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 K6: Focus Session: Ultrafast Probes |
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Chair: C. Lewis Cocke, Kansas State University Room: Minor Hall 125 |
Thursday, May 21, 2009 10:30AM - 11:00AM |
K6.00001: Imaging Ultrafast Dynamics in the Molecular Frame Invited Speaker: Time-Resolved Coincidence Imaging Spectroscopy (TRCIS) is a femtosecond photoelectron probe of Molecular Frame ultrafast dynamics in polyatomic molecules. TRCIS makes use of 3D particle timing-imaging detectors for full 3D recoil momentum vector determination of coincident photoions and photoelectrons as a function of time. One vector correlation is particularly interesting as it permits Time-Angle-Energy resolved photoelectron studies from the Molecular Frame rather than the lab frame point of view. An alternate approach to Molecular Frame ultrafast dynamics is to make use non-resonant laser field pre-alignment. Provided that the molecular dynamics are fast compared to rotational dephasing, this method also permits time-resolved Molecular Frame observations. We experimentally demonstrate both these approaches, comparing and contrasting their relative merits. [Preview Abstract] |
Thursday, May 21, 2009 11:00AM - 11:12AM |
K6.00002: Harmonic Generation from Common Liquid Phase Systems Anthony DiChiara, Emily Sistrunk, Terry Miller, Pierre Agostini, Lou DiMauro Harmonic generation driven by the fundamental frequency of an intense laser field has been the subject of extensive studies in gas phase atomic and molecular systems. Exciting contributions from this diverse field of study include the production of attosecond (10E-18 s) pulses and molecular orbital tomography. Recently, there has been an interest in scaling strong field processes with wavelengths in the mid-infrared (MIR) region. An advantage is that harmonics of moderate order are transparent in air for driving fields in the MIR region. Here we present an experiment where this transparency is exploited to study the generation of harmonics from a liquid at room temperature. A wire guided fluid jet is used to create thin (100-150 microns) flowing samples of various fluids. The laser is a MIR optical parametric amplifier operating at a center wavelength of 3.6 microns with a repetition rate of 1 kHz, a peak pulse power over 700 MW and is focused to field intensities less than 10 terawatts per-square-centimeter. No evidence of supercontinuum generation is observed. Odd harmonics from samples of water, heavy water and several alcohols, including Isopropyl, are examined. For example, Isopropyl alcohol (water) generated up to the ninth (thirteenth) harmonic. For isopropyl the seventh harmonic is 130 times brighter and the overall efficiency of harmonic production is greater by two orders of magnitude as compared to water. [Preview Abstract] |
Thursday, May 21, 2009 11:12AM - 11:24AM |
K6.00003: Ultrafast electronic dynamics in Helium nanodroplets studied by femtosecond time-resolved EUV photoelectron imaging Oliver Gessner, Oleg Kornilov, Chia Wang, Mathew Leonard, Andrew Healy, Stephen Leone, Daniel Neumark Helium nanodroplets constitute a unique cryogenic matrix for the creation, isolation and spectroscopy of regular and exotic species, such as free radicals and molecules in high-spin states. The droplets readily pick up atoms and molecules but interact only very weakly with the respective dopants due to their superfluid nature. Despite the remarkable number of experimental and theoretical studies that have been performed on this new type of matter, neither the electronic structure nor the electron dynamics after EUV excitation are even remotely understood. We have performed the first femtosecond EUV-pump, IR-probe experiment to study the photoionization dynamics of pure Helium nanodroplets below the atomic Helium IP (24.6 eV) in real-time. Using Velocity-Map Imaging (VMI) photoelectron spectroscopy we were able to discern processes with associated timescales ranging from tens of femtoseconds to tens of picoseconds. The results will be discussed in the light of complementary energy-domain studies and theoretical models of the droplet's electronic and nuclear dynamics. [Preview Abstract] |
Thursday, May 21, 2009 11:24AM - 11:36AM |
K6.00004: Attosecond Photoelectron Spectroscopy of Metal Surfaces Chang-hua Zhang, Uwe Thumm Recent attosecond streaking spectroscopy experiments [Cavalieri {\it et al.}, Nature {\bf 449}, 1029 (2007)] using co-propagating extreme ultraviolet (XUV) and infrared (IR) pulses of variable relative delay have measured a delay of approximately 100 attoseconds between photoelectrons emitted by a single XUV photon from localized core states and delocalized conduction-band states of a tungsten surface. We analyze the underlying XUV photoemission - IR streaking mechanism by combining a perturbative description of the XUV photoemission process and the subsequent non-perturbative IR streaking of the photoelectrons. Our calculated time-resolved photoelectron spectra depend on the transport of photo-released electrons inside the solid, agree with the experiments of Cavalieri {\it et al.}, and demonstrate that the observed temporal shift is caused by the interference of core-level photoelectrons that originate in different layers of the solid and experience a temporal modulation induced by the streaking field. [Preview Abstract] |
