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 J4: Free Electron Laser and Ultrahigh Field Physics |
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Chair: Matthias Fuchs, University of Nebraska Room: Union DE |
Wednesday, June 10, 2015 2:00PM - 2:12PM |
J4.00001: A correlation criterion for non-sequential or sequential two--photon double ionization of helium in ultrashort XUV pulses Aihua Liu, Uwe Thumm For sufficiently long XUV pulses, sequential and non-sequential two--photon double ionization (DI) of helium atoms are distinguishable by the central XUV photon energy. If this energy is above the second ionization potential (54.4 eV), emission proceeds sequentially, otherwise non-sequential correlated DI occurs [1]. For ultrashort XUV pulses, this distinction breaks down if their spectrum overlaps the sequential and non-sequential regimes [2]. We investigated joint photoelectron angular distributions for two--photon DI of helium atoms in the sequential and non-sequential regimes. We found that strong correlation results in similar angular distributions at different energy sharing of the photoelectrons. Based on the emission asymmetry in joint angular distributions (ranging from -1 for emission in opposite to 1 for emission into the same hemisphere) we distinguish ``sequential" and ``non-sequential" contributions to DI. E.g., for 50 eV XUV pulses, the sequential DI contribution reaches a maximum value at 650 as pulse duration, which we explain as being due to the competition between increasing temporal and decreasing spectral pulse widths. \\[4pt] [1] A. Liu and U. Thumm, Phys. Rev. A \textbf{89}, 063423 (2014).\\[0pt] [2] J. Feist et al.,Phys. Rev. Lett. \textbf{103}, 063002 (2009). [Preview Abstract] |
Wednesday, June 10, 2015 2:12PM - 2:24PM |
J4.00002: STIRAP with XFEL pulses Antonio Picon, Stephen Southworth The development of the laser in the optical-IR regime stimulated completely new schemes for controlling quantum systems by resorting to the coherence of the light-matter interaction. Along this line, it is interesting to explore these schemes or new ones in the x-ray regime, especially with the advent of x-ray free-electron lasers (XFELs) which deliver spatio-temporal coherent pulses in the femtosecond timescale. The new factor in the x-ray regime with respect to the optical regime is the unavoidable creation and ultrafast decay of core-excited states driven by strong electron correlations that one needs to consider. Hence, X-Ray Quantum Optics opens an interesting field in order to explore strongly correlated systems driven by x-ray pulses, and definitely it will play a crucial role in the development of characterization methods for x-ray pulses at XFELs. Here, we propose a theoretical scheme for STIRAP (Stimulated Raman Adiabatic Passage) in Ne gas for the soft x-ray regime. Experimental feasibility will be discussed. [Preview Abstract] |
Wednesday, June 10, 2015 2:24PM - 2:36PM |
J4.00003: XUV-driven Rabi oscillations with the giant resonance in xenon Stefan Pabst, Daochen Wang, Robin Santra We look into the possibility of driving Rabi oscillations in the XUV regime. To do so, we exploit the unusually large dipole transition strength in xenon around 100 eV, which is also responsible for the well-known giant dipole resonance. We show what kind of (FEL) pulses are required to achieve a Rabi period shorter than the lifetime of the giant resonance states. Ways how to detect these fast Rabi oscillations in experiment are discussed. [Preview Abstract] |
Wednesday, June 10, 2015 2:36PM - 2:48PM |
J4.00004: Controlling the hole alignment with Fano resonances Stefan Pabst, Elisabeth Heinrich-Josties, Robin Santra We study the state-resolved production of neon ion after resonant photoionization of Ne via the $2s$-$3p$ Fano resonance. We find that by tuning the photon energy across the Fano resonance a surprisingly high control over the alignment of the final $2p$ hole along the polarization direction can be achieved. In this way, hole alignments can be created that are otherwise not possible to reach in the XUV regime. This effect survives for high-intensity FEL pulses and strongly influences the ionization behavior close to saturation. [Preview Abstract] |
