Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W19: Ultrafast Dynamics using X-rays and Electrons |
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Sponsoring Units: DCP Chair: Phil Bucksbaum, Stanford University Room: Colorado Convention Center 104 |
Thursday, March 8, 2007 2:30PM - 3:06PM |
W19.00001: Rydberg electrons spy conformational dynamics of hot molecules Invited Speaker: The observation of structural dynamics of flexible molecules at high temperatures is arguably one of the most challenging problems of molecular dynamics. We succeeded in observing conformational dynamics by using electrons in Rydberg orbits as spies of the molecular structure. The time-resolved photoionization from the Rydberg states, providing a purely electronic spectrum that serves to characterize the molecular structure, allows us to follow the molecular motions in real time. The internal rotation about carbon-carbon bonds affords the unsaturated hydrocarbon chain molecules N,N-dimethyl-2-butanamine (DM2BA) and N,N-dimethyl-3-hexanamine (DM3HA) an opportunity to assume multiple conformeric structures. We explore the equilibrium compositions and the dynamics of transitions between such structures. An ultrashort laser pulse rapidly increases the molecule's internal energy and changes the potential energy landscape. The molecules respond by adjusting their shape, i.e. by converting between conformeric molecular structures. For DM2BA at a total internal energy of 1.79 eV, the time constants for interconversion between conformers are 19 ps and 66 ps, respectively. In DM3HA, the respective time constants are 23 and 41 ps. Comparison with a calculated conformational energy landscape reveals the conformeric forms of DM2BA involved in the molecular shape transformation. Thus, for the first time a time-resolved and quantitative view of the conformational dynamics of a flexible hydrocarbon chain at high temperature is revealed. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W19.00002: Attosecond Electron Interferometry Invited Speaker: Attosecond optical pulse generation is electron interferometry. Quantum mechanical tunnelling in an intense laser field splits the electron. After tunnelling, one component of the electron wave function is accelerated away from the ion by the laser field, but returns once the field reverses its sign. The other component remains bound to the ion. These two paths form the two arms of the interferometer. When the two components of the electron wave function overlap, they interfere. The interference leads to an oscillating dipole that produces attosecond optical pulses and simultaneously images molecular orbitals. Interferometry allows sub-wavelength changes in the length of one arm to be measured relative to another. Using rotational wave packets, we show that the high harmonics are very sensitivity to very small molecular motion and local fields. We adapt transient grating spectroscopy with two or more grating elements to observe phase changes to the high harmonics as we rotate a molecule. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W19.00003: Ultrafast Transient Absorption and Photoelectron Spectroscopy with High Order Harmonics Invited Speaker: Laser-produced high order harmonics are used to probe chemical dynamics of atoms and molecules on femtosecond timescales. Two basic methods are developed, ultrafast transient absorption and photoelectron spectroscopy. The high order harmonics are produced with an 800 nm Ti:sapphire laser focused into a capillary or rare gas jet. Both inner shell core levels and outer shell valence states are investigated. The transient absorption of xenon ions produced by high field ionization of neutral xenon atoms is probed by core level spectroscopy. The alignment of the vacancy created in forming the ion is measured as a function of pump-probe delay by promotion of an inner d electron to the vacancy in the outer shell. Small molecules are excited to repulsive dissociative states and individual harmonics are used to obtain time-resolved photoelectron spectra. A wave packet on the dissociative state of bromine molecules is detected, as well as the production of atoms at longer time delays. By the use of velocity map imaging, the angular distributions of outgoing photoelectrons are analyzed. In a new experimental system, carrier-envelope phase-stabilized few-cycle pulses will be used to create attosecond pulses of the high order harmonics, to study electronic-time-scale processes in atoms and molecules. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W19.00004: Observation of Intra-molecular Dynamics using High-Harmonic Generation as a Probe Nick Wagner, Xibin Zhou, Wen Li, Robynne Hooper, Margaret Murnane, Henry Kapteyn We report two observations of intramolecular dynamics using electrons rescattered during the process of high-order harmonic generation. In the first experiment, we excite coherent vibrations in SF$_{6}$ using impulsive Raman scattering. A second, more-intense pulse generates high-order harmonics from the excited molecules, at wavelengths of 20-50nm. The 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 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 harmonic generation is a very sensitive probe of vibrational dynamics, yielding more information simultaneously than conventional ultrafast spectroscopies. In our second experiment, we dissociate CF$_{3}$I with a 266nm pulse, and monitor the dissociation by probing high harmonic emission from the intact and dissociated molecule. Since the de Broglie wavelength of the recolliding electron is on the order of interatomic distances ($\sim $1.5{\AA}), small changes in the shape of the molecule lead to large changes in the high harmonic yield. