Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session P5: Interactions of Ultrashort Intense Light with Atoms, Molecules, and Plasmas |
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Chair: K. Schafer, Louisiana State University Room: TELUS Convention Centre Macleod A3-A4 |
Friday, June 8, 2007 10:30AM - 10:42AM |
P5.00001: High Harmonic Generation from Non-symmetric Molecular Targets in a Hollow-Core Waveguide Brett Sickmiller, Robert Jones We explore high harmonic generation (HHG) from gas-phase molecules in hollow-core waveguide driven by intense 2-color (400nm+800nm) laser pulses. For symmetric molecules and laser fields, parity conservation precludes the generation of even harmonics. However for non-symmetric molecules such as CO, the predicted preference for ionization from the C rather than O site should allow for the generation of even harmonics using a symmetric fundamental laser field. Unfortunately, in a macroscopic sample, the random orientation results in a cancellation of the even-harmonics produced by individual molecules within the ensemble. We are attempting to transiently orient CO molecules in a waveguide by impulsively kicking them with a short, asymmetric 2-color laser pulse, creating an ensemble of rotational wavepackets without definite parity. We monitor the production of even harmonics generated by a 30 fsec, 800 nm laser pulse as a function of its delay after the 2-color pump. Liquid nitrogen cooling of the waveguide improves the achievable degree of alignment and orientation. [Preview Abstract] |
Friday, June 8, 2007 10:42AM - 10:54AM |
P5.00002: Imaging Molecular Orbitals with Ultrashort Intense Laser Pulses Andre D. Bandrauk, Gerard Lagmago Kamta Exact 3-D numerical solutions of the time-dependent Schroedinger equation, TDSE, for the one-electron H2+ molecule in the Born Oppenheimer (fixed nuclei) approximation have been obtained to calculate the angular dependence of ionization rates as a function of internuclear distance in the presence of ultrashort (5 fs) intense (I~10$^{14}$ W/cm$^2$) 800 nm laser pulses. It is shown that the ionization rates reproduce well the molecular orbital symmetries and structures near equilibrium whereas at larger internuclear distances where CREI (Charge Resonance Enhanced Ionization) occurs considerable deviation of orbital structure occurs due to ``off-axis'' tunnelling ionization. The results show that intense low frequency ultrashort pulses can image molecular orbitals provided molecules can be properly aligned. [Preview Abstract] |
Friday, June 8, 2007 10:54AM - 11:06AM |
P5.00003: Coherence Effects in Intense Laser Pulse Dissociative Ionization of H 2+ Andre D. Bandrauk, Sczepan Chelkowski Numerical solutions of the time-dependent Schroedinger equation (TDSE) for non-Born-Oppenheimer H2+(D2+) in 20-100 fs intense laser pulses are presented to interpret structure observed in the Coulomb-Explosion(CE) spectra during dissociative ionization of the above molecules(Staudte-Corkum,NRC) [1].Detailed dissociative ionization and CE simulations with a non Born Oppenheimer code [2] at different wave lengths and intensities allow to establish the mechanism for such structure in the CE spectra as due to interference between coherent one and two photon electronic excitation .Increasing intensity leads to ``localization'' of the active electron at the peaks of the electric field of the pulse. ``Attosecond'' electron ionization at these peaks produces structures in the CE spectra from an interfering coherent superposition of nuclear wave packets on different potential surfaces.The dissociative ionization simulations confirm this as a novel nonlinear coherent electron- nuclear interference phenomenon . [1] A Staudte et al, Phys Rev Lett, in press (2207); [2] A D Bandrauk, S Chelkowski, Phys Rev Lett 84, 3562(2000); 87, 273004(2002). [Preview Abstract] |
Friday, June 8, 2007 11:06AM - 11:18AM |
P5.00004: Relativistic ionization and rescattering Michael Klaiber, Karen Hatsagortsyan, Robert Fischer, Manfred Lein, Mario Verschl, Christoph H. Keitel In the relativistic regime the magnetically induced drift of the ionized electron severely suppresses the probability of the electron revisiting the ionic core and, consequently, the yield of harmonic photons. We propose several methods to increase the efficiency of rescattering in the relativistic regime. In the weakly relativistic regime, we demonstrate that the relativistic drift of the electron can be efficiently harnessed to enhance the recollisions of electrons from molecular orbitals with mirror antisymmetry [1]. In the strong relativistic regime, we show how efficient recollisions are feasible by employing strong laser pulses which are specially tailored as attosecond pulse trains [2]. For experimental realization it is more advantageous to employ counter-propagating attosecond pulse trains. This way the energies of the revisiting electron at the atomic core can reach the MeV domain, thus rendering hard x-ray harmonics and nuclear reactions with single atoms feasible. Other recollision schemes proposed are based on two consecutive counterpropagating laser pulses [3] and magnetic field. \underline {References:} [1] R. Fischer \textit{et al.}, Phys. Rev. Lett. 97, 143901 (2006). [2] M. Klaiber, \textit{et al.}, Phys. Rev. A, 74, 051803(R) (2006). [3] M. Verschl, and C. H. Keitel, J. Phys. B, in press. [Preview Abstract] |