Wednesday, June 10, 2015 2:48PM - 3:00PM |
J4.00005: Two-color x-ray pump x-ray probe study of the core-hole decay dynamics in XeF$_{2}$ Carl Stefan Lehmann, Antonio Picon, Steve Southworth, Gilles Doumy, Anne Marie March, Elliot Kanter, Bertold Kraessig, Dooshaye Moonshiram, Linda Young, Steve Pratt, Christoph Bostedt, Jacek Kryzwinski, Ken Ferguson, Max Bucher, Tais Gorkhover, Timur Osipov, Daniel Rolles, Benjamin Erk, Cedric Bomme, Artem Rudenko, Dipanwita Ray, Nora Berrah, Agostino Marinelli To resolve the femtosecond inner-shell dynamics and the subsequent induced electron transfer in a molecule, the core-hole decay dynamics in XeF$_{2}$ have been directly studied using femtosecond time-resolved x-ray pump x-ray probe ion-ion coincidence imaging. Inner-shell photoionization of Xe triggers core-hole decay that is initially localized but subsequently involves delocalized valence electrons. Charge is distributed to neighboring atoms and the system Coulomb explodes. The processes proceed concurrently on the femtosecond time scale. XeF$_{2}$ is a very interesting molecule, as it allows us to compare the molecular core-hole decay with the atomic case, Xe atom. [Preview Abstract] |
Wednesday, June 10, 2015 3:00PM - 3:12PM |
J4.00006: Controlling explosion dynamics in mixed He/Xe clusters with X-ray double pulses M. Ziemkiewicz, C. Bacellar, A. Chatterley, J. Cryan, O. Gessner, M. Mueller, D. Rupp, T. Moeller, C. Jones, R.M.P. Tanyag, C. Bernando, L. Gomez, J. Kwok, A. Vilesov, K. Ferguson, M. Bucher, T. Gorkhover, S. Carron, J. Krzywinski, A. Lutman, A. Marinelli, T. Maxwell, J. Turner, F.-J. Decker, C. Bostedt Intense X-ray induced fragmentation dynamics of hybrid systems consisting of Xe structures embedded in large superfluid helium nanodroplets are studied by femtosecond time-resolved ion mass spectrometry. The clusters are photoionized by a pair of intense X-ray pulses from the Linac Coherent Light Source (LCLS), resulting in fragmentation and ejection of ions with kinetic energies of up to several keV. The production of He$^{\mathrm{++}}$ is of particular interest as this cation is only formed in the mixed system and not upon X-ray illumination of pure He nanodroplets. It is found that the He$^{++}$ product kinetic energy distribution varies sensitively with the relative timing of the X-ray double pulse, exhibiting complex dynamics as a function of pump-probe delay. Existing models describing similar results for experiments using intense infrared laser pulses are not applicable to this study due to the disparate interactions of intense optical and X-ray fields with matter. Possible phenomena underlying the observed trends are discussed. [Preview Abstract] |
Wednesday, June 10, 2015 3:12PM - 3:24PM |
J4.00007: Wavelength dependent high-energy ion emission from intense mid-IR laser-cluster interaction Hyunwook Park, Zhou Wang, Pierre Agostini, Louis DiMauro We present the first measurements on the wavelength dependence from the near-infrared to mid-infrared of inert gas clusters interacting with an intense, ultrafast pulse. In the experiments, ion energy distributions have been recorded with various wavelength (0.8-2.2 $\mu$m), while all other conditions are fixed. It is found that the wavelength plays a significant role in electron-plasma heating and thus energetic ion production. The maximum energy of the detected ion, E$_{\mathrm{max}}$, decreases with increasing wavelength, reaches a minimum, then increases. We attribute this result to two different electron-heating mechanisms depending on the wavelength- volume (Inverse Bremsstrahlung: IB) and surface (Brunel) heating. In the short wavelength regime (0.8-1.5 $\mu$m), IB heating dominates the production of multiply charged ions, since the electrons are resonantly heated near plasma frequency. As the wavelength is increased, IB heating is progressively suppressed, resulting in a smaller value of E$_{\mathrm{max}}$. Brunel heating, on the other hand, increases due to a quadratic increase of the electrons ponderomotive energy, and becomes dominant in the long wavelength regime (1.7-2.2 $\mu$m). The lowest E$_{\mathrm{max}}$ values would thus occur at the wavelength where the dominant heating mechanism switches from volume to surface. [Preview Abstract] |
