All Publications

  • X-ray induced Coulomb explosion imaging of transient excited-state structural rearrangements in CS<sub>2</sub> COMMUNICATIONS PHYSICS Unwin, J., Allum, F., Britton, M., Gabalski, I., Bromberger, H., Brouard, M., Bucksbaum, P. H., Driver, T., Ekanayake, N., Garg, D., Gougoula, E., Heathcote, D., Howard, A. J., Hockett, P., Holland, D. P., Kumar, S., Lam, C., Lee, J. L., Mcmanus, J., Mikosch, J., Milesevic, D., Minns, R. S., Papadopoulou, C. C., Passow, C., Razmus, W. O., Roeder, A., Rouzee, A., Schuurman, M., Simao, A., Stolow, A., Tul-Noor, A., Vallance, C., Walmsley, T., Rolles, D., Erk, B., Burt, M., Forbes, R. 2023; 6 (1)
  • Femtosecond Electronic and Hydrogen Structural Dynamics in Ammonia Imaged with Ultrafast Electron Diffraction. Physical review letters Champenois, E. G., List, N. H., Ware, M., Britton, M., Bucksbaum, P. H., Cheng, X., Centurion, M., Cryan, J. P., Forbes, R., Gabalski, I., Hegazy, K., Hoffmann, M. C., Howard, A. J., Ji, F., Lin, M. F., Nunes, J. P., Shen, X., Yang, J., Wang, X., Martinez, T. J., Wolf, T. J. 2023; 131 (14): 143001


    Directly imaging structural dynamics involving hydrogen atoms by ultrafast diffraction methods is complicated by their low scattering cross sections. Here we demonstrate that megaelectronvolt ultrafast electron diffraction is sufficiently sensitive to follow hydrogen dynamics in isolated molecules. In a study of the photodissociation of gas phase ammonia, we simultaneously observe signatures of the nuclear and corresponding electronic structure changes resulting from the dissociation dynamics in the time-dependent diffraction. Both assignments are confirmed by ab initio simulations of the photochemical dynamics and the resulting diffraction observable. While the temporal resolution of the experiment is insufficient to resolve the dissociation in time, our results represent an important step towards the observation of proton dynamics in real space and time.

    View details for DOI 10.1103/PhysRevLett.131.143001

    View details for PubMedID 37862660

  • Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge. The journal of physical chemistry letters Gabalski, I., Allum, F., Seidu, I., Britton, M., Brenner, G., Bromberger, H., Brouard, M., Bucksbaum, P. H., Burt, M., Cryan, J. P., Driver, T., Ekanayake, N., Erk, B., Garg, D., Gougoula, E., Heathcote, D., Hockett, P., Holland, D. M., Howard, A. J., Kumar, S., Lee, J. W., Li, S., McManus, J., Mikosch, J., Milesevic, D., Minns, R. S., Neville, S., Papadopoulou, C. C., Passow, C., Razmus, W. O., Röder, A., Rouzée, A., Simao, A., Unwin, J., Vallance, C., Walmsley, T., Wang, J., Rolles, D., Stolow, A., Schuurman, M. S., Forbes, R. 2023: 7126-7133


    Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

    View details for DOI 10.1021/acs.jpclett.3c01447

    View details for PubMedID 37534743

  • Filming enhanced ionization in an ultrafast triatomic slingshot. Communications chemistry Howard, A. J., Britton, M., Streeter, Z. L., Cheng, C., Forbes, R., Reynolds, J. L., Allum, F., McCracken, G. A., Gabalski, I., Lucchese, R. R., McCurdy, C. W., Weinacht, T., Bucksbaum, P. H. 2023; 6 (1): 81


    Filming atomic motion within molecules is an active pursuit of molecular physics and quantum chemistry. A promising method is laser-induced Coulomb Explosion Imaging (CEI) where a laser pulse rapidly ionizes many electrons from a molecule, causing the remaining ions to undergo Coulomb repulsion. The ion momenta are used to reconstruct the molecular geometry which is tracked over time (i.e., filmed) by ionizing at an adjustable delay with respect to the start of interatomic motion. Results are distorted, however, by ultrafast motion during the ionizing pulse. We studied this effect in water and filmed the rapid "slingshot" motion that enhances ionization and distorts CEI results. Our investigation uncovered both the geometry and mechanism of the enhancement which may inform CEI experiments in many other polyatomic molecules.

