Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C02: Focus Session: Advances in AMO science with FELsFocus Live
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Chair: Li Fang, University of Central Florida |
Tuesday, June 1, 2021 10:30AM - 10:42AM Live |
C02.00001: Probing molecular coherences by correlation Raman signals with stochastic FEL pulses Stefano M Cavaletto, Daniel Keefer, Shaul Mukamel Hard-x-ray free-electron lasers (FELs) relying on the self-amplified spontaneous emission (SASE) mechanism generate stochastic x-ray pulses lacking phase control. This has represented a major bottleneck, since most time-resolved multidimensional nonlinear x-ray spectroscopy schemes are based on sequences of coherent, phase-controlled pulses. We show that suitable correlation signals averaged over independent realizations of stochastic FEL pulses can retrieve the same joint temporal and spectral resolutions of signals with phase-controlled pulses [1]. This is demonstrated for TRUECARS, an off-resonant x-ray Raman technique that monitors conical-intersection dynamics by directly accessing molecular coherences in a background-free manner. We apply our protocol to the RNA base uracil, showing that correlation signals at FELs can offer the same physical insight as TRUECARS with phase-controlled pulses. |
Tuesday, June 1, 2021 10:42AM - 11:12AM Live |
C02.00002: Ultrafast Resonant Interatomic Coulombic Decay Induced by Quantum Fluid Dynamics Invited Speaker: Aaron C LaForge Interatomic processes play a crucial role in weakly-bound complexes exposed to ionizing radiation, therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 femtoseconds, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory andab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles which drastically accelerates ICD. Similar processes are likely to occur in other fluids such as water, where the formation of nano-bubbles plays a role in the solvation of electrons and the unfolding and aggregation of proteins. Our results demonstrate the importance of bubble dynamics in interatomic decay processes and open up a new approach for understanding the basic processes causing radiation damage in biological systems. |
Tuesday, June 1, 2021 11:12AM - 11:24AM Live |
C02.00003: High-resolution and Elemental Contrast Fluorescence Imaging with X-ray Free-electron Laser Pulses Phay J Ho, Christopher Knight, Linda Young Coherent diffractive imaging (CDI) with x-ray free-electron laser (XFEL) pulses holds the promise to probe structure and follow the dynamics of non-periodic entities with atomic resolution. However, this approach remains challenging due to sample damage during the pulse. Based on the Hanbury Brown and Twiss effect [1], Classen and coworkers proposed to used fluorescence intensity correlation (FIC) to image three-dimensional arrangement of heavy atoms with XFEL [2]. Our previous work shows that fluorescence dynamics in the nonlinear x-ray regime differs from that in weak x-ray fields and enables higher fluorescence yield and narrower emission time window [3]. Building on this, we theoretically investigate the FICs of Ar clusters and Mo-doped iron oxide nanoparticles exposed to intense, few-fs and sub-fs XFEL pulses for high-resolution and elemental contrast imaging. We present the FIC of Kα and KαH emission in Ar clusters and discuss the impact of sample damage on retrieving high-resolution structural information. Furthermore, we show that the FICs computed from the fluorescence of Mo atoms in Mo-doped iron oxide nanoparticles can be used to image dopant distributions, which are directly connected to the catalytic functionality of these nanoparticles. |
Tuesday, June 1, 2021 11:24AM - 11:36AM Live |
C02.00004: Time-resolved intramolecular photoelectron scattering Luca Argenti, Bejan Ghomashi, Nicolas Douguet When a molecule is ionized by an x-ray photon from a localized core orbital, the emerging photoelectron collides with nearby nuclei giving rise to the well-known interference pattern of EXAFS spectroscopy. The vibrationally-resolved spectrum of core-ionized molecules bears the signature of the energy transferred to the nuclei by the intramolecular scattering process [1,2]. New pulsed x-ray sources, such as XFELS, make it now possible to study this phenomenon resolved in time. In this work, we simulate the real-time dynamics of the CO molecule following the C-1s ionization induced by a coherent soft-x-ray pulse using a simplified 1D analytical model that captures the essential aspects of the process. The vibrationally-resolved photoemission delay bears the signature of electron localization at birth as well as of its resonant confinement by the two nuclei. For short pulses, the ion is created in a partially coherent vibrational state: either in compression or in expansion, depending on the pulse central energy with respect to the nearest confinment resonance. The deviation of the nuclear Wigner distribution maximum from the sudden photoemission expectation value can be interpreted as a vibrational delay due to intramolecular scattering. |
