Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session E4: Short Pulse Processes II |
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Chair: Wendell Hill III, University of Maryland Room: Knoxville Convention Center 301E |
Wednesday, May 17, 2006 1:30PM - 1:42PM |
E4.00001: Direct Measurement of Dynamic Alignment in Strong Femtosecond Fields Kun Zhao, Lee N. Elberson, Getahun M. Menkir, Marcus Laich, Wendell T. Hill, III Coulomb explosion imaging provides a unique window into molecular structure and dynamics on a timescale commensurate with intramolecular motion. This techniques combines the rapid removal of electrons from a molecular target, induced by intense ultrashort laser pulses, with position sensitive detection for coincidence capture of all fragments over $4\pi$ sr. We employed to measure the degree of dynamic alignment (the excess alignment beyond that due to geometric alignment) induced by linearly polarized, 100 fs pulses in the $10^{15}$ W/cm$^2$ intensity range. Exploiting circular polarization to turn off dynamic alignment a quantitative measure of the excess alignment was extracted from the relative atomic ion yields subsequent to Coulomb explosion in linearly and circularly polarized fields for several linear molecules (H$_2$, N$_2$, O$_2$ and CO$_2$). The degree of dynamic alignment was measured to be about 0.90 (H$_2$), 0.16 (N$_2$ and O$_2$) and 0.29 (CO$_2$). The anomalously large value of CO$_2$ implies a torque enhancement that we show is consistent with CO$_2$ interacting with the field longer than N$_2$ and O$_2$ prior to enhanced ionization. These results will be summarized and compared with other approaches and previous measurements. [Preview Abstract] |
Wednesday, May 17, 2006 1:42PM - 1:54PM |
E4.00002: Laser-Cluster Interaction: X-Ray Production by Short Laser Pulses C. Deiss, N. Rohringer, J. Burgd\"orfer, E. Lamour, C. Prigent, J.-P. Rozet, D. Vernhet Recent experiments on the interaction of large rare-gas clusters (number of atoms N $>$10000) with short intense infrared laser-pulses found characteristic x-ray emission at moderate ($\sim $10$^{15}$Wcm$^{-2})$ intensities. The ponderomotive energy of the electrons at these intensities is by far too low to create the K-shell vacancies (2 keV to 4 keV) which are the precursors of x-ray radiation. In order to investigate the mechanisms for efficient heating of quasi-free electrons in a cluster, we developed a mean-field classical transport simulation to describe the electronic dynamics. Our model takes into account the interaction of the electrons with the laser field, the build-up of an overall charge of the cluster due to electrons leaving the cluster, the resulting cluster expansion, polarization of the cluster due to collective electron motion, electron-impact ionization, and elastic scattering of the electrons by the ions inside the cluster. Elastic large-angle backscattering of electrons at ionic cores in the presence of a laser field is shown to be essential at low laser intensities. Results for the absolute x-ray yields are in surprisingly good quantitative agreement with experimental results. [Preview Abstract] |
Wednesday, May 17, 2006 1:54PM - 2:06PM |
E4.00003: Interaction of coherent VUV radiation with xenon clusters Zachary Walters, Robin Santra, Chris H. Greene When a short, intense laser pulse interacts with an atomic cluster, the atom-laser interaction is greatly enhanced by the high density of atoms within the cluster, while the finite size of the cluster prevents energy from escaping the interaction region. Thus hot, dense plasmas can form which can in turn alter the form of the laser-cluster interaction. We present a model of the laser-cluster interaction which takes atomic structure into account, using non-perturbative R-matrix techniques to calculate inverse bremsstrahlung and photoionization cross sections for Herman-Skillman atomic potentials. We describe the evolution of the cluster under the influence of the processes of inverse bremsstrahlung heating, photoionization, collisional ionization and recombination, and expansion of the cluster. We compare results with the experiments of Wabnitz {\em et al} [Nature {\bf 420}, 482 (2002)] and Laarmann {\em et al.} [Phys. Rev. Lett. {\bf 95}, 063402 (2005)]. [Preview Abstract] |
Wednesday, May 17, 2006 2:06PM - 2:18PM |
E4.00004: Electron Momentum Distributions in Short-Pulse Double Ionization -- Agreement with a Classical Model in 3D Stan Haan, Llian Breen, Armin Karim, J.H. Eberly In this talk we will present results from fully classical, three-dimensional ensemble studies of double ionization of helium. We will look at how the time delay between recollision and final double ionization leads to ejection of electrons into opposite momentum hemispheres, and will examine individual trajectories as well as ensemble behavior. We will show that our results compare favorably with experiment. [Preview Abstract] |
Wednesday, May 17, 2006 2:18PM - 2:30PM |
E4.00005: Amplitude and Phase Control of Direct Frequency Comb Spectroscopy by Atomic Filter Goran Pichler, Hrvoje Skenderovi\'{c}, Damir Aumiler, Ticiana Ban, Marin Pichler Frequency comb may be virtually seen in the direct observation of the velocity selective optical pumping of the rubidium ground state hyperfine levels induced by a train of femtosecond pulses \footnote{D. Aumiler, T. Ban, H. Skenderovi\'{c}, G. Pichler, Phys. Rev. Lett. 95, 233001 (2005)}. The accumulation effects can be easily observed by using a weak cw scanning probe laser transmitted collinearly with the femtosecond laser centered on the D2 or D1 resonance lines. However, if the high repetition frequency comb obtains phase and amplitude changes within the Rb hyperfine components, then the Doppler profile will exhibit additional changes. When all the frequency comb lines within the hyperfine structure are completely absorbed the velocity selection will be entirely removed. Experiments with Doppler broadened rubidium hyperfine lines illuminated by the femtosecond laser beam modified by another atomic filter (Rb vapor filled sapphire cell) will be presented and discussed. [Preview Abstract] |
