Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session Q6: Ultra-Fast Probing of Molecular Wave Functions |
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Chair: Tom Weinacht, SUNY Stony Brook Room: Minor Hall 125 |
Friday, May 22, 2009 8:00AM - 8:30AM |
Q6.00001: Time and Space Resolved High Harmonic Imaging of Electron Tunnelling from Molecules Invited Speaker: High harmonic generation in intense laser fields carries the promise of combining sub-Angstrom spatial and attosecond temporal resolution of electronic structures and dynamics in molecules, see e.g. [1-3]. High harmonic emission occurs when an electron detached from a molecule by an intense laser field recombines with the parent ion [4]. Similar to Young's double-slit experiment, recombination to several ``lobes'' of the same molecular orbital can produce interference minima and maxima in harmonic intensities [1]. These minima (maxima) carry structural information -- they occur when the de-Broglie wavelength of the recombining electron matches distances between the centers. We demonstrate both theoretically and experimentally that amplitude minima (maxima) in the harmonic spectra can also have dynamical origin, reflecting multi-electron dynamics in the molecule. We use high harmonic spectra to record this dynamics and reconstruct the position of the hole left in the molecule after ionization. Experimental data are consistent with the hole starting in different places as the ionization dynamics changes from tunnelling to the multi-photon regime. Importantly, hole localization and subsequent attosecond dynamics are induced even in the tunnelling limit. Thus, even ``static'' tunnelling induced by a tip of a tunnelling microscope will generate similar attosecond dynamics in a sample. We anticipate that our approach will become standard in disentangling spatial and temporal information from high harmonic spectra of molecules.\\[4pt] In collaboration with Serguei Patchkovskii, National Research Council, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada; Yann Mairesse, NRC Canada and CELIA, Universit\'e Bordeaux I, UMR 5107 (CNRS, Bordeaux 1, CEA), 351 Cours de la Lib\'eration, 33405 Talence Cedex, France; Nirit Dudovich, NRC Canada and Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel; David Villeneuve, Paul Corkum, NRC Canada; and Misha Yu. Ivanov, NRC Canada and Imperial College of Science, Technology and Medicine, London SW7 2BW, United Kingdom. \\[4pt] [1] Lein, M., \emph{et al}. \emph{Phys. Rev. Lett.} {\bf 88}, 183903 (2002).\\[0pt] [2] Itatani, J. \emph{et al}. \emph{Nature} {\bf 432}, 834 (2004).\\[0pt] [3] Baker, S. \emph{et al} Science {\bf 312}, 424 (2006).\\[0pt] [4] Corkum, P. B.\emph{Phys. Rev. Lett.} {\bf 71}, 1994 (1993). [Preview Abstract] |
Friday, May 22, 2009 8:30AM - 9:00AM |
Q6.00002: Theory of Dynamic Chemical Imaging with infrared laser Pulses Invited Speaker: When an infrared laser pulse interacts with atoms or molecules, electrons which are released from the target earlier may be driven back by the laser field to recollide with the target ions. These recollisions incur the well-known strong field phenomena of high-order harmonic generation (HHG) as well as high-energy above-threshold ionization (HATI) photoelectrons. Since infrared laser pulses with duration of a few femtoseconds are now widely available, it is clear that HHG and HATI spectra can be used to extract the structural change of a transient molecule with temporal resolution of a few femtoseconds in a typical pump-probe setup. Based on the rescattering concept, recently we have established a quantitative rescattering theory (QRS) where the HHG yield can be expressed as the product of a returning electron wave packet with the photo-recombination cross section, and the HATI spectra can be expressed as the product of a similar wave packet with the elastic differential cross sections (DCS) between the target ion with free electrons. The QRS has now been applied to compare with experimental HHG spectra from molecules, from which molecular frame photoionization cross sections and the phases can be extracted. The QRS has also been applied to study the HATI spectra extensively, to retrieve the DCS of electron-ion collisions. Using the extracted DCS for electron energies in the typical range of the returning electrons, we have shown that the target structure can be retrieved. From the returning electron wave packet, we can also obtain the laser parameters, including pulse duration, peak intensity and carrier-envelope-phase, from the measured HATI spectra. [Preview Abstract] |
Friday, May 22, 2009 9:00AM - 9:30AM |
Q6.00003: Multi-Orbital contributions in High Harmonic Generation Invited Speaker: The high harmonic spectrum generated from atoms or molecules in a strong laser field contains information about the electronic structure of the generation medium. In the high harmonic generation (HHG) process, a free electron wave packet tunnel-ionizes from the molecular orbital in a strong laser field. After being accelerated by the laser electric field, the free electron wave packet coherently recombines to the orbital from which is was initially ionized, thereby emitting the harmonic spectrum. Interferences between the free electron wave packet and the molecular orbital will shape the spectrum in a characteristic way. These interferences have been used to tomographically image the highest occupied molecular orbital (HOMO) of N$_2$ [1]. Molecular electronic states energetically below the HOMO should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. We have observed evidence of HHG from multiple orbitals in aligned N$_2$ [2]. The tunneling ionization (and therefore the harmonic generation) is most efficient if the orbital has a large extension in the direction of the harmonic generation polarization. The HOMO with its $\sigma_g$ symmetry therefore dominates the harmonic spectrum if the molecular axis is parallel to the harmonic generation polarization, the lower bound $\pi_u$ HOMO-1 dominates in the perpendicular case. The HOMO contributions appear as a regular plateau with a cutoff in the HHG spectrum. In contrast, the HOMO-1 signal is strongly peaked in the cutoff region. We explain this by semi-classical simulations of the recombination process that show constructive interferences between the HOMO-1 and the recombining wave packet in the cutoff region. The ability to monitor several orbitals opens the route to imaging coherent superpositions of electronic orbitals. \newline [1] J. Itatani \emph{et al.}, Nature \textbf{432}, 867 (2004)\newline [2] B. K. McFarland, J. P. Farrell, P. H. Bucksbaum and M. G\"uhr, Science \textbf{322}, 1232 (2008) [Preview Abstract] |
Friday, May 22, 2009 9:30AM - 10:00AM |
Q6.00004: Molecular dynamics probed using high harmonic generation and strong field ionization Invited Speaker: The attosecond time-scale electron recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We show that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we show that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration, but populates the first excited state at the inner turning point. This state switching mechanism is manifested as bursts of high harmonic light emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals. We also probe the electron rearrangement in a chemical reaction using strong field ionization. Electronic dynamics play the central role in a chemical reaction. It is extremely desirable for a detection technique to have the capability of probing the ultrafast electronic motion. Time resolved photoelectron spectroscopy can provide some insights. However, the most direct information of electronic dynamics such as the electron configurational change remains elusive. Recently, strong field ionization has been demonstrated to probe the static electron density distribution of the HOMO orbital. We report the preliminary results of applying strong field ionization as a probe to study the electron rearrangement in the photodissociation of bromine. This technique provides more complete information on reaction dynamics and is very promising of making \textit{movies} of chemical reactions in atomic level. [Preview Abstract] |
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