Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session X05: Time-Resolved Studies with Advanced Light Facilities |
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Chair: Ruaridh Forbes, SLAC National Accelerator Laboratory Room: 205 |
Friday, June 9, 2023 8:00AM - 8:30AM |
X05.00001: Ultrafast Diffraction and Spectroscopy Studies of Gas-Phase Photochemistry Invited Speaker: Daniel Rolles The continuing progress of ultrafast sources such as X-ray free-electron lasers, high-repetition-rate near-infrared lasers, and Mega-electronvolt ultrafast electron diffraction facilities enable studies of electronic and structural dynamics in gas-phase molecules with unprecedented spatial and temporal resolution. I will present recent examples of experiments utilizing a variety of different spectroscopic techniques such as time-resolved photoelectron spectroscopy [1] and Coulomb explosion imaging [2,3,4]. The results are compared to experiments performed with other ultrafast techniques such as ultrafast electron and X-ray diffraction to highlight strengths and limitations of each technique. |
Friday, June 9, 2023 8:30AM - 8:42AM |
X05.00002: Resonant Double-Core Excitation in N2 Gilles Doumy, Eetu Pelimanni, Adam E Fouda, Phay J Ho, Linda Young, James P Cryan, Dimitris Koulentianos, Iyas Ismail, Maria Novella N Piancastelli, Marc Simon, Tommaso Mazza, Michael Meyer, Ralph Püttner We report on the experimental observation of a molecular resonant double-core excitation process in N2 performed at the SQS endstation of the European XFEL, where a single few-femtosecond soft X-ray pulse sequentially excites two core-level electrons to the same unoccupied molecular orbital. The production of these neutral two-site K−1K−1V2 double core hole states in the molecule are accessible due to few-eV bandwidth of the pulses. Scanning the central photon energy in the vicinity of the 1sσ→ 1π*g resonance in the ground state molecule and recording the electron de-excitation spectrum, the unambiguous signature of the decay of the doubly-excited molecules is observed through the appearance of new emission lines, including the so-called double-participator decay where both excited electrons take part in the de-excitation process. A full discussion of the measured spectra as well as a comparison with calculations is presented. |
Friday, June 9, 2023 8:42AM - 8:54AM |
X05.00003: Impulsive X-ray Raman in liquid water Oliver G Alexander, Elijah S Egun, Douglas Garratt, Laura Rego, James P Cryan, Taran Driver, Jonathan P Marangos We investigate the use of X-ray Raman scattering to excite valence states of neutral atoms in micrometre thick liquid water targets using attosecond XFEL pulses tuned below the oxygen K-edge and with intensity up to 1017 Wcm-² . We measure transmitted X-ray spectra and observe nonlinear emission features approximately 10 eV below the edge. Our numerical calculations based on the polarisation response of molecular water and considering propagation (Maxwell’s equations) show that this emission is consistent with X-ray Raman sidebands. These calculations show that the valence excited states are populated by the X-ray pulse and the final state populations are affected by propagation effects, which may limit the thickness of condensed phase targets for pump-probe experiments. Excitation fractions are low (<0.1%) but increase at higher intensities, where impulsive X-ray Raman could be used to excite and measure valence wavepackets. This opens new avenues for the study of attosecond charge dynamics and the investigation of electron wavepackets for pump-probe experiments, which is typically restricted to cationic wavepackets created via strong-field or impulsive ionisation. Valence state populations are affected by propagation effects for several femtoseconds, in part due to reshaping of the X-ray pulses, which may limit the thickness of condensed matter targets. |
Friday, June 9, 2023 8:54AM - 9:06AM |
X05.00004: Synthetic data-driven machine learning for multi-dimensional spectroscopy of molecules Sajal K Giri, Ulf Saalmann, Jan M Rost In previous work [1, 2] we have trained deep neural networks using synthetic Hamilton matrices (SHM) with quasi-random matrix elements. The matrices represent atomic and molecular systems (which could exist but do not necessarily exist), dipole-coupled to short and noisy laser pulses. This allows us to generate millions of spectra solving the dynamics required for successful training. The trained network is capable of "purifying" noisy nonlinear photo-electron spectra as if they had been generated by a Fourier-limited pulse. |
Friday, June 9, 2023 9:06AM - 9:18AM |
