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 W3: Focus Session: Molecular Photoionization and High Harmonic Generation |
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Chair: Chii-Dong Lin, Kansas State University Room: Imperial West |
Saturday, May 29, 2010 8:00AM - 8:12AM |
W3.00001: Observing electron rearrangement in the simplest chemical reaction Agnieszka Jaron-Becker, Wen Li, Margaret Murnane, Henry Kapteyn, Andreas Becker We present the theory that analyzes the experiment in which an intense ultrashort laser pulse ionizes a dissociating bromine molecule, allowing for the real-time observation of an electron rearrangement during the break-up of a chemical bond. Our results for alignment dependent ionization yields as a function of time during the dissociation as well as the kinetic energy release of the ions agree well with the experimental data. Angular dependent ionization yields strongly depend on the nature of the orbital that has been ionized. This allows us to identify how the changes in the experimental results correspond to the competition of contributions from several dynamically changing valence orbitals. The angular dependent ionization yields also serve as sensitive test for defining the moment of the break-up of the molecular bond. We find that the system remains molecular-like for a much longer time than expected and the electrons do not localize onto the individual atoms until the fragments are quite far apart, in a region where the potential energy curve for the dissociation is essentially flat, and where there is negligible wave function overlap for the two atoms. [Preview Abstract] |
Saturday, May 29, 2010 8:12AM - 8:42AM |
W3.00002: How Initial State Correlation Controls Sequential Laser Tunnel Ionization Invited Speaker: The description of multiphoton ionization by laser-induced tunnelling emerged in the 1960's. Since then, numerous experiments have probed sequential multiphoton ionization, especially in rare gas atoms. Aside from re-collision, each successive ionization stage appeared to loose memory of previous one. We\footnote{P. B. Corkum, A Fleischer, H. W\"{o}rner, L. Liu, D. Villeneuve and A. Staudte} show that, unobserved in all of these studies has been a strong space-time correlation between the two ionized electrons. The correlation arises because each electron must pass through a narrow laser-imposed momentum filter. In many systems this momentum filter only allows a localized hole in the electronic distribution of the atom to be created. A second electron is sensitive to the hole left from the previous stage. We show that the directional entanglement of the continuum electrons evolves with time, driven by spin-orbit coupling in the intermediate ion. Our experiment (on Ne, Ar and HCl) opens a new path to studying multielectron correlations in complex systems -- both in the time domain, (which we demonstrate) or in the frequency domain. [Preview Abstract] |
Saturday, May 29, 2010 8:42AM - 8:54AM |
W3.00003: Bragg Diffraction High Harmonic Spectroscopy Joseph Farrell, Limor Spector, Brian McFarland, Philip Bucksbaum, Markus G\"uhr, Mette Gaarde, Kenneth Schafer High harmonic generation (HHG) spectra contain information about the electronic structure of the generation medium which has proven to be powerful for monitoring molecular and atomic ground states. All HHG experiments on excited atomic or molecular states suffer from a ground state harmonic background, thereby reducing excited state sensitivity. We use a Bragg diffraction scheme to overcome this problem and also obtain spectrally resolved high harmonics without the need for a spectrometer. We imprint a 400nm excited state grating structure on the HHG medium by two counterpropagating 800nm pulses. A strongly focused 800nm probe pulse hits the grating under a shallow angle. The harmonics of order n are scattered into the Bragg angle $\theta_{Bragg}=sin^{-1}(1/n)$. We test the scheme with plasma gratings in argon gas and molecular alignment gratings in $N_2$. The generated harmonics are scattered into their respective Bragg angle and we observe up to 6 spectrally resolved odd harmonics that show enhanced sensitivity to the atomic or molecular excitation. [Preview Abstract] |
Saturday, May 29, 2010 8:54AM - 9:06AM |
