1202-1206

  Abstract

  X-ray detectors that combine two-dimensional spatial resolution with a high time resolution are needed in numerous applications of synchrotron radiation. Most detectors with this combination of capabilities are based on semiconductor technology and are therefore limited in size. Furthermore, the time resolution is often realised through rapid time-gating of the acquisition, followed by a slower readout. Here, a detector technology is realised based on relatively inexpensive microchannel plates that uses GHz waveform sampling for a millimeter-scale spatial resolution and better than 100 ps time resolution. The technology is capable of continuous streaming of time- and location-tagged events at rates greater than 10(7) events per cm(2). Time-gating can be used for improved dynamic range.

  View details for DOI 10.1107/S1600577515010322

  View details for Web of Science ID 000360142400009

  View details for PubMedID 26289271

• Invited Article: A test-facility for large-area microchannel plate detector assemblies using a pulsed sub-picosecond laser REVIEW OF SCIENTIFIC INSTRUMENTS Adams, B., Chollet, M., Elagin, A., Oberla, E., Vostrikov, A., Wetstein, M., Obaid, R., Webster, P. 2013; 84 (6)

  Abstract

  The Large Area Picosecond Photodetector Collaboration is developing large-area fast photodetectors with time resolution

  View details for DOI 10.1063/1.4810018

  View details for Web of Science ID 000321273500001

  View details for PubMedID 23822326