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 M05: Femtochemistry |
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Chair: Rebecca Boll, European XFEL Room: 205 |
Wednesday, June 7, 2023 2:00PM - 2:12PM |
M05.00001: X-ray induced electron and ion fragmentation dynamics in iodine monobromine Phay J Ho, Robert W Dunford, Adam E Fouda, Gilles Doumy, Donald A Walko, Dipanwita Ray, Stefan Lehmann, Xuechen Zheng, Lan Cheng, Linda Young, Stephen Southworth Characterization of the inner-shell decay processes in molecules containing heavy elements is key to understanding x-ray damage of molecules and materials and for medical applications with Auger-electron-emitting radionuclides. The 1s hole states of heavy atoms can be produced by absorption of tunable x-rays and the resulting vacancy decays characterized by recording emitted photons, electrons, and ions. The 1s hole states in heavy elements have large x-ray fluorescence yields that transfer the hole to intermediate electron shells that then decay by sequential Auger-electron transitions that increase the ion's charge state until the final state is reached. In molecules the charge is spread across the atomic sites, resulting in dissociation to energetic atomic ions. We have used x-ray/ion coincidence spectroscopy to measure charge states and energies of I and Br atomic ions following 1s ionization at the I and Br K-edges of iodine monobromine (IBr). We present the charge states and kinetic energies of the two correlated fragment ions associated with core-excited states produced during the various steps of the cascades. We develop a computational model that combines Monte-Carlo/Molecular Dynamics simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments. Our theory shows good agreement with the measured ion data as well as the exiting electron data. Our calculation also predicts that molecules can undergo bond length contraction in femtosecond timescale before dissociation. Future pump-probe studies can enable imaging of the x-ray induced ultrafast electron and dissociation dynamics in molecules. |
Wednesday, June 7, 2023 2:12PM - 2:24PM |
M05.00002: Dissociation branching ratios of electronically excited OD+ transients W. Iskandar, T. N Rescigno, Z. L Streeter, C. W McCurdy, R. R Lucchese, K. A Larsen, D. S Slaughter, Th. Weber, A. E Orel, B. Griffin, J. B Williams, T. Severt, B. Jochim, Itzik Ben-Itzhak We investigate electronically excited OD+ transients formed in sequential fragmentation of heavy water dications following double ionization by a single photon. The dissociation of these excited OD+ states, driven by spin-orbit coupling and charge transfer, leads to either D+ + O(3P) or D(2S) + O+(4S) final products. We show that the branching ratio of these dissociation channels is a direct measure of the key transition probabilities, specifically A3Π → X3Σ− → B3Σ−, and measure it as a function of energy above the dissociation limit for two specific states of OD+. |
Wednesday, June 7, 2023 2:24PM - 2:36PM |
M05.00003: Electric field-resolved nonlinear spectroscopy of aligned molecules Siddhant Pandey, Francis F Walz, Eric H Liu, Liang Tan, Varun S Makhija, Niranjan Shivaram We report the measurement of third-order nonlinear electronic response from impulsively aligned molecules. A moderately strong femtosecond near-infrared laser pulse is used to impulsively align gas-phase molecules, and a set of weaker femtosecond pulses subsequently probe the nonlinear response using degenerate four-wave mixing (DFWM). We show that by measuring both the amplitude and phase of the emitted electric field, additional information about the excited rotational wavepacket can be accessed. By comparing these measurements from two linear molecules, nitrogen and carbon dioxide, we show that our measurement contains information about the electronic symmetries of the molecules, beyond their rotational response which is generally similar for all linear molecules. The emitted electric field is measured using an interferometry technique for measuring ultraweak femtosecond pulses, called TADPOLE, which is further enhanced using a lock-in detection-based spectrometer. We then discuss similar measurements on electronically excited molecules to study their ultrafast dynamics after VUV excitation. |
Wednesday, June 7, 2023 2:36PM - 2:48PM |