Wednesday, June 10, 2015 3:24PM - 3:36PM |
J4.00008: THz-Induced, High-Energy Electron Emission from Nano-Structured Metals Sha Li, Anna Yanchenko, R.R. Jones Single-cycle THz pulses have been used to induce keV electron emission from tungsten nano-tipped wires. Single tips are exposed, in vacuum, to intense THz pulses generated via tilted pulse front optical rectification of 150 fs Ti:Sapphire laser pulses. Electrons emitted from the tip are subsequently driven to very high energies by the locally enhanced THz field. The electron yield follows the Fowler-Nordheim tunneling prediction for weak THz pulses, but falls well below that prediction at high fields. The maximum electron energy is proportional to the THz field component along the tip axis, and reaches several keV for incident THz fields \textless 500 kV/cm. Our results can be understood using a model recently proposed for sub-cycle emission from nano-tips in mid-infrared fields [Herink et al., Nature 483, 190 (2012)], assuming a field enhancement factor of $\sim$ 1000 in the vicinity of the tip. We observe electron energies that are 10 times greater than those produced with mid-infrared pulses even though the THz intensity is five orders of magnitude smaller. Comparison of the maximum electron energy as a function of tip radius and cone angle provides additional insight into the local electron-field interaction. [Preview Abstract] |
Wednesday, June 10, 2015 3:36PM - 3:48PM |
J4.00009: Neutron Production from In-situ Heavy Ice Coated Targets at Vulcan John Morrison, A.G. Krygier, S. Kar, H. Ahmed, A. Alejo, R. Clarke, J. Fuchs, A. Green, D. Jung, A. Kleinschmidt, Z. Najmudin, H. Nakamura, P. Norreys, M. Notley, M. Oliver, M. Roth, L. Vassura, M. Zepf, M. Borghesi, R.R. Freeman Laser based neutron production experiments have been performed utilizing ultra-high intensity laser accelerated ions impinging upon a secondary target. The neutron yield from such experiments may be improved if the accelerated ions were primarily deuterons taking advantage of the d-d cross section. Recent experiments have demonstrated that selective deuteron acceleration from in-situ heavy ice coating of targets can produce ion spectra where deuterons comprise $> 99\%$ of the measured ions. Results will be presented from integrated neutron production experiments from heavy ice targets coated in-situ recently performed on the Vulcan laser at Rutherford Appleton Laboratory. [Preview Abstract] |
Wednesday, June 10, 2015 3:48PM - 4:00PM |
J4.00010: Backward-going MeV electrons and gamma rays from $10^{18}$ W/cm$^2$ laser interactions with water Scott Feister, John T. Morrison, Kyle D. Frische, Chris Orban, Vladimir M. Ovchinnikov, John A. Nees, Drake R. Austin, Enam A. Chowdhury, Richard R. Freeman, W. Melvyn Roquemore Gamma rays with $\sim$1 MeV energy are measured following the relativistic interaction of a 3 mJ, 10$^{18}$ W/cm$^2$ short pulse laser with a 30 $\mu$m diameter flowing water column. Contrary to expectations, radiation emission is peaked in the direction opposite to the normally-incident laser propagation (specular direction). Experimental measurements and particle-in-cell (PIC) simulations of laser-plasma interaction show a pre-formed-plasma-dependent, backward-going, beam-like primary electron source. The MeV component of the electron and gamma ray spectrum, which is more than five times the ponderomotive energy scale of the laser, is highly sensitive to the presence of a nanosecond-timescale laser pre-pulse. [Preview Abstract] |
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