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 S3: Strong Field (Intense Laser) Processes I |
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Chair: Mette Gaarde, Louisiana State University Room: Knoxville Convention Center 301D |
Friday, May 19, 2006 8:00AM - 8:12AM |
S3.00001: Ellicity dependence of Harmonic Generation in $\rm \bf C_{60}$ Guoping Zhang, Thomas F. George C60 shows a large off-resonant third harmonic generation. Its higher harmonic generation is of great interest both scientifically and technologically. We investigate how the laser ellipticity affects the harmonic generation. For the first time, we demonstrate a clear interference effect in the harmonic generation. This clarifies the roles of laser pulse duration and laser intensity on the ellipticity dependence. This should have a profound impact on the future attosecond tomography. This work was supported by ARO. G. P. Zhang, Phys. Rev. Lett. {\bf 95}, 047401 (2005). [Preview Abstract] |
Friday, May 19, 2006 8:12AM - 8:24AM |
S3.00002: Three-Step Model for High-Harmonic Generation in Many-Electron Systems Robin Santra, Ariel Gordon The three-step model (TSM) of high-harmonic generation (HHG) is generalized to atomic and molecular many-electron systems. Using many-body perturbation theory, corrections to the standard TSM due to exchange and electron--electron correlations are derived. It is shown that canonical Hartree-Fock orbitals represent the most appropriate set of one-electron states for calculating the HHG spectrum. To zeroth order in many-body perturbation theory, a HHG experiment allows direct access in general to a combination of occupied Hartree-Fock orbitals rather than to the highest occupied molecular orbital by itself. [Preview Abstract] |
Friday, May 19, 2006 8:24AM - 8:36AM |
S3.00003: Alignment dependence of high-order harmonic generation from CO$_{2}$ Anh-Thu Le, Xiao-Min Tong, C. D. Lin High-order harmonic generation (HHG) from aligned CO$_{2}$ is studied within framework of the strong-field approximation (SFA). The results are in qualitative agreements with recent pump-probe experiments. The experimentally observed inverted modulation in HHG signals as a function of pump-probe delay time has been attributed to the quantum interference from the two oxygen centers. Our results, however, indicate that this is not necessary for the inverted modulation alone. The angular dependence of the HHG and the evolution of the HHG yield as functions of delay time are influenced strongly by the depletion of the ground state and, therefore, are sensitive to the probe laser intensity. [Preview Abstract] |
Friday, May 19, 2006 8:36AM - 8:48AM |
S3.00004: Phase Dependence of Molecular High Order Harmonic Generation at Long-Range Andre D. Bandrauk, Samira Barmaki, Gerard Lagmago-Kamta The physics of high-order harmonic generation in single-electron molecular ions at long-range(large molecular internuclear distances ) is compared for the two proton system H2+ [1] vs the three proton system H(+)-H2(+) via exact numerical solutions of the 3-D time-dependent Schroedinger equation in intense laser fields (I~ 10**14 W/cm2).These numerical solutions show that photon energies can be produced exceeding the atomic cut-off law Ip + 3.17 Up, where Ip is the ionization potential and Up the ponderomotive energy [2]. It is shown that electron recollision with the parent ion and collision with neighboring ions can be controlled by the absolute phase(carrier envelope phase)of a few cycle ultrashort laser pulse.Furthermore the new cut-off law is a sensitive function of internuclear distance thus allowing in principle for imaging of electron states in molecules separated by large internuclear distances R~ 30-50 a.u. [1] A D Bandrauk,H Yu, J Phys B 31,4243(1998) [2] A D Bandrauk,H Yu, E Constant,Phys Rev A 56,2537(1997) [Preview Abstract] |
Friday, May 19, 2006 8:48AM - 9:00AM |
S3.00005: Observation of Intra-molecular Vibrational Dynamics using High-Harmonic Generation as a Probe Nick Wagner, Andrea W\"uest, Margaret Murnane, Henry Kapteyn Here we report one of the first observations of intramolecular dynamics using electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF$_{6}$ using impulsive Raman scattering with a short laser pulse. A second, more-intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of $\sim $20-50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all the Raman-active modes of SF$_{6}$, with an asymmetric breathing mode most visible. This is in contrast to conventional impulsive stimulated Raman spectroscopy where only the symmetric breathing mode of the molecule is easily observed. The data also show evidence of relaxation dynamics following impulsive excitation of the molecule. Our results indicate that high harmonic generation is a sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Since the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e. $\sim $1.5 {\AA}, small changes in the shape of the molecule lead to large changes in the high harmonic yield. [Preview Abstract] |
Friday, May 19, 2006 9:00AM - 9:12AM |