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W19.00005: Ultrafast x-ray pulses emitted from a liquid mercury laser target Christopher Laperle, Christian Reich, Brian Ahr, Xiaodi Li, Frank Benesch-Lee, Christoph Rose-Petruck We report the generation of ultrashort, hard x-ray pulses from a liquid mercury target irradiated by 5-kHz laser pulses. The new x-ray source is designed for time-resolved x-ray absorption spectroscopy as well as imaging applications. This marks the first laser-driven plasma x-ray source that continuously recycles the target material, facilitating maintenance-free operation. Theoretical calculations show mercury targets emit shorter x-ray pulses than targets of lighter elements under identical illumination and x-ray detection conditions. The plasma-physical properties of mercury are very well suited for sub-50-fs hard x-ray pulse generation. The x-ray emission properties of this source have been simulated by a combination of particle-in-cell (PIC) and Monte-Carlo (MC) calculations of the laser target interaction and the resulting electron dynamics. All calculations were performed for p-polarized, 100-fs, 800-nm laser pulses with an incidence angle of 45 degrees. The calculated x-ray yields are in good agreement with the measured emission spectra. The simulated x-ray pulses have a width of 60 fs (fwhm), as short as the driving laser pulse width. Applications of laser-pump x-ray probe measurements are presented. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W19.00006: Control and interrogation of electronic dynamics by above-threshold ionization Mark Abel, Thomas Pfeifer, Phil Nagel, Daniel Neumark, Stephen Leone While direct interrogation of coherent nuclear dynamics has been possible for some time, only recently have the motions of valence- and core-level electrons become experimentally accessible. This access is provided by strong-field physics, through the application of high harmonic generation to ultrafast x-ray pulse synthesis. We show that another phenomenon from strong-field physics, above-threshold ionization (ATI), can yield information about electronic states and electronic dynamics without using subfemtosecond x-ray pulses. In particular, quantum beating in Xe atoms and in a 1-dimensional argon atom model show that electronic motion can be excited and interrogated in a pump-probe ATI experiment. Measurements in molecular gases show that this technique is also applicable to ro-vibrational dynamics. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W19.00007: Vibrational Modulation of High Harmonic Generation in SF$_{6}$ Zachary Walters, Stefano Tonzani, Chris H. Greene In a recent experiment performed at JILA (N. Wagner et al, PNAS {\bf 103} 13279, 2006), a molecule is hit by two pulses: the first stimulates Raman-active vibrations while the second generates high-order harmonics. The harmonic intensity oscillates as a function of delay time between the two pulses, with oscillation frequencies equal to those of the Raman-active modes. We interpret this oscillation as a form of quantum interference between neighboring vibrational states of the molecule. Nonzero derivatives of the ionization and recombination amplitudes with respect to nuclear coordinates give the molecule some amplitude to change vibrational states during the high harmonic process. We present a theoretical description of vibrational high harmonic modulation and compare with the experimental results of Wagner {\em et al.} [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W19.00008: Strong-field ionization of Xe probed by femtosecond high-order harmonic absorption spectroscopy Zhi-Heng Loh, Munira Khalil, Raoul E. Correa, Robin Santra, Stephen R. Leone Recent experiments on strong-field ionization of atoms and ions have led to conflicting conclusions regarding the existence of orbital alignment in the ionized species. Using table-top, femtosecond high-order harmonic absorption spectroscopy, we have measured the alignment of Xe$^{+}$ formed via strong-field ionization. High-order harmonics generated by focusing an intense 800 nm pulse into a Ne-filled capillary are spatially overlapped with an optical pump pulse in a sample gas cell before they are spectrally dispersed in an extreme ultraviolet spectrometer. Probing the transition from the 4$d$ core level to the $^{2}P_{3/2}$ state of Xe$^{+}$ at 55.4 eV yields a polarization anisotropy of 0.12 $\pm $ 0.01, in good agreement with the theoretical value of 0.1. This result suggests that strong-field ionization exclusively populates the $m_{J}=\pm $1/2 sub-levels in the Xe$^{+} \quad ^{2}P_{3/2}$ state. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W19.00009: Using high-order harmonics with momentum imaging techniques to study atomic and molecular dynamics Arvinder Sandhu, Etienne Gagnon, Ariel Paul, Margaret Murnane, Henry Kapteyn Laser-generated high-order harmonics provide a source of extreme-ultraviolet radiation with unique capabilities for probing atomic and molecular dynamics. Here we present the first studies that employ high harmonics in conjunction with coincidence momentum imaging (COLTRIMS) techniques for studies of molecular dynamics. We generate pulse at $\sim $ 43 eV photon energy by upconverting intense ($>$ 10$^{14 }$Wcm$^{-2})$ 25 fs laser pulses in an argon filled waveguide. These photons illuminate a cold molecular beam of CO, CO$_{2}$ or N$_{2}$, with the ion and electron products from ionization/dissociation detected using time-and-position resolved detectors. We obtain count rates as high as 0.25 per harmonic pulse, sufficient for a variety of studies. By employing pump-probe techniques, we can launch molecules into highly excited states near the molecular double-ionization threshold, from where the dynamics unfold along different channels. We also employ field-free impulsive molecular alignment to demonstrate for the first time the use of single photon excitation to obtain ion and electron angular distributions in the lab frame. [Preview Abstract] |
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