Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session R5: Ultrafast Atomic and Molecular Dynamics |
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Chair: Thomas Weinacht, Stony Brook University Room: Arboretum I-III |
Friday, May 28, 2010 10:30AM - 10:42AM |
R5.00001: Femtosecond time-resolved study of the dissociation of small molecules using a two-color vacuum ultraviolet pump and x-ray probe technique. A. Belkacem, T. Allison, C. Khurmi, T. Wright, A. Stooke We developed a unique two-color ultraviolet (UV) pump and extreme ultraviolet (EUV) probe capability to study molecular dissociation and non-adiabatic molecular dynamics of small to complex molecules excited in the UV regime. This capability revolves around the development of a very high intensity high harmonics source in combination with a split-mirror technique. The pump-probe delay has an interferometric stability of better than 100 attoseconds. We used this system to probe the femtosecond internal conversion of excited ethylene, water and oxygen molecules pumped with the 5th harmonic ($\sim$7.75 eV) and probed with the 19th harmonic ($\sim$29.45 eV). The results of these measurements will be presented. [Preview Abstract] |
Friday, May 28, 2010 10:42AM - 10:54AM |
R5.00002: Femtosecond time-resolved EUV photoionization studies of ultrafast dynamics in helium nanodroplets Oliver Gessner, Oleg Kornilov, Oliver Buenermann, Stephen Leone, Daniel Neumark By combining a femtosecond high harmonic generation (HHG) light source with state-of-the art photoelectron and ion imaging techniques, a new set of tools has been created to probe electronic and nuclear dynamics in EUV excited atoms, molecules and clusters in real-time. The new technique has been applied to the study of electronic and nuclear dynamics of electronically excited helium nanodroplets (He$_{N}$, $<$N$>\sim $2x10$^{6})$. Femtosecond time-resolved photoelectron imaging experiments reveal two relaxation timescales, 280 fs and 2.8 ps, that we associate with an intraband transition and relaxation between two separate electronic bands, respectively. Rapid emission of Rydberg atoms is indicated by transient photoelectron spectra and preliminary results from femtosecond time-resolved ion imaging experiments. Mass- and momentum-resolved transient ion spectra reveal complex dynamics in the production of electronically excited cluster fragments. The observations are compared with first results of ab-initio calculations on the electronically excited states of the parent cluster and ionization dynamics of cluster fragments. [Preview Abstract] |
Friday, May 28, 2010 10:54AM - 11:06AM |
R5.00003: All-active-electron nonadiabatic time-dependent calculations on diatomics using MCTDHF Daniel Haxton, Thomas Rescigno, C. William McCurdy Calculations on energies, properties, and excitations of various diatomic molecules in a non-Born-Oppenheimer framework are presented. These calculations use the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method and are performed in the nonorthogonal coordinate system of prolate spheroidal coordinates for the electrons, and the bond length R. An interpolating polynomial basis is used in the electronic and nuclear degrees of freedom, and the wavefunction is expanded in terms of slater determinants of electronic orbitals times the full primitive basis in R. Ionization is accounted for by complex scaling of the radial prolate spheriodal variable; dissociation may be similarly treated. We present calculations aimed towards describing pump-probe experiments within an all-active-electron framework, in which we prepare ground states, excite with a pulse and then extract spectral information via further propagation of the wavefunction. [Preview Abstract] |
Friday, May 28, 2010 11:06AM - 11:18AM |
R5.00004: \textit{Ab Initio} 6D Treatment of the Time-Evolution Multiphoton Dynamics for Two-Electron Systems in Few-Cycle XUV Laser Pulses John Heslar, Shih-I Chu We present a 6D \textit{time-dependent generalized pseudospectral} (TDGPS) approach in \textit{spherical coordinates} for fully \textit{ab initio} nonperturbative treatment of multiphoton dynamics of atomic systems in intense laser fields. Based on the generalized pseudospectral method, an optimal space discretization is developed for the radial and angular grids in spherical coordinates. The procedure is applied to the investigation of high-order-harmonic generation (HHG) of helium atoms in intense ultrashort laser pulses. The 6D time-dependent Schrodinger equation equations are discretized and solved efficiently and accurately by means of the TDGPS method. This method is accurate and efficient and beyond the TD-hyperspherical coordinate methods. We extend the new approach to the nonperturbative treatment of double ionization of the helium atom in intense high-frequency xuv laser pulses. [Preview Abstract] |
Friday, May 28, 2010 11:18AM - 11:30AM |
R5.00005: Self-Imaging of aligned molecules from the diffraction spectra by laser-induced rescattering electrons Junliang Xu, Zhangjin Chen, Anh-Thu Le, Chii-Dong Lin According to the quantitative rescattering (QRS) theory, the high-energy Above-threshold-ionization (HATI) electron spectra from a molecule M can be used to extract the differential cross section (DCS) at large scattering angles for e- + M+ collisions. Using mid-infrared (MIR) lasers the rescattering electrons can reach energies from 50 eV to a few hundred eV's where the independent atom model (IAM) is accurate enough to describe the DCS for large scattering angles. Within this model, we show that the interatomic separations can be accurately retrieved from the DCS, thus paving the way for using HATI spectra for time-dependent gaseous electron diffraction (TDGED) of transient molecules. [Preview Abstract] |