S3.00006: High harmonic generation from ions in a capillary discharge David M. Gaudiosi, Brendan Reagan, Tenio Popmintchev, Michael Grisham, Mark Berrill, Oren Cohen, Barry C. Walker, Margaret M. Murnane, Henry C. Kapteyn, Jorge J. Rocca We demonstrate a significant extension of the high harmonic cutoff observed in xenon, up to 150 eV, by generating harmonics from ions in a capillary discharge plasma. Usually, the highest harmonics that can be generated from an atom or ion are limited by ionization-induced defocusing, beam attenuation of the laser, and a lack of phase matching of the frequency conversion process. Using a pre-ionized plasma generated by the capillary discharge, we dramatically reduce ionization-induced defocusing and energy loss of the driving laser due to ionization, allowing higher photon energies to be generated from xenon ions. We also demonstrate enhancement of the harmonic flux of nearly two orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a capillary discharge plasma for high harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths. [Preview Abstract] |
Friday, May 19, 2006 9:12AM - 9:24AM |
S3.00007: Effective one-electron approaches to calculate high harmonic generation Nina Rohringer, Robin Santra The single-active electron approach (SAE) is frequently applied to calculate high harmonic generation in atoms and consists in solving a one-particle Schr\"odinger equation in an appropriate model potential. As an ad hoc approach it is difficult to be systematically improved. Starting with the time-dependent configuration interaction singles (TDCIS) technique we derive a new class of effective one-electron approaches. The resulting one-electron equations are in general non-local and non-unitary. A local approximation to TDCIS can be derived by restricting the total many-body Hamiltonian to a local mean-field Hamiltonian (those usually used in SAE calculations). The resulting equations are similar to traditional SAE approaches but include an additional term which destroys the unitarity of the time-evolution. We show that this correction term is essential and improves on traditional SAE approaches. Numerical tests show that this improved SAE method gives dipole-moments in better agreement with exact results than time-dependent Hartree Fock. The test system is a one-dimensional model of helium which allows for a straightforward numerical solution and therefore provides a benchmark to assess the quality of the different approximations. [Preview Abstract] |
Friday, May 19, 2006 9:24AM - 9:36AM |
S3.00008: Quantum origin of plateau structures in strong laser-atom processes M.V. Frolov, N.L. Manakov, A.A. Khuskivadze, Anthony F. Starace The characteristic plateau structures in the high-energy spectra of strong laser-atom phenomena (such as ATI and HHG) are shown to originate from the properties of the exact wave function of an initially bound electron subjected to a strong monochromatic laser field of frequency $\omega$. Using time-dependent effective range theory [1], we obtain the spatial dependence of the Fourier-harmonic components (for frequencies $n\omega$) of the quasistationary, quasienergy state (QQES) corresponding to the initial bound state. This dependence reveals a remarkable plateau-like behavior over spatial distances up to the amplitude of free-electron oscillations in the laser field. This behavior is typical for Fourier-harmonics with energies $n\hbar\omega$ up to 3.17$U_{p}$ (where $U_{p}$ is the ponderomotive energy). Our numerical analysis of ATI and HHG spectra demonstrates that the high-energy plateaus in these spectra (which have been interpreted in terms of the semiclassical rescattering scenario) originate from the above-mentioned plateau features of the Fourier-harmonics of the QQES wave function. [1] M.V. Frolov et al., \textit{Phys. Rev. Lett.} \textbf{91}, 053003 (2003). [Preview Abstract] |
Friday, May 19, 2006 9:36AM - 9:48AM |
S3.00009: Coherent states of the periodically driven Rydberg atom Luz Vianey Vela-Arevalo A methodology to calculate generalized coherent states for a periodically driven system is presented. We study wave packets constructed as a linear combination of suitable Floquet states of the three-dimensional Rydberg atom in a microwave field. The driven coherent states show classical space localization, spreading and revivals, and remain localized along the classical trajectory. The microwave strength and frequency have a great effect in the localization of Floquet states, since quasienergy avoided crossings produce delocalization of the Floquet states, showing that tuning of the parameters is very important. Using wavelet based time-frequency analysis, the classical phase space structure is determined, which allows us to show that the driven coherent state is located in a large regular region in which the z coordinate is in resonance with the external field. The expectation values of the wave packet show that the driven coherent state evolves along the classical trajectory. [Preview Abstract] |
Friday, May 19, 2006 9:48AM - 10:00AM |
S3.00010: Creating more stable molecular dications in strong laser fields X. Wang, C. Guo Metastable channel of some doubly ionized molecules were not commonly seen in previous strong-field experiments at visible wavelength region, but were readily observable using single high-energy-photon/electron excitation. In this work, we perform a study to understand the experimental conditions of obtaining more metastable dictations in strong laser fields. Our results show that electron collective motion plays a role in the ionization and dissociation dynamics. [Preview Abstract] |
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