Tuesday, June 1, 2021 11:36AM - 11:48AM Live |
C02.00005: Charging and Ion Ejection Dynamics of Large Helium Nanodroplets Exposed to Intense Femtosecond Soft X-Ray Pulses Catherine A Saladrigas, Alexandra J Feinberg, Michael Ziemkiewicz, Camila Bacellar, Daniel M Neumark, Christoph Bostedt, Andrey F Vilesov, Oliver Gessner Ion ejection from helium nanodroplets exposed to intense femtosecond soft x-ray pulses is studied via single-pulse ion time-of-flight (TOF) spectroscopy in coincidence with small-angle x-ray scattering. Scattering images encode droplet size and absolute photon flux incident on each droplet. Ion TOF spectra report on maximum ion kinetic energies (KE) of He+n=1-4 fragments. Measurements span HeN droplet sizes from N ≈ 107 to 1010 and droplet charges from ~9×10-5 to ~4×10-3 e/atom, spanning Coulomb explosion conditions to substantial frustration of outer ionization. The combination of absolute x-ray intensities, cluster sizes, and ion KE on an event-by-event basis reveals the correlations between ionization conditions and ejection dynamics of the fragments. The observed maximum He+ KE is compared to estimates of the maximum based on the cluster Coulomb potential resulting from unscreened ions and considering the impact of ion-atom collisions for ions emerging from within the cluster. The maximum He+ KE is found to be governed by Coulomb repulsion from unscreened cations across all expansion regimes. Findings are consistent with the emergence of a charged spherical shell around a quasi-neutral plasma core with increasing frustration. |
Tuesday, June 1, 2021 11:48AM - 12:00PM Live |
C02.00006: Probing core level ionization in solvated transition metal complexes using X-ray pump, X-ray probe absorption spectroscopy: Computations and proposed XFEL experiment Robert B Weakly, Chelsea Liekhus Schmaltz, Phay J Ho, Andrew Aquila, Robert W Schoenlein, Munira Khalil, Niranjan Govind Femtosecond X-ray pump X-ray probe experiments are currently possible at free electron lasers such as the Linac Coherent Light Source (LCLS), which opens new opportunities for studying systems in condensed and solvated phases. In order to make the most effective use of these experiments, it is necessary to determine which chemical properties an X-ray pump pulse will measure. We combine Monte-Carlo electron cascade calculations and excited-state time-dependent density functional theory calculations to first predict the initial state prepared by an X-ray pump and then determine the subsequent X-ray absorption spectra at the Fe K-edge in the solvated model transition metal complexes, K4FeII(CN)6 and K3FeIII(CN)6. Final states with 3p holes produce unique shifts from the Fe K-β energy as a function of 3p and 3d electron interactions. The initial hole in a t2g orbital of the FeIII complex leads to more highly ionized probed states, and alters the distribution of 3p transitions relative to the FeII complex. We find several key spectral features that report on the ligand-field splitting, solvent-solute interactions, and 3p -3d electron interactions. Finally, we show how these features could be measured in an experiment. |
Tuesday, June 1, 2021 12:00PM - 12:30PM Live |
C02.00007: Photon-recoil imaging: A new route to study nonlinear x-ray physics Invited Speaker: Ulli Eichmann With the advent of XFEL radiation it has been a central goal to merge the merits of x-ray spectroscopy, especially its ability to probe atom-specific local electron dynamics, with nonlinear techniques developed in the optical regime. Among the important nonlinear x-ray phenomena is the fundamental stimulated x-ray Raman scattering (SXRS) process near an inner-shell resonance, which is crucial for the development of any nonlinear x-ray method. Stimulated emission and stimulated Raman scattering have been observed in dense media by measuring the scattered x-ray radiation, and in molecules, where the final excited states have been detected via photoionization. In both cases, it is cumbersome to separate SXRS from spontaneous Raman processes. Moreover, for x-ray detection the stimulated light is inherently emitted in the forward direction coinciding with the driving x-ray laser beam, identification of stimulated Raman scattering using x-ray detection techniques is per se largely impeded. |
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