Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session J3: Focus Session: Ultrafast X-ray ProcessesFocus
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Chair: Stephen Southworth, Argonne National Laboratory Room: Ballroom D |
Wednesday, May 25, 2016 2:00PM - 2:30PM |
J3.00001: Probing Transient Electron Dynamics Using Ultrafast X Rays Invited Speaker: Philip Bucksbaum Linear x-ray absorption in atoms or molecules creates highly excited multi-electron quantum systems, which relax rapidly by fluorescence or Auger emission. These relaxation rates are usually less than a few femtoseconds in duration, and so they can reveal transient elecronic states in molecules as they undergo photo-induced transformations. I will show recent results from femtosecond x-ray experiments that display this phenomenon. There are efforts underway to push the temporal resolving power of ultrafast x-ray pulses into the attosecond regime, using stronger fields to initiate nonlinear absorption processes such as transient stimulated electronic Raman scattering. I will discuss current progress and future prospects for research in this area. [Preview Abstract] |
Wednesday, May 25, 2016 2:30PM - 2:42PM |
J3.00002: Transient lattice contraction in the solid to plasma transition of x-ray heated xenon clusters C. Bostedt, K. Ferguson, T. Gorkhover, P.H. Bucksbaum, S. Boutet, J.E. Koglin, A. Lutman, A. Marinelli, J. Turner, M. Bucher, P. Ho, C. Knight, L. Young, H. Fukuzawa, Y. Kumagai, K. Ueda, K. Nagaya, M. Messerschmidt, G. Williams Any sample in the focus of intense x-ray pulses will be transformed into a nanoplasma within femtoseconds. We have employed the novel two-color two-pulse mode available at the Linac Coherent Light Source free-electron laser to investigate the structural dynamics in nanoparticles upon x-ray exposure. We find that the nanoparticle transiently contracts within the first 80 fs following x-ray irradiation before ultimately disintegrating in a rapid hydrodynamic expansion. The contraction can be attributed to the massive x-ray induced electronic excitation that induces a collective change in the bond character of the nanoparticles. Alternative explanations for the contraction include a compression wave stemming from a rapid surface explosion of the nanoparticle. Computer simulations under way can elucidate the dominant contraction mechanism and yield further insight into the complex x-ray induced dynamics in nanoscale samples. [Preview Abstract] |
Wednesday, May 25, 2016 2:42PM - 2:54PM |
J3.00003: Time-resolved x-ray imaging of x-ray induced dynamics in Xe clusters M. Bucher, K. Ferguson, T. Gorkhover, S. Carron, J. Cryan, J. Krzywinski, A. Lutman, A. Marinelli, C. Bacellar, A. Chatterley, M. Ziemkiewicz, C. Bernando, L. Gomez, C. Jones, J Kwok, R.M.P. Tanyag, M. Mueller, D. Rupp, T. M\"oller, O. Gessner, A. Vilesov, C. Bostedt Nanoparticles irradiated by intense x-ray pulses from the LCLS free-electron laser are immediately transformed into a highly excited nanoplasma. Within the first few femtoseconds of the x-ray pulse the particles are ionized and on longer time scales they disintegrate due to Coulomb forces. We performed an x-ray pump / x-ray probe experiment to investigate how the nanoplasma creation and disintegration changes the particle’s diffraction response. As samples we used pristine Xe clusters as well as Xe clusters embedded in He droplets. The data show that for pristine clusters the higher diffraction orders diminish first and vanish completely as the nanoplasma expansion progresses. This effect is less prominent in the embedded clusters. We compare our results to previous studies on optically pumped clusters (T. Gorkhover et al., Nat. Photonics, 2016). [Preview Abstract] |
Wednesday, May 25, 2016 2:54PM - 3:24PM |
J3.00004: Ultrafast ionization and fragmentation dynamics of molecules at high x-ray intensity Invited Speaker: Sang-Kil Son X-ray free-electron lasers (XFEL) open a new era in science and technology, offering many unique opportunities that have not been conceivable with conventional light sources. Because of their very high x-ray photon fluence within very short pulse duration, materials interacting with XFEL undergo significant radiation damage --- they possibly become highly ionized and then explode. To comprehend underlying physics, it is crucial to understand detailed ionization and fragmentation dynamics of atoms and molecules during intense XFEL pulses. We have developed the XMOLECULE toolkit to describe molecular x-ray-induced processes and to simulate radiation damage dynamics of molecules. In this talk, I will present a theoretical framework of XFEL-matter interaction, namely x-ray multiphoton absorption. Then I will discuss recent results of ultrafast x-ray-induced explosion of methyl iodide (CH$_3$I) molecules. Charge state distribution and kinetic energy releases of fragments are calculated to probe ionization and fragmentation dynamics, and compared with recent experimental results. It will be demonstrated that ionization of heavy-atom-containing molecules at high x-ray intensity is much enhanced in comparison with the isolated atomic case, due to ultrafast charge rearrangement during x-ray multiphoton absorption. [Preview Abstract] |