    View details for DOI 10.1038/s42004-023-00882-w

    View details for PubMedID 37106058

    View details for PubMedCentralID PMC10140156

  • Transient vibration and product formation of photoexcited CS2 measured by time-resolved x-ray scattering. The Journal of chemical physics Gabalski, I., Sere, M., Acheson, K., Allum, F., Boutet, S., Dixit, G., Forbes, R., Glownia, J. M., Goff, N., Hegazy, K., Howard, A. J., Liang, M., Minitti, M. P., Minns, R. S., Natan, A., Peard, N., Rasmus, W. O., Sension, R. J., Ware, M. R., Weber, P. M., Werby, N., Wolf, T. J., Kirrander, A., Bucksbaum, P. H. 2022; 157 (16): 164305


    We have observed details of the internal motion and dissociation channels in photoexcited carbon disulfide (CS2) using time-resolved x-ray scattering (TRXS). Photoexcitation of gas-phase CS2 with a 200nm laser pulse launches oscillatory bending and stretching motion, leading to dissociation of atomic sulfur in under a picosecond. During the first 300fs following excitation, we observe significant changes in the vibrational frequency as well as some dissociation of the C-S bond, leading to atomic sulfur in the both 1D and 3P states. Beyond 1400fs, the dissociation is consistent with primarily 3P atomic sulfur dissociation. This channel-resolved measurement of the dissociation time is based on our analysis of the time-windowed dissociation radial velocity distribution, which is measured using the temporal Fourier transform of the TRXS data aided by a Hough transform that extracts the slopes of linear features in an image. The relative strength of the two dissociation channels reflects both their branching ratio and differences in the spread of their dissociation times. Measuring the time-resolved dissociation radial velocity distribution aids the resolution of discrepancies between models for dissociation proposed by prior photoelectron spectroscopy work.

    View details for DOI 10.1063/5.0113079

    View details for PubMedID 36319419

  • Multichannel photodissociation dynamics in CS2 studied by ultrafast electron diffraction. Physical chemistry chemical physics : PCCP Razmus, W. O., Acheson, K., Bucksbaum, P., Centurion, M., Champenois, E., Gabalski, I., Hoffman, M. C., Howard, A., Lin, M., Liu, Y., Nunes, P., Saha, S., Shen, X., Ware, M., Warne, E. M., Weinacht, T., Wilkin, K., Yang, J., Wolf, T. J., Kirrander, A., Minns, R. S., Forbes, R. 2022


    The structural dynamics of photoexcited gas-phase carbon disulfide (CS2) molecules are investigated using ultrafast electron diffraction. The dynamics were triggered by excitation of the optically bright 1B2(1Sigmau+) state by an ultraviolet femtosecond laser pulse centred at 200 nm. In accordance with previous studies, rapid vibrational motion facilitates a combination of internal conversion and intersystem crossing to lower-lying electronic states. Photodissociation via these electronic manifolds results in the production of CS fragments in the electronic ground state and dissociated singlet and triplet sulphur atoms. The structural dynamics are extracted from the experiment using a trajectory-fitting filtering approach, revealing the main characteristics of the singlet and triplet dissociation pathways. Finally, the effect of the time-resolution on the experimental signal is considered and an outlook to future experiments provided.

    View details for DOI 10.1039/d2cp01268e

    View details for PubMedID 35707953

  • X-ray scattering signatures of early-time accelerations in iodine dissociation JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Gabalski, I., Ware, M. R., Bucksbaum, P. H. 2020; 53 (24)