All Publications

  • Multi-Particle Three-Dimensional Covariance Imaging: "Coincidence" Insights into the Many-Body Fragmentation of Strong-Field Ionized D2O. The journal of physical chemistry letters Allum, F., Cheng, C., Howard, A. J., Bucksbaum, P. H., Brouard, M., Weinacht, T., Forbes, R. 2021: 8302-8308


    We demonstrate the applicability of covariance analysis to three-dimensional velocity-map imaging experiments using a fast time stamping detector. Studying the photofragmentation of strong-field doubly ionized D2O molecules, we show that combining high count rate measurements with covariance analysis yields the same level of information typically limited to the "gold standard" of true, low count rate coincidence experiments, when averaging over a large ensemble of photofragmentation events. This increases the effective data acquisition rate by approximately 2 orders of magnitude, enabling a new class of experimental studies. This is illustrated through an investigation into the dependence of three-body D2O2+ dissociation on the intensity of the ionizing laser, revealing mechanistic insights into the nuclear dynamics driven during the laser pulse. The experimental methodology laid out, with its drastic reduction in acquisition time, is expected to be of great benefit to future photofragment imaging studies.

    View details for DOI 10.1021/acs.jpclett.1c02481

    View details for PubMedID 34428066

  • Strong-field ionization of water. II. Electronic and nuclear dynamics en route to double ionization PHYSICAL REVIEW A Cheng, C., Streeter, Z. L., Howard, A. J., Spanner, M., Lucchese, R. R., McCurdy, C., Weinacht, T., Bucksbaum, P. H., Forbes, R. 2021; 104 (2)
  • Dissecting subcycle interference in photoelectron holography PHYSICAL REVIEW A Werby, N., Maxwell, A. S., Forbes, R., Bucksbaum, P. H., Faria, C. 2021; 104 (1)
  • Unmasking the cis-Stilbene Phantom State via Vacuum Ultraviolet Time-Resolved Photoelectron Spectroscopy and Ab Initio Multiple Spawning. The journal of physical chemistry letters Williams, M., Forbes, R., Weir, H., Veyrinas, K., MacDonell, R. J., Boguslavskiy, A. E., Schuurman, M. S., Stolow, A., Martinez, T. J. 2021: 6363-6369


    We present the first vacuum ultraviolet time-resolved photoelectron spectroscopy (VUV-TRPES) study of photoisomerization dynamics in the paradigmatic molecule cis-stilbene. A key reaction intermediate in its dynamics, known as the phantom state, has often been invoked but never directly detected in the gas phase. We report direct spectral signatures of the phantom state in isolated cis-stilbene, observed and characterized through a combination of VUV-TRPES and ab initio multiple spawning (AIMS) nonadiabatic dynamics simulations of the channel-resolved observable. The high VUV probe photon energy tracks the complete excited-state dynamics via multiple photoionization channels, from initial excitation to its return to the "hot" ground state. The TRPES was compared with AIMS simulations of the dynamics from initial excitation, to the phantom-state intermediate (an S1 minimum), through to the ultimate electronic decay to the ground state. This combination revealed the unique spectral signatures and time-dependent dynamics of the phantom-state intermediate, permitting us to report here its direct observation.

    View details for DOI 10.1021/acs.jpclett.1c01227

    View details for PubMedID 34231356

  • Disentangling the subcycle electron momentum spectrum in strong-field ionization PHYSICAL REVIEW RESEARCH Werby, N., Natan, A., Forbes, R., Bucksbaum, P. H. 2021; 3 (2)
  • Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with time-resolved recoil-frame covariance imaging. Faraday discussions Allum, F., Anders, N., Brouard, M., Bucksbaum, P., Burt, M., Downes-Ward, B., Grundmann, S., Harries, J., Ishimura, Y., Iwayama, H., Kaiser, L., Kukk, E., Lee, J., Liu, X., Minns, R. S., Nagaya, K., Niozu, A., Niskanen, J., O'Neal, J., Owada, S., Pickering, J., Rolles, D., Rudenko, A., Saito, S., Ueda, K., Vallance, C., Werby, N., Woodhouse, J., You, D., Ziaee, F., Driver, T., Forbes, R. 2021


    The photodissociation dynamics of strong-field ionized methyl iodide (CH3I) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources.

    View details for DOI 10.1039/d0fd00115e

    View details for PubMedID 33629700

  • Time-resolved site-selective imaging of predissociation and charge transfer dynamics: the CH3I B-band JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Forbes, R., Allum, F., Bari, S., Boll, R., Borne, K., Brouard, M., Bucksbaum, P. H., Ekanayake, N., Erk, B., Howard, A. J., Johnsson, P., Lee, J. L., Manschwetus, B., Mason, R., Passow, C., Peschel, J., Rivas, D. E., Roerig, A., Rouzee, A., Vallance, C., Ziaee, F., Rolles, D., Burt, M. 2020; 53 (22)
  • Momentum-resolved above-threshold ionization of deuterated water PHYSICAL REVIEW A Cheng, C., Forbes, R., Howard, A. J., Spanner, M., Bucksbaum, P. H., Weinacht, T. 2020; 102 (5)
  • Tracking the ultraviolet-induced photochemistry of thiophenone during and after ultrafast ring opening. Nature chemistry Pathak, S., Ibele, L. M., Boll, R., Callegari, C., Demidovich, A., Erk, B., Feifel, R., Forbes, R., Di Fraia, M., Giannessi, L., Hansen, C. S., Holland, D. M., Ingle, R. A., Mason, R., Plekan, O., Prince, K. C., Rouzee, A., Squibb, R. J., Tross, J., Ashfold, M. N., Curchod, B. F., Rolles, D. 2020


    Photoinduced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic structure and molecular dynamics calculations of the excited- and ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state are shown to be driven by ballistic S-C bond extension and to be complete within 350 fs. Theory and experiment also enable visualization of the rich ground-state dynamics that involve the formation of, and interconversion between, ring-opened isomers and the cyclic structure, as well as fragmentation over much longer timescales.

    View details for DOI 10.1038/s41557-020-0507-3

    View details for PubMedID 32690894

  • Spectroscopic and Structural Probing of Excited-State Molecular Dynamics with Time-Resolved Photoelectron Spectroscopy and Ultrafast Electron Diffraction PHYSICAL REVIEW X Liu, Y., Horton, S. L., Yang, J., Nunes, J. F., Shen, X., Wolfe, T. A., Forbes, R., Cheng, C., Moore, B., Centurion, M., Hegazy, K., Li, R., Lin, M., Stolow, A., Hockett, P., Rozgonyi, T., Marquetande, P., Wang, X., Weinacht, T. 2020; 10 (2)