All Publications

  • 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

  • Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time. Nature communications Liu, Y., Sanchez, D. M., Ware, M. R., Champenois, E. G., Yang, J., Nunes, J. P., Attar, A., Centurion, M., Cryan, J. P., Forbes, R., Hegazy, K., Hoffmann, M. C., Ji, F., Lin, M., Luo, D., Saha, S. K., Shen, X., Wang, X. J., Martinez, T. J., Wolf, T. J. 2023; 14 (1): 2795


    Electrocyclic reactions are characterized by the concerted formation and cleavage of both sigma and pi bonds through a cyclic structure. This structure is known as a pericyclic transition state for thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule alpha-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated pi bonds. The sigma bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.

    View details for DOI 10.1038/s41467-023-38513-6

    View details for PubMedID 37202402

  • 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

  • Single- and multi-photon-induced ultraviolet excitation and photodissociation of CH3I probed by coincident ion momentum imaging PHYSICAL CHEMISTRY CHEMICAL PHYSICS Ziaee, F., Borne, K., Forbes, R., Raju, P., Malakar, Y., Kaderiya, B., Severt, T., Ben-Itzhak, I., Rudenko, A., Rolles, D. 2023; 25 (14): 9999-10010


    The UV-induced photodissociation dynamics of iodomethane (CH3I) in its A-band are investigated by time-resolved coincident ion momentum imaging using strong-field ionization as a probe. The delay-dependent kinetic energy distribution of the photofragments resulting from double ionization of the molecule maps the cleavage of the carbon-iodine bond and shows how the existence of a potential well in the di-cationic potential energy surfaces shapes the observed distribution at small pump-probe delays. Furthermore, the competition between single- and multi-photon excitation and ionization of the molecule is studied as a function of the intensity of the UV-pump laser pulse. Two-photon excitation to Rydberg states is identified by tracking the transformation of the delay-dependent singly-charged iodomethane yield from a pure Gaussian distribution at low intensity to a Gaussian with an exponentially decaying tail at higher intensities. Dissociative ionization induced by absorption of three UV photons is resolved as an additional delay-dependent feature in the kinetic energy of the fragment ions detected in coincidence.

    View details for DOI 10.1039/d3cp00498h

    View details for Web of Science ID 000956071900001

    View details for PubMedID 36960727

  • Multiparticle Cumulant Mapping for Coulomb Explosion Imaging. Physical review letters Cheng, C., Frasinski, L. J., Moğol, G., Allum, F., Howard, A. J., Rolles, D., Bucksbaum, P. H., Brouard, M., Forbes, R., Weinacht, T. 2023; 130 (9): 093001


    We extend covariance velocity map ion imaging to four particles, establishing cumulant mapping and allowing for measurements that provide insights usually associated with coincidence detection, but at much higher count rates. Without correction, a fourfold covariance analysis is contaminated by the pairwise correlations of uncorrelated events, but we have addressed this with the calculation of a full cumulant, which subtracts pairwise correlations. We demonstrate the approach on the four-body breakup of formaldehyde following strong field multiple ionization in few-cycle laser pulses. We compare Coulomb explosion imaging for two different pulse durations (30 and 6 fs), highlighting the dynamics that can take place on ultrafast timescales. These results have important implications for Coulomb explosion imaging as a tool for studying ultrafast structural changes in molecules, a capability that is especially desirable for high-count-rate x-ray free-electron laser experiments.

    View details for DOI 10.1103/PhysRevLett.130.093001

    View details for PubMedID 36930921

  • Photon energy-resolved velocity map imaging from spectral domain ghost imaging NEW JOURNAL OF PHYSICS Wang, J., Driver, T., Allum, F., Papadopoulou, C. C., Passow, C., Brenner, G., Li, S., Duesterer, S., Tul Noor, A., Kumar, S., Bucksbaum, P. H., Erk, B., Forbes, R., Cryan, J. P. 2023; 25 (3)
  • 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

