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

  • A plano-convex thick-lens velocity map imaging apparatus for direct, high resolution 3D momentum measurements of photoelectrons with ion time-of-flight coincidence REVIEW OF SCIENTIFIC INSTRUMENTS Davino, M., McManus, E., Helming, N. G., Cheng, C., Mogol, G., Rodnova, Z., Harrison, G., Watson, K., Weinacht, T., Gibson, G. N., Saule, T., Trallero-Herrero, C. A. 2023; 94 (1): 013303


    Since their inception, velocity map imaging (VMI) techniques have received continued interest in their expansion from 2D to 3D momentum measurements through either reconstructive or direct methods. Recently, much work has been devoted to the latter of these by relating electron time-of-flight (TOF) to the third momentum component. The challenge is having a timing resolution sufficient to resolve the structure in the narrow (<10 ns) electron TOF spread. Here, we build upon the work in VMI lens design and 3D VMI measurement by using a plano-convex thick-lens (PCTL) VMI in conjunction with an event-driven camera (TPX3CAM) providing TOF information for high resolution 3D electron momentum measurements. We perform simulations to show that, with the addition of a mesh electrode to the thick-lens geometry, the resulting plano-convex electrostatic field extends the detectable electron cutoff energy range while retaining the high resolution. This design also extends the electron TOF range, allowing for a better momentum resolution along this axis. We experimentally demonstrate these capabilities by examining above-threshold ionization in xenon, where the apparatus is shown to collect electrons of energy up to ∼7 eV with a TOF spread of ∼30 ns, both of which are improved compared to a previous work by factors of ∼1.4 and ∼3.75, respectively. Finally, the PCTL-VMI is equipped with a coincident ion TOF spectrometer, which is shown to effectively extract unique 3D momentum distributions for different ionic species in a gas mixture. These techniques have the potential to lend themselves to more advanced measurements involving systems where the electron momentum distributions possess non-trivial symmetries.

    View details for DOI 10.1063/5.0129900

    View details for Web of Science ID 000908414000005

    View details for PubMedID 36725611

  • Stable excited dication: trapping on the S-1 state of formaldehyde dication after strong field ionization PHYSICAL CHEMISTRY CHEMICAL PHYSICS Singh, V., Cheng, C., Weinacht, T., Matsika, S. 2022; 24 (35): 20701-20708


    Combined theoretical and experimental work examines the dynamics of dication formaldehyde produced by strong field ionization. Trajectory surface hopping dynamics on the first several singlet electronic states of the formaldehyde dication are used to examine the relaxation pathways and dissociation channels, while kinetic energy distributions after strong field ionization of formaldehyde and deuterated formaldehyde are used to confirm the theoretical predictions. We find that the first excited state of the formaldehyde dication is stable, neither decays to the ground state nor dissociates, even though the ground state and higher lying states are directly dissociative. The stability of the first excited state is explained by its symmetry which does not allow for radiative or nonradiative transitions to the ground state and by large barriers to dissociate on the excited state surface.

    View details for DOI 10.1039/d2cp02604j

    View details for Web of Science ID 000830569200001

    View details for PubMedID 35894510

  • 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)
  • 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)
  • 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)