I am a Postdoctoral Scholar affiliated with the PULSE institute and the LCLS Laser Science Division. My research interests focus on the ultrafast photodynamics of isolated molecules in the gas phase, as studied by a range of techniques typically incorporating charged particle imaging, photoionization spectroscopy or diffractive imaging. I am also interested in developing new approaches to studying ultrafast photochemistry, through, for instance, the generation of broad bandwidth optical pulses and new data analysis techniques to extract additional information from rich and complex datasets.

Prior to joining SLAC in October 2021, I studied for my PhD at the University of Oxford under the supervision of Mark Brouard. My doctoral research included a range of studies into ultrafast photodissociation dynamics using velocity-map imaging, both in a laboratory setting and at international FEL facilities.

Stanford Advisors

All Publications

  • 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

  • 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)
  • Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study JOURNAL OF PHYSICAL CHEMISTRY A Allum, F., McManus, J., Denby, O., Burt, M., Brouard, M. 2022: 8577-8587


    The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

    View details for DOI 10.1021/acs.jpca.2c04379

    View details for Web of Science ID 000885280500001

    View details for PubMedID 36351075

  • 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

  • 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

  • The kinetic energy of PAH dication and trication dissociation determined by recoil-frame covariance map imaging. Physical chemistry chemical physics : PCCP Lee, J. W., Tikhonov, D. S., Allum, F., Boll, R., Chopra, P., Erk, B., Gruet, S., He, L., Heathcote, D., Kazemi, M. M., Lahl, J., Lemmens, A. K., Loru, D., Maclot, S., Mason, R., Müller, E., Mullins, T., Passow, C., Peschel, J., Ramm, D., Steber, A. L., Bari, S., Brouard, M., Burt, M., Küpper, J., Eng-Johnsson, P., Rijs, A. M., Rolles, D., Vallance, C., Manschwetus, B., Schnell, M. 2022


    We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. In our experiments, ions are formed by interaction with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS2 multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision, ranging 1.94-2.60 eV and 2.95-5.29 eV for the dications and trications, respectively. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) simulations.

    View details for DOI 10.1039/d2cp02252d

    View details for PubMedID 35876592

  • Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy FRONTIERS IN PHYSICS Garg, D., Lee, J. L., Tikhonov, D. S., Chopra, P., Steber, A. L., Lemmens, A. K., Erk, B., Allum, F., Boll, R., Cheng, X., Duesterer, S., Gruet, S., He, L., Heathcote, D., Johny, M., Kazemi, M. M., Koeckert, H., Lahl, J., Loru, D., Maclot, S., Mason, R., Mueller, E., Mullins, T., Olshin, P., Passow, C., Peschel, J., Ramm, D., Rompotis, D., Trippel, S., Wiese, J., Ziaee, F., Bari, S., Burt, M., Kuepper, J., Rijs, A. M., Rolles, D., Techert, S., Eng-Johnsson, P., Brouard, M., Vallance, C., Manschwetus, B., Schnell, M. 2022; 10
  • A localized view on molecular dissociation via electron-ion partial covariance COMMUNICATIONS CHEMISTRY Allum, F., Music, V., Inhester, L., Boll, R., Erk, B., Schmidt, P., Baumann, T. M., Brenner, G., Burt, M., Demekhin, P., Doerner, S., Ehresmann, A., Galler, A., Grychtol, P., Heathcote, D., Kargin, D., Larsson, M., Lee, J. L., Li, Z., Manschwetus, B., Marder, L., Mason, R., Meyer, M., Otto, H., Passow, C., Pietschnig, R., Ramm, D., Schubert, K., Schwob, L., Thomas, R. D., Vallance, C., Vidanovic, I., Schmising, C., Wagner, R., Walter, P., Zhaunerchyk, V., Rolles, D., Bari, S., Brouard, M., Ilchen, M. 2022; 5 (1)
  • UV-induced dissociation of CH2BrI probed by intense femtosecond XUV pulses JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Kockert, H., Lee, J. L., Allum, F., Amini, K., Bari, S., Bomme, C., Brausse, F., Brouard, M., Burt, M., de Miranda, B., Duesterer, S., Eng-Johnsson, P., Erk, B., Geleoc, M., Geneaux, R., Gentleman, A. S., Guillemin, R., Goldsztejn, G., Holland, D. P., Ismail, I., Journel, L., Kierspel, T., Kuepper, J., Lahl, J., Mackenzie, S. R., Maclot, S., Manschwetus, B., Mereshchenko, A. S., Mullins, T., Olshin, P. K., Palaudoux, J., Penent, F., Piancastelli, M., Rompotis, D., Rouzee, A., Ruchon, T., Rudenko, A., Schirmel, N., Simon, M., Techert, S., Travnikova, O., Trippel, S., Vallance, C., Wang, E., Wiese, J., Ziaee, F., Marchenko, T., Rolles, D., Boll, R. 2022; 55 (1)
  • Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime NATURE COMMUNICATIONS Lee, J. L., Tikhonov, D. S., Chopra, P., Maclot, S., Steber, A. L., Gruet, S., Allum, F., Boll, R., Cheng, X., Duesterer, S., Erk, B., Garg, D., He, L., Heathcote, D., Johny, M., Kazemi, M. M., Koeckert, H., Lahl, J., Lemmens, A. K., Loru, D., Mason, R., Mueller, E., Mullins, T., Olshin, P., Passow, C., Peschel, J., Ramm, D., Rompotis, D., Schirmel, N., Trippel, S., Wiese, J., Ziaee, F., Bari, S., Burt, M., Kuepper, J., Rijs, A. M., Rolles, D., Techert, S., Eng-Johnsson, P., Brouard, M., Vallance, C., Manschwetus, B., Schnell, M. 2021; 12 (1): 6107


    Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10-100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

    View details for DOI 10.1038/s41467-021-26193-z

    View details for Web of Science ID 000709466400001

    View details for PubMedID 34671016

    View details for PubMedCentralID PMC8528970

  • 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

  • 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)
  • Post extraction inversion slice imaging for 3D velocity map imaging experiments MOLECULAR PHYSICS Allum, F., Mason, R., Burt, M., Slater, C. S., Squires, E., Winter, B., Brouard, M. 2021; 119 (1-2)
  • Coulomb explosion imaging of CH3I and CH2CII photodissociation dynamics JOURNAL OF CHEMICAL PHYSICS Allum, F., Burt, M., Amini, K., Boll, R., Kockert, H., Olshin, P. K., Bari, S., Bomme, C., Brausse, F., de Miranda, B., Duesterer, S., Erk, B., Geleoc, M., Geneaux, R., Gentleman, A. S., Goldsztejn, G., Guillemin, R., Holland, D. P., Ismail, I., Johnsson, P., Journel, L., Kuepper, J., Lahl, J., Lee, J. L., Maclot, S., Mackenzie, S. R., Manschwetus, B., Mereshchenko, A. S., Mason, R., Palaudoux, J., Piancastelli, M., Penent, F., Rompotis, D., Rouzee, A., Ruchon, T., Rudenko, A., Savelyev, E., Simon, M., Schirmel, N., Stapelfeldt, H., Techert, S., Travnikova, O., Trippel, S., Underwood, J. G., Vallance, C., Wiese, J., Ziaee, F., Brouard, M., Marchenko, T., Rolles, D. 2018; 149 (20): 204313


    The photodissociation dynamics of CH3I and CH2ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH2ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.

    View details for DOI 10.1063/1.5041381

    View details for Web of Science ID 000451745000028

    View details for PubMedID 30501230