Wednesday, May 17, 2006 2:30PM - 2:42PM |
E4.00006: Stabilizing carrier-envelope phase of Kansas Light Source Chengquan Li, Eric Moon, Zuoliang Duan, Jason Tackett, Kristan L. Corwin, Brian R. Washburn, Zenghu Chang The Kansas Light Source is a kilohertz Ti:Sapphire laser system that produces 6 fs pulses with 0.8 mJ energy. In the past, the carrier envelope phase of the laser pulses changes randomly from shot to shot. By stabilizing the carrier envelope offset frequency of the oscillator to a quarter of its repetition rate, the fast variation of carrier-envelope phase of amplified pulses can be eliminated for over three hours. There is a remaining slow carrier envelope phase drift that was measured with a colinear $f$-to-2$f$ interferometer. The phase drift was converted into a low frequency AC signal (mHz) that was sent to the phase locking electronics that controls the carrier envelope phase of oscillator. The residual drift was less than 500 mrad over 40 minutes. Variables factors that contributed to the long term phase stability are investigated. The phase stabilized pulses are used for studying attosecond pulse generation, ionization of atoms and dissociation of molecular ions at the J. R. Macdonald laboratory. [Preview Abstract] |
Wednesday, May 17, 2006 2:42PM - 2:54PM |
E4.00007: Creating inner shell vacancy by inelastic X-ray photons scattering S.A. Novikov The absolute values and shapes of the differential cross section for the process of inelastic X-ray photons scattering by neon atom accompanied by the creation of 1s-vacancy are calculated. The effects of the rearrangement of electron shells of the atomic residues in the field of creating virtual vacancies and the decay of vacancies via Auger and/or radiative channels are taken into account. Velocity form have been used for matrix elements of electromagnetic field interaction operator in both dipole and non-dipole approximations. [Preview Abstract] |
Wednesday, May 17, 2006 2:54PM - 3:06PM |
E4.00008: Using attosecond pulses to probe ultrafast electronic motions inside atoms L. A. Collins, S. X. Hu With using an efficient and accurate parallel solver for the time-dependent Schr\"odinger equation, we have performed full-dimensional numerical simulations of the proposed attosecond pump-probe for {\it exploring} the extremely fast motion of an electronic wave packet inside atoms. Pumped by a broadband femtosecond UV pulse, one electron of ground-state Helium can be launched into a superposition of low-lying excited states, thus forming a bound wavepacket oscillating relative to the atomic core. A time-delayed attosecond EUV (probe) pulse then ionizes the atom causing three-body breakup. Measuring either the energy sharing of the ionized electrons or the total ionization probability as a function of the time delay traces out the internal motion of the excited electron. Our simulations have shown that an ultrashort oscillating period of 2 $fs$ can be followed for several cylces. This opens the prospect of a wealth of similar pump-probe experiments to examine ultrafast {\em{electronic}} motions. [Preview Abstract] |
Wednesday, May 17, 2006 3:06PM - 3:18PM |
E4.00009: Ultra-Strong Field Ionization From Ne to Xe Barry Walker, Isaac Ghebregziabher, Anthony DiChiara, Sasi Palaniyappan We report the ionization of Ne, Ar, Kr and Xe in ultrastrong laser fields from 10$^{16}$ W/cm$^{2}$ to 10$^{18}$ W/cm$^{2}$. At 10$^{18}$ W/cm$^{2}$ the photoelectron has an instantaneous kinetic energy equal to 60 percent of its rest mass. All species studied, including charge states as high as Xe$^{11+}$, exhibit correlated multielectron ionization, which increases with the atomic number. High order, i.e. (e,4e), electron correlation processes in Ar, Kr, and Xe are shown to exist even between electrons in different shells with a higher yield than observed for (e,2e) correlated ionization in He under similar experimental conditions. The data is compared to a 3D relativistic tunneling ionization and ``rescattering'' model, which is a physical mechanism behind correlated, nonsequential ionization and high harmonic generation in nonrelativisitic, strong fields $<$ 10$^{16}$ W/cm$^{2}$. In rescattering the electron that is tunnel ionized by the strong laser field is driven back into the parent ion by the oscillating laser field. The model captures several features of ultrastrong field ionization, indicating that rescattering is still a prominent physical mechanism, but the details are not currently known. Possible excitation mechanisms in ultrastrong field rescattering, including the generation of inner shell holes, are discussed. [Preview Abstract] |
Wednesday, May 17, 2006 3:18PM - 3:30PM |
E4.00010: Quantum Diffusion Monte Carlo Method for strong field time dependent problems Matt Kalinski We formulate the Quantum Diffusion Quantum Monte Carlo (QDMC) method for the solution of the time-dependent Schr{\"o}dinger equation for atoms in strong laser fields. Unlike for the normal diffusion Monte Carlo the wave function is represented by walkers with two kinds or colors which solve two coupled and nonlinear diffusion equations. Those diffusion equations are coupled by the potentials similar to those introduced by Shay which must be added to Schr{\"o}dingers equation to obtain classical dynamics equivalent to the quantum mechanics [1]. The potentials are calculated semi-analytically similarly to smoothing methods of smooth particle electrodynamics (SPD) with Gaussian smoothing kernels. We apply this method to strong field two electron ionization of Helium. We calculate two electron double ionization rate in full six-dimensional configuration space quantum mechanically. Comparison with classical mechanics and the low dimensional grid models is also provided. \vspace{1cm} [1] D. Shay, Phys. Rev A {\bf 13}, 2261 (1976) [Preview Abstract] |
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