X05.00005: Probing photochemical reactivity of aromatic carbonyls with time-resolved X-ray absorption spectroscopy at Oxygen K-edge Yusong Liu, Martin Graßl, Alice Green, Felix Allum, Kurtis D Borne, Martin Centurion, xinxin Cheng, Taran Driver, Ruaridh Forbes, James M Glownia, Erik Isele, Andrei Kamalov, Kirk Larsen, Xiang Li, Ming-Fu Lin, Razib Obaid, Adam M Summers, Jun Wang, Thomas Weinacht, James P Cryan, Thomas J Wolf We studied UV-induced ultrafast photochemistry of aromatic carbonyls using time-resolved soft X-ray absorption spectroscopy. We focused on the internal conversion and subsequently interplay between the singlet and triplet states leading to the photodissociation of one of the smallest aromatic carbonyls, Acetophenone (C6H5COCH3), via a Norrish Type-I reactions. Our experiment was carried out in LCLS soft X-ray beamline at the TMO experimental end-station, and we make use of the specific sensitivity (resonance transition between core to empty valence orbitals) of near edge X-ray absorption fine structure spectroscopy (NEXAFS) to the electronic character of the excited states (e.g., ππ* and nπ* states). Gas phase acetophenone was excited with a 266 nm pump pulse, followed by the probing of a soft X-ray pulse from the free-electron laser (FEL) around O K-edge (~530 eV). By scanning the X-ray photo energy across the pre-edge and edge region (~520 to 540 eV), we monitored the X-ray absorbance through the measure of Auger-Meitner electron yield together with the photon spectrum. From our measured time-resolved NEXAF spectra, we observed the pre-edge absorption features at early time (<1ps) which are attributed to the transition from the initial populated ππ* to the nπ* state. The subsequent slower features (>10ps) in the spectra indicate the dynamics to the triplet manifolds before dissociation. This work highlights the unique element and site specificity with utilizing X-ray spectroscopy on ultrafast photochemistry. |
Friday, June 9, 2023 9:18AM - 9:30AM |
X05.00006: Picosecond Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy at a Synchrotron Using a Fast Time-Tagging Approach Zachery Donnellan, Sahan Neelakanni Mudiyanselage, Lars Hoffmann, Friedrich Roth, Wolfgang Eberhardt, Oliver Gessner Time-resolved X-ray photoemission spectroscopy (TRXPS) is a powerful tool that provides quantitative information about transient electron and hole populations at surfaces and interfaces with unique site- and element-specificity. The technique uses a “pump” pulse to initiate electronic and/or nuclear dynamics, the evolution of which is monitored by X-ray “probe” pulses. Using synchrotrons for picosecond TRXPS experiments is commonly associated with the major challenge of a significant disparity between the repetition rates of laser pump and synchrotron probe pulses. One option to address this challenge is to suppress XPS signals from all X-ray pulses save for one at the same repetition rate as the pump pulse. However, this approach often leads to impractically long data acquisition times and significant sample damage issues as all X-ray pulses damage a sample, but only a small fraction contributes to the XPS signal. We present a different approach, implemented at the Advanced Light Source (ALS), that utilizes the combination of a hemispherical analyzer with a fast delay line detector to record the kinetic energy and emission time of every photoelectron in the 500 MHz X-ray multi-bunch pattern. The time-tagging approach allows the use of all X-ray pulses simultaneously, dramatically reducing data acquisition times and X-ray induced sample damage. The surface photovoltage effect in n-Si and electron transfer dynamics in Au nanoparticle sensitized TiO2 are used as model systems to demonstrate the capabilities and limitations of the technique. |
Friday, June 9, 2023 9:30AM - 9:42AM |
X05.00007: Viewing time-resolved X-ray scattering data in a maximally sparse basis Ian Gabalski, Malick Sere, Kyle Acheson, Felix Allum, Sebastien Boutet, Gopal Dixit, Ruaridh Forbes, James M Glownia, Nathan Goff, Kareem Hegazy, Andrew J Howard, Mengning Liang, Michael Minitti, Russell S Minns, Adi Natan, Nolan Peard, Weronika O Razmus, Roseanne J Sension, Matthew Ware, Peter M Weber, Nicholas Werby, Thomas J Wolf, Adam Kirrander, Philip H Bucksbaum Time-resolved X-ray scattering (TRXS) data on photoexcited molecules contains rich spatial and temporal information that can yield unique insights into ultrafast chemical dynamics. Recent advances in data analysis methodology have shown promise in reducing molecular motion to sparse data features, such as frequency-resolved X-ray scattering (FRXS) for temporal sparsity and natural scattering kernels (NSK) for spatial sparsity. However, these techniques fail to achieve sparsity along both axes simultaneously, posing a challenge for directly extracting molecular structure and dynamics. |
Friday, June 9, 2023 9:42AM - 9:54AM |
X05.00008: Influence of the shot-to-shot variation of sub-fs soft X-Ray free electron laser pulses on pump-probe measurements of attosecond charge migration in aromatic molecules Gilbert Grell, Piero Decleva, Alicia Palacios, Fernando Martín
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