W3.00004: High Harmonic Generation in Sulfur Dioxide Limor S. Spector, Joseph P. Farrell, Brian K. McFarland, Philip H. Bucksbaum, Markus Guehr We show results of high harmonic generation (HHG) on the triatomic molecule sulfur dioxide. This molecule is one of the simplest systems having a conical intersection (CI), a region of electron degeneracy in which the Born-Oppenheimer approximation breaks down. A CI of particular interest exists between the excited electronic $^{1}$B$_{1}$ and $^{1}$A$_{2}$ states in the sulfur dioxide molecule. This CI can be reached via a direct Frank-Condon transition from the electronic ground state. The ionization and recombination steps of HHG render HHG a sensitive probe of electronic structure. HHG thus allows us to monitor electronic symmetry changes within the CI region, which has been shown to contain contributions from multiple orbitals [McFarland et al. \textit{Science} 2008]. We spectrally resolve the high harmonics of the ground and excited states of sulfur dioxide to characterize the CI, paving the way for a more complete understanding of non-Born-Oppenheimer dynamics. [Preview Abstract] |
Saturday, May 29, 2010 9:06AM - 9:36AM |
W3.00005: Extraction of Fixed-in-Space photoionization cross section and phase with high-order harmonic generation from aligned molecules Invited Speaker: It has been known for over two decades that high-order harmonic generation (HHG) process occurs when an electron released earlier from an atom/molecule by an intense laser field returns to recombine with the parent ion. This relation between photo-recombination (or its time-reverse, photoionization process) and HHG has been firmly established on the quantitative level recently by the quantitative rescattering theory (QRS) [1]. According to the QRS, HHG signal can be expressed as a product of a returning electron wave packet and the \textit{laser-free} photo-recombination differential cross section. The QRS has been carefully tested against available exact numerical solutions of the time-dependent Schroedinger equation. Here we report comparisons with recent experimental data for magnitude, phase, polarization state, and ellipticity of the emitted harmonics for aligned molecules, from which the molecular frame photoionization cross sections and phases can be probed in great details [2]. We also address the issue about the contribution from inner molecular orbitals [3,4]. Finally, we will discuss the possibility for future dynamic chemical imaging with femto-second temporal resolution on an example of probing molecular dynamics of vibrationally excited N$_{2}$O$_{4}$ [5]. In collaboration with C.D. Lin, R.R. Lucchese, T. Morishita, C. Jin.\\[4pt] [1] A.T. Le \textit{et al}, Phys. Rev. A. \textbf{80}, 013401 (2009).\\[0pt] [2] X. Zhou \textit{et al}, Phys. Rev. Lett. \textbf{102}, 073902 (2009).\\[0pt] [3] B.K. McFarland \textit{et al}, Science \textbf{322}, 1232 (2008).\\[0pt] [4] O. Smirnova \textit{et al}, Nature \textbf{460}, 972 (2009).\\[0pt] [5] W. Li \textit{et al}, Science \textbf{322}, 1207 (2008). [Preview Abstract] |
Saturday, May 29, 2010 9:36AM - 9:48AM |
W3.00006: Time Reversing High Harmonic Generation: Towards table top XUV spectroscopy Carlos Trallero-Herrero, Bruno Schmidt, Andrew Shiner, Jean-Claude Kieffer, Paul Corkum, Francois Legare, David Villeneuve We produce high harmonics in atoms and molecules with a few cycle 1800nm source. Due to the long wavelength of the driving field we are able to extend the maximum energy of the emitted XUV photons in low ionization potential atoms to reveal new features in the harmonic structure. In molecules, due to the small energy spacing between harmonics and the few cycle nature of our laser source, we are able to distinguish very sharp features in the harmonic spectrum. Such features can be traced back to the recombination dipole moment which is a time reversal of the photo-ionization dipole. This assumption is confirmed by direct comparison with the photo-ionization cross section. [Preview Abstract] |
Saturday, May 29, 2010 9:48AM - 10:00AM |
W3.00007: High order harmonic generation in a strongly-driven periodic solid Shambhu Ghimire, Anthony DiChiara, Emily Sistrunk, Louis Dimauro, Pierre Agostini, David Reis We present results of high order harmonic generation from the wide band gap (3.2eV) semiconductor ZnO driven by mid-infrared (0.34 eV) laser pulses in the strongly non-perturbative limit where the field approaches a band-gap per lattice constant. We measure harmonics up to 17$^{th}$ order\textit{---extending well above the band edge}. The spectrum comprises odd harmonics when the electric field is polarized along a centrosymmetric orientation and both odd and even harmonic when polarized along a non-centrosymmetric orientation. We present a simple semi-classical model for emission based on the nonlinear driven oscillation of laser-induced carriers in a periodic lattice potential and discuss its limitations. In addition, we study the laser-dressed band structure using dynamical absorption of a time delayed frequency doubled Ti: Sapphire (3.0 eV) probe. [Preview Abstract] |
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