Friday, May 28, 2010 11:30AM - 11:42AM |
R5.00006: Study of electron diffraction structure of atomic targets from laser-induced photoelectron momentum spectra D. Ray, S. De, Z. Chen, A.T. Le, C.D. Lin, C.L. Cocke, M.F. Kling We measure the full vector momentum spectra of high energy photoelectrons from atomic targets (Xe, Ar and Ne) generated by intense laser pulses. Prior experimental investigations and comparison with Quantitative Rescattering Theory$^{[1]}$ had confirmed that accurate elastic differential scattering cross-sections can be retrieved from electrons rescattered with maximum energy of 10Up at different peak intensities. Present studies show that information about the target structure, namely elastic differential cross-sections for the scattering of free electrons from the corresponding ionic core, can be retrieved over a range of energies, from 4Up up to 10Up, independent of the peak intensity at which the photoelectron spectra have been measured. \\[4pt] [1] Z. Chen, A.T. Le, T. Morishita, and C.D. Lin, PRA 79, 033409 (2009) [Preview Abstract] |
Friday, May 28, 2010 11:42AM - 11:54AM |
R5.00007: Clusters in Intense XUV ultrafast pulses: effects of cluster size on expansion dynamics and ionization Edward Ackad, Nicolas Bigaouette, Lora Ramunno Recent experiments in the XUV are probing the expansion of clusters. Of primary interest is the expansion of the target during the pulse. XUV- cluster interactions are dominated by atomic processes, such as photo-ionization. We present results using our many body molecular dynamics simulations of clusters in intense XUV pulses which includes augmented collisional ionization processes. The increase in emergence probability of high charge states with respect to cluster size is analyzed using our model. The detailed expansion of clusters throughout the laser pulse and at much later times is explained, for clusters spanning two orders of magnitude in size. Understanding the expansion of clusters is a first step in being able to image biomolecules in ultra intense laser pulses. [Preview Abstract] |
Friday, May 28, 2010 11:54AM - 12:06PM |
R5.00008: Atomic and nuclear coherence excited by optical pulses Yuri Rostovtsev Recent progress in ultrashort, e.g. attosecond, laser technology allows researchers nowdays to obtain ultra-strong fields which can be the same order of magnitude as the electric field created by an atomic nucleus. Interaction of such strong and broadband field with atomic systems even under the action of a far-off resonance strong pulse of laser radiation should be revisited and as we have shown that such pulses can excite remarkable coherence on high frequency transitions. We have found and analyzed analitical solutions for various pulse shapes. We discuss possible applications of obtained results to cooperative generation of XUV and nuclear radiation. For a sample with density of atoms or ions of the order of $10^{16}-10^{19}$ cm$^{-3}$, the density of excited atoms is $10^2-10^5$ cm$^{-3}$, and all nuclei, that are coherently oscillating in phase produce cooperative burst of gamma ray radiation that is 10-1000 time faster that the relaxation time, 100 ns for $^{57}$Fe. [Preview Abstract] |
Friday, May 28, 2010 12:06PM - 12:18PM |
R5.00009: Femtosecond Coherent Dynamics of Bromine Molecules in Clathrates Zoe-Elizabeth Sariyanni, V. Ara Apkarian We simulate the dynamics of vibrational coherence in the excited electronic states of bromine molecules enclosed in clathrate hydrates. The molecules are probed by three femtosecond pulses to induce Raman coherence among the vibrational modes of the first excited electronic level and generate Anti-Stokes signal. The theoretical results are compared with the experimental data. [Preview Abstract] |
Friday, May 28, 2010 12:18PM - 12:30PM |
R5.00010: Rovibrational Wave-Packet Dispersion during Femtosecond Laser Filamentation in Air Johanan Odhner, Dmitri A. Romanov, Robert J. Levis An impulsive, femtosecond filament-based Raman technique producing high quality Raman spectra over a broad spectral range (1554.7--4155 cm) is presented. The filamentation dynamics of a loosely focused high-energy femtosecond pulse in air shortens the pulse, leading to impulsive excitation of all vibrational and rotational Raman-active modes present in the filament channel. For particular Raman modes, impulsive excitation requires pulse duration on the order of the characteristic oscillation periods of the mode, thus revealing the degree of pulse-shortening. The temperature of gas-phase molecules can be recovered by measuring the dispersion of the impulsively excited vibrational wave-packets using a delayed narrow-band pulse. This technique reveals that the initial rovibrational temperature in the filament is 300 K for both N$_{2}$ and O$_{2}$. The temperature-dependent wave-packet dynamics are interpreted using an analytic anharmonic oscillator model. The wave-packets reveal a 1/e dispersion time of 3.9 ps for N$_{2}$ and 2.8 ps for O$_{2}$. Pulse self-shortening up to 8 fs temporal features within the filament is directly measured by impulsive vibrational excitation of H$_{2}$. [Preview Abstract] |
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