Thursday, May 21, 2009 11:36AM - 12:06PM |
K6.00005: Benchmark measurements of H$_{3}^{+}$ fragmentation in intense ultrashort laser pulses Invited Speaker: A fundamental grasp of how molecules respond to short intense laser pulses is essential to advance applications in laser- molecular science. Basic research on this topic lays the foundation for many vibrant areas of physics. Traditionally research begins on the simplest system and gradually expands to more complex systems as knowledge and understanding grows. For this reason H$_{2}^{+}$ is considered the prototype diatomic molecule and can be credited for the discovery of many of the strong field molecular phenomena known to us. In a similar manner H$_{3}^{+}$ is considered the prototype polyatomic molecule --- composed of three protons, bound by two electrons, in an exotic triangular configuration. Modeling the full quantum mechanical response of H$_{3}^{+}$ to intense lasers is a serious challenge for theory, thus experiments can help provide the necessary simplifying assumptions. To date, though, this system remains experimentally unexplored for a combination of reasons. At this meeting we present the first measurements of the fragmentation of H$_{3}^{+}$ and its isotopologues in intense fields using coincidence 3D momentum imaging. The imaging gives kinetic energy release and angular distributions that provide information on the breakup mechanisms. For example, we find that three-body dissociative ionization of H$_{3}^{+}$ is strongly enhanced when the molecular plane aligns to the laser polarization and, moreover, when one of the nuclei within the plane aligns to the polarization. It is hoped that these benchmark measurements will guide future theoretical and experimental work on H$_{3}^ {+}$ and larger polyatomic molecules. [Preview Abstract] |
Thursday, May 21, 2009 12:06PM - 12:18PM |
K6.00006: Accurate Retrieval of Satellite Pulses of Single Isolated Attosecond Pulses Michael Chini, He Wang, Sabih Khan, Chao Wang, Zenghu Chang Single isolated pulses less than 300 attoseconds have been generated by amplitude gating, polarization gating and double optical gating. However, pre- and post-pulses always accompany the main pulse, separated by a half or full cycle of the driving laser field. Accurate characterization of the contrast of isolated attosecond pulses is crucial for improving the pulse quality and for experimental applications of such unique sources. Typically, isolated attosecond pulses are measured using the streaking method, and the resulting spectrogram can be analyzed using Frequency-Resolved Optical Gating techniques to retrieve the pulse shape and phase, a technique known as CRAB (Complete Reconstruction of Attosecond Bursts). However, at least two factors can lead to error in the reconstruction of the satellite pulses. Due to the low photoelectron count rate, the CRAB trace is susceptible to shot noise. Also, distortions of the trace occur due to limited energy resolution of the electron detector. These two factors can mask the presence of interference fringes in the spectrogram and lead to underestimation of satellite pulses. We discuss the effects of these two factors on the retrieval of attosecond pulses for several pulse shapes and phases, and we suggest lower limits to the number of photoelectron counts and the energy resolution necessary for accurate reconstruction of the satellite pulses. [Preview Abstract] |
Thursday, May 21, 2009 12:18PM - 12:30PM |
K6.00007: Improving count rate of attosecond streak camera Ximao Feng, Steve Gilbertson, Hiroki Mashiko, He Wang, Sabih Khan, Michael Chini, Zenghu Chang We report the measurement of 140-attosecond single isolated extreme ultraviolet pulses generated with 9-fs carrier-envelope phase stabilized lasers. The temporal shape and phase of the isolated attosecond pulses were characterized using the attosecond streaking technique, whereby electrons ionized by the attosecond pulse are momentum-shifted by a near-infrared laser field. Due to the limited flux of attosecond pulses, low electron count rate is a major problem in the streaking measurement. In our setup, the streaked photoelectrons were detected by a COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) type time-of-flight detector. A uniform magnetic field was applied along the flight axis between the detection gas jet and the MCP to increase the full acceptance angle to more than 28 degrees. Instead of measuring the photoelectron energy, the electron momentum was measured, which allows us to examine the effect of the acceptance angle on the accuracy of the pulse characterization. Differential pumping was successfully applied, which allows high gas target pressure for increasing the XUV photoionization efficiency. When the isolated attosecond pulses were generated with argon gas and detected with neon gas, the improved count rate allowed the streaking measurement to be done in the time period that the laser could maintain carrier-envelope phase locking. [Preview Abstract] |
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