Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 65, Number 4
Monday–Friday, June 1–5, 2020; Portland, Oregon
Session S04: FOCUS: Attosecond and High-Field AMO with XFELsFocus Live
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Chair: Taisia Gorkhover, SLAC Room: D137-138 |
Friday, June 5, 2020 8:00AM - 8:30AM Live |
S04.00001: Attosecond Science at the Linac Coherent Light Source Invited Speaker: Marinelli Agostino The motion of electrons in molecules and solids occurs on the sub-femtosecond timescale. Consequently, the study of ultrafast electronic phenomena with pump/probe experiments requires the generation of laser pulses shorter than 1 fs and of sufficient intensity to interact with their target with high probability. Since the first lasing of the Linac Coherent Light Source much progress has been made to improve the time resolution of X-ray free-electron laser (XFEL) experiments, reducing it by one order of magnitude over the initial 100 fs resolution. In my talk I will discuss the frontier of time-resolved pump/probe experiments with XFELs: the attosecond regime. I will present our recent experimental results on attosecond pulse generation$^{\mathrm{1\thinspace }}$as well as the use of attosecond pulses for non-linear X-ray spectroscopy, photoemission delay measurements and single-shot imaging. I will also discuss our ongoing efforts to perform the first X-ray attosecond pump/probe experiments at LCLS-II using two-color attosecond X-ray pulses$^{\mathrm{2}}$ and electron beam-based techniques. 1) Duris, J., Li, S., Driver, T. et al. Tunable isolated attosecond X-ray pulses with gigawatt peak power from a free-electron laser. Nat. Photonics 14, 30--36 (2020) 2) Zhang, Zhen, et al. "Double chirp-taper x-ray free-electron laser for attosecond pump-probe experiments."~Physical Review Accelerators and Beams~22.5 (2019): 050701. [Preview Abstract] |
Friday, June 5, 2020 8:30AM - 8:42AM Live |
S04.00002: Spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy Siqi Li, Joseph Duris, Thomas J. Lane, Agostino Marinelli, Daniel Ratner, Elio Champenois, James Cryan, Taran Driver, Oliver Alexander, Thomas Barillot, Douglas Garratt, Jon Marangos With the advent of an X-ray free-electron laser, scientists can probe femtosecond ultrafast molecular dynamics with atomic-site specificity. One well-established technique is transient absorption spectroscopy, where transient sample absorption is determined by scanning the central photon energy and recording the resultant photoproducts. When using sub-femtosecond pulses, this method is limited in spectral resolution due to the large energy bandwidth inherent to the short pulses. Here, we present a novel technique based on the principle of ghost imaging, and demonstrate sub-bandwidth absorption spectroscopy measurements with sub-femtosecond pulses. We show both one-dimensional and two-dimensional results. Our method is applicable to any spectral measurement limited by the bandwidth or noise level of the probing source, provided the probe spectrum is known at each shot. [Preview Abstract] |
Friday, June 5, 2020 8:42AM - 8:54AM Live |
S04.00003: Probing core-hole wavepacket dynamics in molecules using angle-resolved photoelectron spectroscopy Siddhartha Chattopadhyay, Ludger Inhester, Robin Santra, Artem Rudenko, Daniel Rolles, Loren Greenman Attosecond dynamics have been predicted following core ionization, including charge migration and core-hole dynamics. The angular correlations in photoelectron angular distributions from the core-hole states gives direct access to probe ultrafast quantum dynamics. Attosecond x-ray pump-probe measurements, which will be available with LCLS-II, first core ionize a molecule and then probes its dynamics. In the present work, we use variational scattering calculations for the continuum states of the $\mathrm{N}_2$ molecule. Our scheme is based on two different approximation, first using the atomic cross-section the angular correlation between two photo-electrons shows prominent oscillations with respect to the time delay between the two pulses. In the second approach, we use the electronic structure of the neutral and the ionized molecule in the Hartree-Fock approximation to obtain a more accurate description. This calculation shows that the dynamics can be observed for both single-site and two-site double-core-hole states in contrast to the atomic approach. The calculation based on the molecular potential allows us to study charge migration in complex polyatomic molecules after core-ionization. [Preview Abstract] |
Friday, June 5, 2020 8:54AM - 9:06AM Live |