  • Probing two-path electron quantum interference in strong-field ionization with time-correlation filtering PHYSICAL REVIEW A Werby, N., Maxwell, A. S., Forbes, R., Faria, C., Bucksbaum, P. H. 2022; 106 (3)
  • Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis. Physical chemistry chemical physics : PCCP McManus, J. W., Walmsley, T., Nagaya, K., Harries, J. R., Kumagai, Y., Iwayama, H., Ashfold, M. N., Britton, M., Bucksbaum, P. H., Downes-Ward, B., Driver, T., Heathcote, D., Hockett, P., Howard, A. J., Kukk, E., Lee, J. W., Liu, Y., Milesevic, D., Minns, R. S., Niozu, A., Niskanen, J., Orr-Ewing, A. J., Owada, S., Rolles, D., Robertson, P. A., Rudenko, A., Ueda, K., Unwin, J., Vallance, C., Burt, M., Brouard, M., Forbes, R., Allum, F. 2022


    We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (12 ions shot-1). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.

    View details for DOI 10.1039/d2cp03029b

    View details for PubMedID 36106844

  • 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

  • Efficient generation of the 7th harmonic of Ti:sapphire (114.6 nm) vacuum ultraviolet pulses with 60 fs duration by non-collinear four-wave mixing in argon OPTICS LETTERS Forbes, R., Hockett, P., Lausten, R. 2022; 47 (10): 2410-2413


    The recent advances in femtosecond vacuum UV (VUV) pulse generation, pioneered by the work of Noack et al., has enabled new experiments in ultrafast time-resolved spectroscopy. Expanding on this work, we report the generation of 60 fs VUV pulses at the 7th harmonic of Ti:sapphire with more than 50 nJ of pulse energy at a repetition rate of 1 kHz. The 114.6 nm pulses are produced using non-collinear four-wave difference-frequency mixing in argon. The non-collinear geometry increases the phase-matching pressure, and results in a conversion efficiency of ∼10-3 from the 200 nm pump beam. The VUV pulses are pre-chirp-compensated for material dispersion with xenon, which has negative dispersion in this wavelength range, thus allowing almost transform-limited pulses to be delivered to the experimental chamber.

    View details for DOI 10.1364/OL.457070

    View details for Web of Science ID 000798087600011

    View details for PubMedID 35561363

  • Auger electron angular distributions following excitation or ionization from the Xe 3d and F 1s levels in xenon difluoride PHYSICAL CHEMISTRY CHEMICAL PHYSICS Forbes, R., Hockett, P., Powis, I., Bozek, J. D., Pratt, S. T., Holland, D. P. 2022; 24 (3): 1367-1379


    Linearly polarized synchrotron radiation has been used to record polarization dependent, non-resonant Auger electron spectra of XeF2, encompassing the bands due to the xenon M45N1N45, M45N23N45, M45N45N45 and M45N45V and fluorine KVV transitions. Resonantly excited Auger spectra have been measured at photon energies coinciding with the Xe 3d5/2 → σ* and the overlapped Xe 3d3/2/F 1s → σ* excitations in XeF2. The non-resonant and resonantly excited spectra have enabled the Auger electron angular distributions, as characterized by the βA parameter, to be determined for the M45N45N45 transitions. In the photon energy range over which the Auger electron angular distributions were measured, theoretical results indicate that transitions into the εf continuum channel dominate the Xe 3d photoionization in XeF2. In this limit, the theoretical value of the atomic alignment parameter (A20) characterizing the core ionized state becomes constant. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the βA parameters extracted from our non-resonant Auger spectra. For a particular Auger transition, the electron kinetic energy measured in the resonantly excited spectrum is higher than that in the directly ionized spectrum, due to the screening provided by the electron promoted into the σ* orbital. The interpretation of the F KVV Auger band in XeF2 has been discussed in relation to previously published one-site populations of the doubly charged ions (XeF22+). The experimental results show that the ionization energies of the doubly charged states predominantly populated in the decay of a vacancy in the F 1s orbital in XeF2 tend to be higher than those populated in the decay of a vacancy in the Xe 4d level in XeF2.

    View details for DOI 10.1039/d1cp04797c

    View details for Web of Science ID 000733922500001

    View details for PubMedID 34951418

  • 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)