Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session H4: Invited Session: Nonlinear X-ray Science |
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Chair: James Cryan, SLAC National Accelerator Laboratory Room: Union DE |
Wednesday, June 10, 2015 10:30AM - 11:00AM |
H4.00001: Multi-photon ionization of atoms in intense short-wavelength radiation fields Invited Speaker: Michael Meyer The unprecedented characteristics of XUV and X-ray Free Electron Lasers (FELs) have stimulated numerous investigations focusing on the detailed understanding of fundamental photon-matter interactions in atoms and molecules. In particular, the high intensities (up to 10$^{\mathrm{16\thinspace }}$W/cm$^{\mathrm{2}})$ giving rise to non-linear phenomena in the short wavelength regime. The basic phenomenology involves the production of highly charged ions via electron emission to which both sequential and direct multi-photon absorption processes contribute. The detailed investigation of the role and relative weight of these processes under different conditions (wavelength, pulse duration, intensity) is the key element for a comprehensive understanding of the ionization dynamics. Here the results of recent investigations are presented, performed at the FELs in Hamburg (FLASH) and Trieste (FERMI) on atomic systems with electronic structures of increasing complexity (Ar, Ne and Xe). Mainly, electron spectroscopy is used to obtain quantitative information about the relevance of various multi-photon ionization processes. For the case of Ar, a variety of processes including above threshold ionization (ATI) from 3p and 3s valence shells, direct 2p two-photon ionization and resonant 2p-4p two-photon excitations were observed and their role was quantitatively determined comparing the experimental ionization yields to ab-initio calculations of the cross sections for the multi-photon processes. Using Ar as a benchmark to prove the reliability of the combined experimental and theoretical approach, the more complex and intriguing case of Xe was studied. Especially, the analysis of the two-photon ATI from the Xe 4d shell reveals new insight into the character of the 4d giant resonance, which was unresolved in the linear one-photon regime. Finally, the influence of intense XUV radiation to the relaxation dynamics of the Ne 2s-3p resonance was investigated by angle-resolved electron spectroscopy, especially be observing the intensity dependent variation of the angular distribution patterns for the sequential ionization process. [Preview Abstract] |
Wednesday, June 10, 2015 11:00AM - 11:30AM |
H4.00002: Multiphoton Ionization of Atoms and Molecules with Soft and Hard X-rays Invited Speaker: Daniel Rolles We have recently extended our previous investigations of the multiphoton ionization of heavy atoms, such as Kr and Xe [1, 2], and of high-Z atom containing molecules [3] from the soft into the hard X-ray range as well as into the XUV regime. Using the 100-nm focus environment at LCLS, we were able to reach peak intensities up to 10$^{19}$W/cm$^{2}$ at photon energies between 5 to 9 keV. This allows studying atomic and molecular ionization processes under unprecedented X-ray intensities and, in particular, under the identical conditions where typical coherent diffractive imaging experiments are performed. Our results are thus important benchmarks for calculating radiation damage effects in FEL-based X-ray imaging experiments. Using new micro-focusing capabilities at FLASH, we also extended our studies into the XUV range between 70 and 200 eV photon energy and observed significantly higher charge states than previously reported. I will present the results from our recent measurements at LCLS and FLASH and discuss the different multiphoton ionization mechanisms that play a role in the XUV, soft, and hard X-ray range. \\[4pt] [1] B. Rudek \textit{et al.}, Nature Photon. \textbf{6}, 858 (2012)\\[0pt] [2] B. Rudek \textit{et al.} Phys. Rev. A \textbf{87}, 023413 (2013)\\[0pt] [3] B. Erk \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 053003 (2013) [Preview Abstract] |
Wednesday, June 10, 2015 11:30AM - 12:00PM |
H4.00003: Imaging ultrafast processes in nanometer sized clusters Invited Speaker: Christoph Bostedt Free-electron lasers deliver extremely intense, coherent x-ray flashes with femtosecond pulse length. With the intense x-ray pulses single nanoscale objects can be imaged wirth single shots, opening the door for spatially and time resolved investigations of transient states and dynamic processes. Imaging of individual He droplets allows the unambiguous identification of quantum vortices. Ultrafast scattering of small highly excited nanoplasma carries information about their transient electronic states. With pump-probe techniques the electronic and structural evolution of highly excited clusters and nanoplasmas far from equilibrium can be investigated with femtosecond time and nanometer spatial resolution. These examples showcase that there are exciting new opportunities for Atomic, Molecular and Cluster Physics using ultrafast and ultraintense x-ray pulses. [Preview Abstract] |
Wednesday, June 10, 2015 12:00PM - 12:30PM |
H4.00004: Nonlinear X-Ray and Auger Spectroscopy at X-Ray Free-Electron Laser Sources Invited Speaker: Nina Rohringer X-ray free-electron lasers (XFELs) open the pathway to transfer non-linear spectroscopic techniques to the x-ray domain. A promising all x-ray pump probe technique is based on coherent stimulated electronic x-ray Raman scattering, which was recently demonstrated in atomic neon. By tuning the XFEL pulse to core-excited resonances, a few seed photons in the spectral tail of the XFEL pulse drive an avalanche of resonant inelastic x-ray scattering events, resulting in exponential amplification of the scattering signal by of 6-7 orders of magnitude. Analysis of the line profile of the emitted radiation permits to demonstrate the cross over from amplified spontaneous emission to coherent stimulated resonance scattering. In combination with statistical covariance mapping, a high-resolution spectrum of the resonant inelastic scattering process can be obtained, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and a realistic feasibility study of stimulated electronic x-ray Raman scattering in CO will be presented. Challenges to realizing stimulated electronic x-ray Raman scattering at present-day XFEL sources will be discussed, corroborated by results of a recent experiment at the LCLS XFEL. Due to the small gain cross section in molecular targets, other nonlinear spectroscopic techniques such as nonlinear Auger spectroscopy could become a powerful alternative. Theory predictions of a novel pump probe technique based on resonant nonlinear Auger spectroscopic will be discussed and the method will be compared to stimulated x-ray Raman spectroscopy. [Preview Abstract] |
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