Bulletin of the American Physical Society
50th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 64, Number 4
Monday–Friday, May 27–31, 2019; Milwaukee, Wisconsin
Session H03: FOCUS: Science with Xray and free electron lasers |
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Chair: Daniel Rolles, Kansas State University Room: Wisconsin Center 101CD |
Wednesday, May 29, 2019 8:00AM - 8:30AM |
H03.00001: Probing ultrafast x-ray induced inner-shell processes. Invited Speaker: Gilles Doumy Short and intense pulses produced by x-ray free electron lasers (XFELs) initiate complex processes when they interact with matter through inner shell ionization, resonant excitation or even non-linear interaction. The ability to follow these processes in time is a continuing challenge, but it is of high importance to be able to understand quantitatively the basic mechanisms at play during any measurement using intense x-rays. In this talk, I will illustrate progress towards that goal using two separate approaches involving detection of direct photoelectrons and/or Auger electrons to track the inner-shell decay mechanism. The first approach takes advantage of the ability to produce pairs of x-ray pulses with a well-defined delay and different colors. By measuring the time-dependent chemical shifts, one can access the changing chemical environment due to electronic and nuclear rearrangement following inner shell interaction with a few-femtosecond resolution. The second approach exploits the electron energy modulation in the presence of a strong linearly polarized laser field. Using a self-referenced analysis, decay mechanisms in atoms and small molecules have been studied with sub-femtosecond resolution. Both approaches are poised to take advantage of the upcoming LCLS-2, with full wavelength tuning ability in two-pulse mode, high repetition rate and reliable attosecond pulse generation. [Preview Abstract] |
Wednesday, May 29, 2019 8:30AM - 9:00AM |
H03.00002: Attosecond X-ray Experiments at Free Electron Laser Facilities Invited Speaker: James Cryan This talk will discuss the potential of using x-ray free electron lasers (XFELs) for applications in attosecond science. I will show recent results on the production and measurement of sub-femtosecond soft x-ray pulses at the Linac Coherent Light Source (LCLS) XFEL facility. Our measurement technique exploits phase-dependent energy modulation of a photoelectron ionized in the presence of a strong laser field with circular polarization. Beyond pulse characterization, this attoclock method can be applied to study molecular photoionization dynamics in the time domain, including the ultrafast decay of core-excited states produced via core-shell ionization. We have also made use of the broad bandwidth of attosecond x-ray pulses to demonstrate coherent electronic population transfer via stimulated X-ray Raman scattering in small molecular systems. [Preview Abstract] |
Wednesday, May 29, 2019 9:00AM - 9:12AM |
H03.00003: Characterizing multiphoton excitation using time-resolved x-ray scattering Philip Bucksbaum, Matthew Ware, Noor Al-Sayyad, Adi Natan, J. Michael Glownia Molecular iodine was photoexcited by a strong 800 nm laser, driving Raman excitation of vibrational states on the ground state as well as multiphoton dissociation. The subsequent motion following photoexcitation is observed using time-resolved x-ray scattering (TRXS) and analyzed using the temporal Fourier transform of TRXS, called frequency-resolved x-ray scattering (FRXS). The FRXS signal identifies vibrations with a beat frequency of 40.3$+$-1.0 THz oscillating about an equilibrium position of 0.28$+$-0.01nm, which match the expected beat frequency and equilibrium position for Raman excited vibrations on the ground state. Molecular dissociation also has a distinct signature in FRXS, which may be used to identify both velocities and initial positions of the dissociation channels. [Preview Abstract] |
Wednesday, May 29, 2019 9:12AM - 9:24AM |