M05.00004: Intersystem crossing dynamics in thiobases studied by time-resolved photoelectron spectroscopy: tautomer aspects Susanne Ullrich, Bijay Duwal Sulfur substitution of an oxygen atom in the canonical nucleobases deactivates ultrafast internal conversion pathways back to the ground state and instead triplet states are populated with high quantum yields. To date gas-phase studies into the photophysics of thiobases have primarily focused on thiouracils and revealed a central role of the thiocarbonyl group in mediating the electronic relaxation dynamics. Extending this work to thiocytosine, which exists purely in its thiol form in a gas-phase environment, we investigate the photophysics of a thiobase from a different tautomer perspective. |
Wednesday, June 7, 2023 2:48PM - 3:00PM |
M05.00005: Excited State Dynamics of o-Nitrophenol Studied with UV-Pump, VUV-Probe Time Resolved Photoelectron and Photoion Spectroscopy Samuel A McClung, Dakshitha Abeygunewardane, Spiridoula C Matsika, Thomas Weinacht Time resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255 nm (4.9 eV) and probed with VUV pulses at 153 nm (8.1 eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump-probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation, through a process of internal conversion and intersystem crossing. |
Wednesday, June 7, 2023 3:00PM - 3:12PM |
M05.00006: Mass-selected Ion-molecule Cluster Beam Apparatus for Ultrafast Photofragmentation Studies: CF3I·I- as an example Xiaojun Wang, Mahmudul Hasan, Yibo Wang, Lin Fan, Daniel S Slaughter, Martin Centurion We describe the design and implementation of an apparatus to investigate the charger transfer process and the resulting fragments after photoexcitation of the ion-molecule cluster. The apparatus includes the generation of the ion-molecule clusters in source chamber, separation and selection of the parent ion-molecule cluster in time of flight (TOF) chamber and photofragments detection after excitation of ion-molecule cluster in detection chamber. In our apparatus, the mass filter is employed to select the parent anion cluster which ensures only parent anion cluster exists to be photoexcited in the detection chamber. In addition, the linear-plus-quadratic reflectron (LPQR) is used to resolve the anion photofragments with the focus of the reflectron for a certain energy unchanged while the potentials applied to the reflectron varying. The performance of the apparatus is shown in experiments of ion-molecule cluster CF3I· I- photoexcited by 266 nm UV beam. Anion fragments I- and CF3I- are detected and identified with neutral fragments collected together. The results are compared with existed calculation and photoelectron spectrum of CF3I· I-. Detection of CF3I- directly verifies that the charge transfer inside CF3I·I- cluster happens when it is excited by 4.66 eV photons and implies the existence of charge-transfer excited state of CF3I·I-. |
Wednesday, June 7, 2023 3:12PM - 3:24PM |
M05.00007: Pulse Shape Spectroscopy of Strong Field Molecular Ionization Gonenc Mogol, Brian M Kaufman, Chuan Cheng, Thomas Weinacht We make use of pulse shape spectroscopy and coincidence velocity map imaging to study strong field molecular ionization. We carry out momentum resolved measurements of fragment ions in coincidence and covariance arising from the strong field light-molecule interaction. The measurements are made as a function of pulse shape (with pulses as short as 6 fs), making use of several pulse shape parameterizations that highlight different molecular dynamics, including dissociation, isomerization and concerted elimination. Our velocity map imaging system can be switched to collect both ions and electrons in coincidence/covariance, giving us the ability to explore both structural changes and electronic dynamics driven by the strong laser fields. |
Wednesday, June 7, 2023 3:24PM - 3:36PM |
M05.00008: Breakup of OD+ molecular ions in a strong laser field* Chandan Bagdia, Naoki Iwamoto, Kevin D Carnes, ITZHAK BENITZHAK We investigate the dissociation and ionization processes of OD+ molecular ions in intense ultrashort laser pulses using coincidence three-dimensional momentum imaging of a fast (keV) ion beam. One of the main goals is to investigate the dissociation dynamics near threshold by measuring the competition (branching ratio) between the lowest two dissociation channels O+(4S) + D(1S) and O(3P) + D+. The intensity and the wavelength dependence of these channels is studied, by measuring the kinetic energy release and angular distributions, to unravel the dissociation pathways. Dissociative ionization of OD+, i.e., OD+ → O+ + D+ + e- , will also be discussed. |
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