I am an associate staff scientist in the Laser Science Department of the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. I work on developing tunable few-cycle UV light sources using soliton dynamics and resonant dispersive wave emission in hollow core fibers. I am involved in photoinjector research for LCLS-II, focusing on temporal x-ray pulse shaping to support the extension of attosecond capabilities to high repetition rate. I also work on developing plasma based attosecond XUV light sources at FACET-II.

Education & Certifications

  • BS, University of Oregon (2014)
  • PhD, University of California, Berkeley (2020)

All Publications

  • Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science (New York, N.Y.) Li, S., Lu, L., Bhattacharyya, S., Pearce, C., Li, K., Nienhuis, E. T., Doumy, G., Schaller, R. D., Moeller, S., Lin, M. F., Dakovski, G., Hoffman, D. J., Garratt, D., Larsen, K. A., Koralek, J. D., Hampton, C. Y., Cesar, D., Duris, J., Zhang, Z., Sudar, N., Cryan, J. P., Marinelli, A., Li, X., Inhester, L., Santra, R., Young, L. 2024: eadn6059


    Attosecond-pump/attosecond-probe experiments have long been sought as the most straightforward method to observe electron dynamics in real time. Although numerous successes have been achieved with overlapped near infrared femtosecond and extreme ultraviolet attosecond pulses combined with theory, true attosecond-pump/attosecond-probe experiments have been limited. We used a synchronized attosecond x-ray pulse pair from an x-ray free electron laser to study the electronic response to valence ionization in liquid water via all x-ray attosecond transient absorption spectroscopy (AX-ATAS). Our analysis showed that the AX-ATAS response is confined to the subfemtosecond timescale, eliminating any hydrogen atom motion and demonstrating experimentally that the 1b1 splitting in the x-ray emission spectrum is related to dynamics and is not evidence for two structural motifs in ambient liquid water.

    View details for DOI 10.1126/science.adn6059

    View details for PubMedID 38359104

  • Compact single-shot soft X-ray photon spectrometer for free-electron laser diagnostics OPTICS EXPRESS Larsen, K. A., Borne, K., Obaid, R., Kamalov, A., Liu, Y., Cheng, X., James, J., Driver, T., Li, K., Liu, Y., Sakdinawat, A., David, C., Wolf, T. A., Cryan, J. P., Walter, P., Lin, M. 2023; 31 (22): 35822-35834


    The photon spectrum from free-electron laser (FEL) light sources offers valuable information in time-resolved experiments and machine optimization in the spectral and temporal domains. We have developed a compact single-shot photon spectrometer to diagnose soft X-ray spectra. The spectrometer consists of an array of off-axis Fresnel zone plates (FZP) that act as transmission-imaging gratings, a Ce:YAG scintillator, and a microscope objective to image the scintillation target onto a two-dimensional imaging detector. This spectrometer operates in segmented energy ranges which covers tens of electronvolts for each absorption edge associated with several atomic constituents: carbon, nitrogen, oxygen, and neon. The spectrometer's performance is demonstrated at a repetition rate of 120 Hz, but our detection scheme can be easily extended to 200 kHz spectral collection by employing a fast complementary metal oxide semiconductor (CMOS) line-scan camera to detect the light from the scintillator. This compact photon spectrometer provides an opportunity for monitoring the spectrum downstream of an endstation in a limited space environment with sub-electronvolt energy resolution.

    View details for DOI 10.1364/OE.502105

    View details for Web of Science ID 001106418000001

    View details for PubMedID 38017746

  • Efficiency of charge transfer in changing the dissociation dynamics of OD<SUP>+</SUP> transients formed after the photo-fragmentation of D<sub>2</sub>O JOURNAL OF CHEMICAL PHYSICS Iskandar, W., Rescigno, T. N., Orel, A. E., Severt, T., Larsen, K. A., Streeter, Z. L., Jochim, B., Griffin, B., Call, D., Davis, V., Mccurdy, C. W., Lucchese, R. R., Williams, J. B., Ben-Itzhak, I., Slaughter, D. S., Weber, T. 2023; 159 (9)


    We present an investigation of the relaxation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV. We focus on the very rare D+ + O+ + D reaction channel in which the sequential fragmentation mechanisms were found to dominate the dynamics. Aided by theory, the state-selective formation and breakup of the transient OD+(a1Δ, b1Σ+) is traced, and the most likely dissociation path-OD+: a1Δ or b1Σ+ → A 3Π → X 3Σ- → B 3Σ--involving a combination of spin-orbit and non-adiabatic charge transfer transitions is determined. The multi-step transition probability of this complex transition sequence in the intermediate fragment ion is directly evaluated as a function of the energy of the transient OD+ above its lowest dissociation limit from the measured ratio of the D+ + O+ + D and competing D+ + D+ + O sequential fragmentation channels, which are measured simultaneously. Our coupled-channel time-dependent dynamics calculations reproduce the general trends of these multi-state relative transition rates toward the three-body fragmentation channels.