H03.00004: Discerning Molecular Dynamics using Time Resolved X-ray Scattering James Glownia, Matthew Ware, Noor Al-Sayyad, Jordan O'Neal, Philip Bucksbaum Time Resolved X-ray Scattering (TRXS) has emerged as an important tool to observe ultrafast time resolved molecular motion. Two of the large challenges associated with this technique are inverting the scattered image from scattering space to real space and to resolve dynamics that occur simultaneously on multiple electronic states. Here, we show a TRXS experiment performed with 520 nm photoexcited molecular iodine at the Linac Coherent Light Source.~We use the method of Frequency Resolved X-ray Scattering (FRXS) to extract and identify the sources of molecular motion.~With this technique, the analysis is performed in the x-ray scattering, Q, domain and a temporal Fourier transform is performed on the pump-probe delay dimension. The well-known bound B-electronic state vibrational wavepackets are observed in addition to single photon and multi-photon induced molecular dissociation.~This analysis resolves the equilibrium position, amplitude of motion, and vibrational period information for the bound states, and the initial position and velocities of dissociation for the unbound states, allowing for the precise determination of the origins of coherent motion in the molecule. [Preview Abstract] |
Wednesday, May 29, 2019 9:24AM - 9:36AM |
H03.00005: Observing K-shell pre-edge resonances in Kr, Xe, and XeF$_2$ Stephen Southworth, Robert Dunford, Elliot Kanter, Gilles Doumy, Phay Ho, Anne Marie March, Linda Young, Lan Cheng Rydberg transitions below the K-shell ionization thresholds of Kr and Xe are obscured in x-ray absorption spectra due to core-hole lifetime broadening. However, excitation of these resonances produce spectator electrons as the core holes decay by radiative and radiationless transitions. We report on ion charge-state distributions of Kr and Xe in coincidence with x-ray fluorescence as the absorbed x-ray energy is scanned through pre-edge resonances and ionization thresholds. Shifts of ion yields from higher to lower charge states are observed in the resonance regions that we attribute to spectator electrons. In XeF$_2$, the F ligands modify the valence electron charge distribution, resulting in a chemical shift of the Xe 1s ionization energy. Strong excitation of the lowest unoccupied molecular orbital (LUMO) is also observed and modifies the ion spectra due to molecular alignment. The measured LUMO and ionization energies are compared with relativistic coupled-cluster calculations that treat relativistic and electron correlation interactions on the same footing. [Preview Abstract] |
Wednesday, May 29, 2019 9:36AM - 9:48AM |
H03.00006: Time Resolved X-ray Pump/X-ray Photoelectron Spectroscopy Probe Andre Al Haddad, A Picon, G Doumy, T Gorkhover, S Pratt, A Luttman, A Marinelli, S Moeller, D Ratner, P Walter, L Young, S Southworth, C Bostedt Recently, X-ray Free Electron Lasers proved the ability to produce two intense femtosecond x-ray pulses with controlled time delay and color. Combining these unique capabilities with X-ray photoelectron spectroscopy (XPS) probing, we can probe electronic and nuclear dynamics at a specific atomic site with femtosecond resolution. In our initial study we used carbon monoxide, i.e., a small heteronuclear molecule that can be well described with theoretical models. The pump pulse excites a core-electron resonantly at the oxygen K-edge and then we probe the molecular response with photoemission from the carbon core levels. The data gives a glimpse of the electronic and nuclear relaxation pathways proceeding on time scales \textless 40 fs upon x-ray excitation on the oxygen site.~Such experiments lay the ground for future time resolved core level spectroscopy at FELs that can be combined with more complex pulse modes and non-linear x-ray excitation modes. [Preview Abstract] |
Wednesday, May 29, 2019 9:48AM - 10:00AM |
H03.00007: Time-resolved imaging of XUV-induced fragmentation dynamics in halomethanes B. Kaderiya, X. Li, D. Rolles, A. Rudenko, S. Meister, G. Schmid, S. Augustin, K. Schnorr, Y. Liu, H. Lindenblatt, F. Trost, R. Moshammer, M. Braune Halomethanes are extensive used as model systems for understanding laser-driven and laser-controlled chemistry in the optical and ultraviolet domain. Here, we extend these studies to photo-fragmentation dynamics of CH$_{\mathrm{2}}$ICl and CH$_{\mathrm{3}}$I triggered by femtosecond extreme ultraviolet (XUV) pulses at 24 eV photon energy. We employ a combination of an XUV-pump - XUV-probe arrangement and a dedicated coincident momentum imaging spectrometer installed at the FLASH-II free-electron laser facility. By measuring yields, kinetic energies and angular distribution of ionic fragments resulting from the breakup of doubly- and triply-charged final states as a function of time delay between two XUV pulses, we disentangle different dissociative ionization pathways triggered by the absorption of 24 eV XUV photon(s). For both molecular systems, the results are compared to the outcome of a similar measurement of the molecular fragmentation dynamics driven by intense near-infrared laser pulses with comparable pulse duration. [Preview Abstract] |
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