Wednesday, May 25, 2016 3:24PM - 3:36PM |
J3.00005: Ultrafast molecular processes at the short-wavelength regime A. Picon, C.S. Lehmann, C. Bostedt, A. Rudenko, D. Rolles, A. Marinelli, L. Young, S.T. Pratt, S.H. Southworth Fundamental molecular processes that underlie chemical reactivity and biological processes typically involve intramolecular dynamics consisting of nuclear motion and the flow of charge and energy across atomic sites. Examples include photosynthesis, electron transfer in biomolecules, and molecular fragmentation. Molecular phenomena initiated by the absorption of an XUV/x-ray photon is one of the most challenging questions for the new generation of XUV/x-ray sources. New capabilities at accelerator-based are continuously being developed, being possible to nowadays generate two-color XUV/x-ray pulses with controlled time delay. The site-specificity of those photons allow the excitation of inner-shell electrons in a particular site of the molecule and, with a controlled time delay, the probing of the induced intramolecular dynamics in another site of the same molecule, opening the door to the unexplored field of intramolecular processes initiated by short-wavelength photons. Also, novel XUV/x-ray sources allow the generation of two-color pulses with a high spatio-temporal degree of coherence, suitable for quantum control schemes involving inner-shell electrons. In this talk, we present new theoretical and experimental results towards this direction. [Preview Abstract] |
Wednesday, May 25, 2016 3:36PM - 3:48PM |
J3.00006: Theoretical and Experimental Evidence of Hydrogen Migration rather than Isomerization in the Acetylene Dication Chelsea Liekhus-Schmaltz, Zheng Li, Vladimir Petrovic, Todd Martinez, Phil Bucksbaum Theoretical calculations and experimental results in the acetylene dication have long agreed that isomerization after x-ray excitation occurs in the first singlet state, where the carbon-carbon bond lives long enough for isomerization to complete [1]. These same calculations predict that a large barrier to isomerization exists that would cause isomerization to occur in about a picosecond, while there is some evidence for ultrafast isomerization in under 100 fs.~However, new ab initio calculations of the acetylene dication reveal that ultrafast isomerization after x-ray excitation is unlikely.~In this talk, we present evidence that signatures of hydrogen migration observed in recent time resolved LCLS data [2] are mostly due to hydrogen migration in an excited state which dissociates too quickly for isomerization to complete. [1] T. S. Zyubina, Y. A. Dyakov, S. H. Lin, A. D. Bandrauk, and A. M. Mebel., ``Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states,'' J. Chem. Phys. \textbf{123}, 134320 (2005). [2]~C. E. Liekhus-Schmaltz, et al, ``Ultrafast isomerization initiated by X-ray core ionization,'' Nat. Commun. \textbf{6}, 8199 (2015). [Preview Abstract] |
Wednesday, May 25, 2016 3:48PM - 4:00PM |
J3.00007: Collective resonances of atomic xenon from the linear to the nonlinear regime Yi-Jen Chen, Stefan Pabst, Robin Santra We explain the origin of the two collective sub-resonances of the $4d$ giant dipole resonance of atomic Xe recently discovered by nonlinear spectroscopy. In the case of one-photon absorption, while a change in the resonant-like feature in the cross section upon the inclusion of electronic correlations has been commonly attributed to a change of the resonance parameters of a single resonance state, we show that this modification is a result of switching between the relative visibilities of the underlying resonance states. In addition, we predict hitherto undiscovered collective $4d$ resonance states in Xe that can only be accessed through multiphoton absorption. Unlike any known collective feature in atoms, these resonances are exceptionally long-lived (more than 100 attoseconds), thus opening up possibilities to probe new collective effects in atoms with modern XUV light sources. [Preview Abstract] |
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