Friday, June 8, 2007 11:18AM - 11:30AM |
P5.00005: Intramolecular Dynamics Probed using High Harmonic Generation Robynne Hooper, Xibin Zhou, Wen Li, Nick Wagner, Henry Kapteyn, Margaret Murnane We observed intramolecular dynamics as a modulation in high harmonic emission. We excite coherent vibrations in CF$_{3}$Cl using impulsive Raman scattering with a short laser pulse. A second laser pulse generates high harmonics. The harmonic yield is observed to oscillate at frequencies corresponding to three vibrational modes of CF$_{3}$Cl. In a second experiment, we used UV light to excite and dissociate CF$_{3}$I, and follow the dynamic evolution by monitoring the harmonic yield. We observe a large modulation of the harmonic yield, likely due to resonance excitation and subsequent dissociation of the molecule. We speculate that the less-than full baseline recovery after the UV pulse is due to ionization, and that the harmonic yield is sensitive to the bond length during dissociation. By these two experiments, we confirm that high harmonic generation is a sensitive probe of intramolecular dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. [Preview Abstract] |
Friday, June 8, 2007 11:30AM - 11:42AM |
P5.00006: ABSTRACT HAS BEEN MOVED TO Q2.00009 |
Friday, June 8, 2007 11:42AM - 11:54AM |
P5.00007: Does Wave-Packet Spreading Influence Light Scattering from Free Electrons in an Intense Laser Field? Justin Peatross Arguments are presented against treating electron quantum probability current as a classical current distribution for purposes of computing light emission. This procedure, apparently widely accepted within the high-intensity laser community, leads to the startling prediction that light scattered from a free electron quickly diminishes when the electron wave packet spreads to the scale of the driving-field wavelength. One draws the absurd conclusion that a single electron phase-matches (and phase-mismatches) with itself when emitting light. This question can be put to the test: The outcome of this dispute dramatically impacts the amount of light expected to scatter out the side of a relativistic laser beam. [Preview Abstract] |
Friday, June 8, 2007 11:54AM - 12:06PM |
P5.00008: Electromagnetically induced transparency for x-rays Christian Buth, Robin Santra, Linda Young We discuss electromagnetically induced transparency~(EIT) for x-rays in laser-dressed neon. We use a newly devised \emph{ab initio} theory to calculate the x-ray photoabsorption cross section and the x-ray polarizability of a neon atom dressed by an optical laser ($800 \, \mathrm{nm}$, $10^{13} \, \mathrm{\frac{W}{cm^2}}$) with a photon energy close to the $1s^{-1} \, 3s \to 1s^{-1} \, 3p$~transition. The results are investigated further in terms of an exactly solvable $\Lambda$-type three-level model where we point out the need for an intense dressing laser due to the femtosecond lifetime of core-excited neon. Practical experimental realization of EIT for x-rays is discussed and potential applications are outlined. This work opens new opportunities for research with ultrafast x-ray sources. [Preview Abstract] |
Friday, June 8, 2007 12:06PM - 12:18PM |
P5.00009: A Direct, Time Dependent, Lanczos Propagation Method for Non-Orthogonal Basis Sets Barry I. Schneider, Xiaoxu Guan, Johannes Feist, Klaus Bartschat, Cliff Noble, Oleg Zatsarinny We have developed an efficient approach for solving the time-dependent Schroedinger equation for the interaction of a strong laser pulse with a general atom, when the many-electron basis set is non-orthogonal. The propagation equations have the form, iS dC(t) / dt = HC(t) where S and H are respectively the overlap and Hamiltonian matrices in the many-electron space. By a succession of Lanczos orthogonalizations, the Hamiltonian is reduced to tri-diagonal form, but the overlap matrix remains full in the small, Lanczos basis. Thus, we are faced with solving a small, generalized eigenvalue problem at each step of the Lanczos recursion. The approach is still dominated by the need to find an efficient way to multiply the H and S matrix on a vector. Some examples of the new method will be presented in the talk. [Preview Abstract] |
Friday, June 8, 2007 12:18PM - 12:30PM |
P5.00010: Vibrational stabilization of cold molecules using a phase coherent train of ultrashort pulses Avi Pe'er, Evgeny Shapiro, Matthew C. Stowe, Moshe Shapiro, Jun Ye Ultracold molecules can be created from atoms by Feshbach resonance techniques. While these molecules are in the electronic ground state, they are highly excited vibrationally. As a result, these molecules are unstable due to virational quenching. We present a general and highly efficient scheme for vibration stabilization using a coherent train of weak pump-dump pairs of shaped ultrashort pulses to perform narrow-band Raman transitions between vibrational levels. The use of weak pulses permits an analytic description within the framework of coherent control in the perturbative regime, while coherent accumulation of many pulse pairs enables near unity transfer efficiency with a high spectral selectivity, thus forming a powerful combination of pump-dump control schemes and the precision of the frequency comb. The feasibility and robustness of this concept is verified by realistic simulations of the molecular dynamics. [Preview Abstract] |
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