    View details for DOI 10.1063/5.0159300

    View details for Web of Science ID 001063483100004

    View details for PubMedID 37668253

  • Strongly coupled intermediate electronic states in one-color two-photon single valence ionization of O2. The Journal of chemical physics Larsen, K. A., Bello, R. Y., Lucchese, R. R., McCurdy, C. W., Slaughter, D. S., Weber, T. 2023; 158 (2): 024303


    We present an experimental and theoretical energy- and angle-resolved investigation on the non-dissociative photoionization dynamics of near-resonant, one-color, two-photon, single valence ionization of neutral O2 molecules. Using 9.3eV femtosecond pulses produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and O2 + cations produced from one-color, two-photon ionization in coincidence. The measured and calculated photoelectron angular distributions show agreement, which indicates that a superposition of two intermediate electronic states is dominantly involved and that wavepacket motion on those near-resonantly populated intermediate states does not play a significant role in the measured two-photon ionization dynamics. Here, we find greater utility in the diabatic representation compared to the adiabatic representation, where invoking a single valence-character diabat is sufficient to describe the underlying two-photon ionization mechanism.

    View details for DOI 10.1063/5.0128846

    View details for PubMedID 36641397

  • Step-by-step state-selective tracking of fragmentation dynamics of water dications by momentum imaging. Nature communications Severt, T., Streeter, Z. L., Iskandar, W., Larsen, K. A., Gatton, A., Trabert, D., Jochim, B., Griffin, B., Champenois, E. G., Brister, M. M., Reedy, D., Call, D., Strom, R., Landers, A. L., Dorner, R., Williams, J. B., Slaughter, D. S., Lucchese, R. R., Weber, T., McCurdy, C. W., Ben-Itzhak, I. 2022; 13 (1): 5146


    The double photoionization of a molecule by one photon ejects two electrons and typically creates an unstable dication. Observing the subsequent fragmentation products in coincidence can reveal a surprisingly detailed picture of the dynamics. Determining the time evolution and quantum mechanical states involved leads to deeper understanding of molecular dynamics. Here in a combined experimental and theoretical study, we unambiguously separate the sequential breakup via D++OD+ intermediates, from other processes leading to the same D++D++O final products of double ionization of water by a single photon. Moreover, we experimentally identify, separate, and follow step by step, two pathways involving the b1Sigma+ and a 1Delta electronic states of the intermediate OD+ ion. Our classical trajectory calculations on the relevant potential energy surfaces reproduce well the measured data and, combined with the experiment, enable the determination of the internal energy and angular momentum distribution of the OD+ intermediate.

    View details for DOI 10.1038/s41467-022-32836-6

    View details for PubMedID 36050308

  • Mechanisms and dynamics of the NH2+ + H+ and NH+ + H+ + H fragmentation channels upon single-photon double ionization of NH3 JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Larsen, K. A., Rescigno, T. N., Streeter, Z. L., Iskandar, W., Heck, S., Gatton, A., Champenois, E. G., Severt, T., Strom, R., Jochim, B., Reedy, D., Call, D., Moshammer, R., Dorner, R., Landers, A. L., Williams, J. B., McCurdy, C., Lucchese, R. R., Ben-Itzhak, I., Slaughter, D. S., Weber, T. 2020; 53 (24)
  • Role of dipole-forbidden autoionizing resonances in nonresonant one-color two-photon single ionization of N-2 PHYSICAL REVIEW A Larsen, K. A., Bello, R. Y., Lucchese, R. R., Rescigno, T. N., McCurdy, C., Slaughter, D. S., Weber, T. 2020; 102 (6)
  • Photoelectron and fragmentation dynamics of the H+ PHYSICAL REVIEW RESEARCH Larsen, K. A., Rescigno, T. N., Severt, T., Streeter, Z. L., Iskandar, W., Heck, S., Gatton, A., Champenois, E. G., Strom, R., Jochim, B., Reedy, D., Call, D., Moshammer, R., Doerner, R., Landers, A. L., Williams, J. B., McCurdy, C., Lucchese, R. R., Ben-Itzhak, I., Slaughter, D. S., Weber, T. 2020; 2 (4)
  • Distinguishing resonance symmetries with energy-resolved photoion angular distributions from ion-pair formation in O-2 following two-photon absorption of a 9.3 eV femtosecond pulse JOURNAL OF CHEMICAL PHYSICS Larsen, K. A., Lucchese, R. R., Slaughter, D. S., Weber, T. 2020; 153 (2): 021103


    We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O2 following resonant two-photon absorption of a 9.3 eV femtosecond pulse, where the resulting O+ ions are detected using 3D momentum imaging. Ion-pair formation states of Σg-3 and 3Πg symmetry are accessed through predissociation of optically dark continuum Rydberg states converging to the B Σg-2 ionic state, which are resonantly populated via a mixture of both parallel-parallel and parallel-perpendicular two-photon transitions. This mixture is evident in the angular distribution of the dissociation relative to the light polarization and varies with the kinetic energy release (KER) of the fragmenting ion pair. The KER-dependent photoion angular distribution reveals the underlying two-photon absorption dynamics involved in the ion-pair production mechanism and indicates the existence of two nearly degenerate continuum resonances possessing different symmetries, which can decay by coupling to ion-pair states of the same total symmetry through internal conversion.

