Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session T4: Intense Field Physics |
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Chair: Barry Walker, University of Delaware Room: A704 |
Friday, June 17, 2011 8:00AM - 8:30AM |
T4.00001: Strong-Field nonperturbative effects in solids Invited Speaker: We present recent results of nonperturbative high harmonic generation\footnote{ S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis. Observation of high-order harmonic generation in a bulk crystal. \textit{Nat Phys}, 7(2):138-141, 2011.} and below gap absorption\footnote{S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis, Strong-field induced optical absorption in ZnO, CLEO/QELS 2011.} in single crystal ZnO driven by mid-infrared pulses in a new regime where the field approaches the bandgap per lattice constant. In this limit, the field cannot be considered as producing a small perturbation to the crystal, instead the material properties are transiently yet nondestructively altered. Here we observe harmonics up to 25th order with a cutoff that scales linearly with applied field. The results are consistent with a simple model for tunnel ionization and radiation from nonlinear and attosecond acceleration of carriers in a band. In addition, we observe dramatic sub-band gap absorption of light nearly 10\% below the gap, due to photon assisted tunneling. We discuss the relationship between these two effects. [Preview Abstract] |
Friday, June 17, 2011 8:30AM - 9:00AM |
T4.00002: Photoelectron Holography Invited Speaker: New techniques using High Harmonic Generation (HHG) or attosecond pulses have proven to be successful in following the ultrafast motion of electrons and holes inside a molecule [1, 2]. We introduce a complementary technique; photoelectron holography [3]. This uses the phase and amplitude of the rescattered electrons to encode the structure and dynamics of the studied atom or molecule. Since photoelectron holography benefits from longer wavelengths, i.e. small photon energies, it is very suitable for systems with a small ionization potential. To demonstrate photoelectron holography, both measurements and calculations on atomic Xenon will be shown. Metastable Xenon was ionized with 7$\mu$m light from the FELICE-free electron laser [4]. The three dimensional momentum distribution of the photoelectrons was recorded by a Velocity Map Imaging (VMI)-spectrometer. In these momentum maps interference structures are observed that can be identified as an interference of direct and scattered electrons; a hologram of Xenon. Semi-classical calculations have demonstrated that in the hologram dynamical information of the electron and the atom is stored with a femtosecond to attosecond time resolution.\\[4pt] [1] O. Smirnova et al. Nature 460 972 (2009)\\[0pt] [2] G. Sansone et al. Nature 465 763 (2010)\\[0pt] [3] Y. Huismans et al. Science 331 61 (2011)\\[0pt] [4] J. M. Bakker et al., J. Chem. Phys. 132 074305 (2010) [Preview Abstract] |
Friday, June 17, 2011 9:00AM - 9:30AM |
T4.00003: Multiple ionization bursts in laser-driven hydrogen molecular ion Invited Speaker: The emission of an electron from an atom or molecule is presumably one of the simplest but most central processes in physics and chemistry. The release of an electron induced by an intense laser pulse is often understood in terms of the quasistatic tunnel ionization picture. According to this picture it is assumed that the electron leaves the parent ion with largest probability at the peaks of the oscillating electric field, when the tunnel barrier is thinnest. But, results of numerical simulations for the hydrogen molecular ion interacting with an intense infrared laser field reveal multiple bursts of ionization within a half-cycle of the laser field. These bursts are found to be induced by a sometimes counter-intuitive dynamics of the electron inside the molecule. The dynamics is due to a phase difference of the wave function between the two potential wells induced by the laser electric field and accumulated over time. Two experimental schemes to probe the intramolecular electron dynamics on the attosecond time scale will be discussed. [Preview Abstract] |
Friday, June 17, 2011 9:30AM - 10:00AM |
T4.00004: Strong-field coherent control of molecular-ion-beam fragmentation Invited Speaker: We have studied laser-induced fragmentation of molecular-ion beams using coincidence 3D momentum imaging, with direct separation of all the reaction products measured simultaneously. These measurements provide detailed kinetic energy release and angular distributions of the different fragmentation processes. We mainly focus on the fundamental H$_{2}^{+}$ and H$_{3}^{+}$ molecules (in 7-50 fs laser pulses having 10$^{12}$-10$^{16}$ W/cm$^{2}$ peak intensity) as models for more complex systems. We use deuterium tagging to distinguish different final products and thus study how to control the interference between several fragmentation pathways responsible for the preference of one outcome over another. The preference for HD$^{+}$ to dissociate into either H$^{+}$+D or H+D$^{+}$ is a good example of this kind of control -- usually referred to as channel asymmetry. We expect the H$^{+}$+D channel, associated with the HD$^{+}$ electronic ground state, to dominate over dissociation into H+D$^{+}$ for very slow fragmentation of HD$^{+}$ produced by a two-photon process called (net) zero-photon dissociation. Our experimental results confirm this prediction. The mechanisms of the strong-field control we demonstrate in these comparatively simple systems are applicable to more complex molecules, although identifying all of the important fragmentation pathways becomes more challenging. Nevertheless, the principles of determining the important control parameters remain the same. [Preview Abstract] |
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