S04.00004: Time resolved Coulomb explosion imaging of multi-channel non-adiabatic photodissociation dynamics Ruaridh Forbes, Daniel Rolles, Taran Driver, Michael Burt, Mark Brouard, Philip Bucksbaum We utilize soft X-ray Free Electron Laser (FEL) pulses to track non-adiabatic and photodissociation dynamics in a series of iodide containing hydrocarbons. A time-stamping camera and a hexanode delay line were utilized at the FLASH and SACLA FEL facilities, respectively, to correlate the ion vector momenta following excitation across the deep ultraviolet. In the FLASH results, predissociation of the B-state in methyl iodide is investigated and photofragment angular distributions are extracted from sliced ion velocity map images. At SACLA, the photoexcitation energy dependence of the A-band in methyl iodide and iodobenzene is explored by using tunable pulses from an optical parametric amplifier. Highlighted is the competition between both direct (excited state) and indirect (ground state) photodissociation pathways, with latter mediated by internal conversion. [Preview Abstract] |
Friday, June 5, 2020 9:06AM - 9:18AM Live |
S04.00005: Resonant nonlinear X-ray spectroscopy of acetyl fluoride for probing electronic dynamics with space selectivity Stefano M. Cavaletto, Shaul Mukamel Ultrashort, coherent X-ray pulses from X-ray free-electron lasers (FELs) enable direct access to core excited states in ultrafast time scales. They can be used to extend multidimensional nonlinear spectroscopy into the X-ray regime, and probe the dynamics of a large number of valence and core excited states thanks to their broad bandwidths. Here, we present resonant X-ray sum-frequency-generation (XSFG) signals in order to access novel information on molecular dynamics. In XSFG, the dynamics of a valence electronic wave packet, initiated by an optical or ultraviolet pulse, are subsequently probed by a coherent resonant X-ray pulse with a properly set delay. By resonantly exciting different element-specific core states, the X-ray probe can be employed to monitor the molecular dynamics with high spatial selectivity. Due to their short wavelengths, hard-X-ray FEL pulses are also sensitive to spatial variations within the size of the molecule. Using the minimal-coupling Hamiltonian, we show how XSFG can reveal effects going beyond a description of the light-matter interaction based on the multipole expansion. [Preview Abstract] |
Friday, June 5, 2020 9:18AM - 9:48AM Live |
S04.00006: Coulomb Explosion Imaging of Polyatomic Molecules: Towards Molecular Movies Invited Speaker: Rebecca Boll Recording images of individual molecules with ultrashort ”exposure times” has been a long-standing dream in molecular physics, chemistry, and biology, and was one of the driving forces for the development of X-ray FELs. We have recently used femtosecond soft X-ray pulses from the European XFEL to record snapshot images of a molecule with 11 atoms, including all hydrogens, by Coulomb explosion imaging in a reaction microscope. Contrary to the general expectation that, to image polyatomic molecules, it is necessary to record all charged fragments in coincidence, we demonstrate that three-fold ion coincidences can be sufficient to image the full structure. The X-ray intensity of up to 10$^{13}$photons/$\mu$m$^2$ is high enough to produce extreme charge states in heavy atoms (e.g. up to 42+ in Xe atoms), and to Coulomb-explode molecules very quickly, such that the initial molecular structure is preserved in the recorded momenta. The intriguingly clear momentum images allow us to identify each atom’s position in the molecule unambiguously, and to trace the rearrangement of charge within the molecule. By combining our experimental results with state-of-the-art molecular dynamics calculations, we can follow the charge-up dynamics of the molecule within the femtosecond X-ray pulse. These results can, in the next step, be employed to time-resolved imaging of molecular structure – thus bringing the dream of recording molecular movies of photo-chemical reactions into reach. [Preview Abstract] |
Friday, June 5, 2020 9:48AM - 10:00AM Live |
S04.00007: Potential benefits of electonic damage in x-ray imaging Stephan Kuschel, Andre Haddad, Phay Ho, Felix Zimmermann, Leonie Flueckiger, Matthew Ware, Joseph Duris, James MacArthur, Alberto Lutmann, Peter Walter, Ming-Fu Lin, Xiang Li, Jeff Aldrich, Linda Yeung, Christoph Bostedt, Agostino Marinelli, Tais Gorkhover Ionization is generally considered detrimental to the quality of single exposure images recorded with ultra bright modern X-ray sources, such as X-ray free electron lasers (XFELs). We conducted a X-ray coherent diffraction imaging (CDI) study at the Linac Coherent Light Source (LCLS) on single rare gas nanoparticles. Our results indicate that the X-ray scattering cross section may increase due to transient ionic resonances before structural damage degrades the image. We also observe that the samples become increasingly transparent if the pulse parameters correspond to typical values of single particle imaging experiments. Dynamical electronic structure calculations attribute the observed effects to electronic damage and predict amplification of X-ray coherent scattering of up to two orders of magnitude compared to the neutral scattering cross section might be possible. We also demonstrate that ionic resonances are present in images recorded with sub-fs pulses which proves that transient resonances can be exploited before ionic damage. [Preview Abstract] |
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