    View details for DOI 10.1063/5.0013485

    View details for Web of Science ID 000553097500001

    View details for PubMedID 32668943

  • Angle-resolved nonresonant two-photon single ionization of argon using 9.3-eV photons produced via high-order harmonic generation PHYSICAL REVIEW A Larsen, K. A., Slaughter, D. S., Weber, T. 2020; 101 (6)
  • Symmetry breaking in the body-fixed electron emission pattern due to electron-retroaction in the photodissociation of H-2(+) and D-2(+) close to threshold PHYSICAL REVIEW RESEARCH Heck, S., Gatton, A., Larsen, K. A., Iskandar, W., Champenois, E. G., Strom, R., Landers, A., Reedy, D., Dailey, C., Williams, J. B., Severt, T., Jochim, B., Ben-Itzhak, I., Moshammer, R., Doerner, R., Slaughter, D. S., Weber, T. 2019; 1 (3)
  • Tracing intermolecular Coulombic decay of carbon-dioxide dimers and oxygen dimers after valence photoionization PHYSICAL REVIEW A Iskandar, W., Gatton, A. S., Gaire, B., Sturm, F. P., Larsen, K. A., Champenois, E. G., Shivaram, N., Moradmand, A., Williams, J. B., Berry, B., Severt, T., Ben-Itzhak, I., Metz, D., Sann, H., Weller, M., Schoeffler, M., Jahnke, T., Doerner, R., Slaughter, D., Weber, T. 2019; 99 (4)
  • Ultrafast photodissociation dynamics and nonadiabatic coupling between excited electronic states of methanol probed by time-resolved photoelectron spectroscopy JOURNAL OF CHEMICAL PHYSICS Champenois, E. G., Greenman, L., Shivaram, N., Cryan, J. P., Larsen, K. A., Rescigno, T. N., McCurdy, C., Belkacem, A., Slaughter, D. S. 2019; 150 (11)

    View details for DOI 10.1063/1.5079549

    View details for Web of Science ID 000462014500017

  • Ultrafast photodissociation dynamics and nonadiabatic coupling between excited electronic states of methanol probed by time-resolved photoelectron spectroscopy. The Journal of chemical physics Champenois, E. G., Greenman, L., Shivaram, N., Cryan, J. P., Larsen, K. A., Rescigno, T. N., McCurdy, C. W., Belkacem, A., Slaughter, D. S. 2019; 150 (11): 114301


    The electronic and nuclear dynamics in methanol, following 156 nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260 nm photoionization probe pulse to produce photoelectrons that are analyzed by velocity map imaging. The yields of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20 fs pump and probe pulse durations. When combined with ab initio calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond time scale. Within 15 fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 21A (S2) electronic state, through a conical intersection with the 11A' (S3) state, which reveals paths to photodissociation following C-O stretch and C-O-H angle opening.

    View details for PubMedID 30902015

  • Resonance signatures in the body-frame valence photoionization of CF4 PHYSICAL CHEMISTRY CHEMICAL PHYSICS Larsen, K. A., Trevisan, C. S., Lucchese, R. R., Heck, S., Iskandar, W., Champenois, E., Gatton, A., Moshammer, R., Strom, R., Severt, T., Jochim, B., Reedy, D., Weller, M., Landers, A. L., Williams, J. B., Ben-Itzhak, I., Doerner, R., Slaughter, D., McCurdy, C. W., Weber, T., Rescigno, T. N. 2018; 20 (32): 21075-21084


    We present a combined experimental and theoretical investigation of the electron dynamics and body-frame angular dependence of valence photo-single ionization of CF4 and subsequent dissociation into CF3+ and F. Ionization from a valence t2 orbital shows overlapping shape resonances close to threshold that couple to the same total symmetry, leading to striking changes in the photoelectron angular distributions when viewed in the body-frame.

    View details for DOI 10.1039/c8cp03637c

    View details for Web of Science ID 000447367900033

    View details for PubMedID 30074041

  • VUV and XUV reflectance of optically coated mirrors for selection of high harmonics OPTICS EXPRESS Larsen, K. A., Cryan, J. P., Shivaram, N., Champenois, E. G., Wright, T. W., Ray, D., Kostko, O., Ahmed, M., Belkacem, A., Slaughter, D. S. 2016; 24 (16): 18209-18216


    We report the reflectance, ~1° from normal incidence, of six different mirrors as a function of photon energy, using monochromatic vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation with energies between 7.5 eV and 24.5 eV. The mirrors examined included both single and multilayer optical coatings, as well as an uncoated substrate. We discuss the performance of each mirror, paying particular attention to the potential application of suppression and selection of high-order harmonics of a Ti:sapphire laser.

    View details for DOI 10.1364/OE.24.018209

    View details for Web of Science ID 000384716000061

    View details for PubMedID 27505785