Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session T3: Focus Session: Particle Spectroscopy |
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Chair: Philip Richerme, Harvard University Room: Grand Ballroom E |
Friday, June 8, 2012 8:00AM - 8:30AM |
T3.00001: Fully Differential Study of Fragmentation Dynamics of Li by Ion Impact Using a MOTRemi Apparatus Invited Speaker: Daniel Fischer The study of atomic fragmentation processes due to charged particle impact provides insight in the dynamics of correlated few-particle Coulomb-systems, and thus advances our understanding of the fundamentally important few-body problem. In this respect, fully differential data represent the most sensitive test of theoretical models. For ion-atom collisions such data became available only in the last decade exploiting the technique of ``Reaction Microscopes,'' often referred to as cold target recoil ion momentum spectroscopy (COLTRIMS). These kinematically complete experiments almost exclusively focussed on the fragmentation of helium and other rare gas atoms, because these targets can easily be prepared with temperatures below 1K using supersonic gasjets. Magneto-optically trapped (MOT) targets of alkaline metals have also been used. However, so far complete studies of ionization have not been possible as magnetic stray fields in the MOT hampered the momentum resolved electron detection. Here we report on the first fully-equipped and functional MOTRemi, i.e. a Reaction Microscope with a MOT target. This setup is currently implemented in the ion storage ring TSR at the MPIK in Heidelberg that can provide ion beams with high intensities and very low emittances. Lithium is used as a target which is particularly interesting for its simple, but at the same time asymmetric structure with only one weakly bound outer shell electron and two strongly correlated K-shell electrons. Due to the low temperatures ($<$ 1mK) in the MOT, the momentum resolution achieved in our experiment is drastically improved compared to earlier measurements. We studied single ionization of lithium in collisions with 3 Mev protons and 1.5 Mev/amu O$^{8+}$ ions. Due to the high resolution and by means of optical excitation, for the first time initial state selective cross sections for ion impact ionization became available. Fully differential cross sections of the ionization of 1s, 2s and 2p electrons will be presented. [Preview Abstract] |
Friday, June 8, 2012 8:30AM - 9:00AM |
T3.00002: Pseudostate Methods for Treating Atomic Fragmentation Processes Induced by Heavy-Particle Impact Invited Speaker: James Walters A method for calculating fully differential cross sections that is able to describe any aspect of coincidence measurements involving heavy projectiles will be presented. The method is based upon impact parameter close coupling with pseudostates. Examples from ionization by antiprotons, protons, C$^{6+}$, and gold ions will be shown and compared with experiment. [Preview Abstract] |
Friday, June 8, 2012 9:00AM - 9:12AM |
T3.00003: Measuring optical nonlinearities with pump-probe intracavity phase spectroscopy David Carlson, Jason Jones A passive femtosecond enhancement cavity (fsEC) is used to make sensitive, time-resolved measurements of phase shifts due to optical nonlinearities. In pump-probe intracavity phase spectroscopy, a strong pump pulse train resonant with a fsEC induces a nonlinear response in a sample which is then detected as a shift of the cavity resonance for a weak counter-propagating probe pulse. Recording this resonant shift allows precise determination of the nonlinear phase shift of the pump pulse relative to the probe. When fsECs are used for high harmonic generation (HHG), a static background plasma can accumulate and frustrate HHG phase matching. To improve the achievable HHG power in these systems it is important to understand the plasma levels and decay timescales. Here we demonstrate the pump-probe technique by measuring the decay of this plasma formed by the ionization of a xenon gas target by the pump pulse. [Preview Abstract] |
Friday, June 8, 2012 9:12AM - 9:24AM |
T3.00004: Molecule-surface interactions probed by optimized surface-enhanced coherent Raman spectroscopy Dmitri Voronine, Alexander Sinyukov, Xia Hua, Guowan Zhang, Wenlong Yang, Kai Wang, Pankaj Jha, George Welch, Alexei Sokolov, Marlan Scully Nanoscale molecular sensing is carried out using a time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy with optimized laser pulse configurations. This novel technique combines the advantages of an improved spectral resolution, suppressed non-resonant background and near-field surface enhancement of the Raman signal. We detect two species of pyridine in a vicinity of aggregated gold nanoparticles and measure their vibrational dephasing times which reveal the effects of surface environment and molecule-surface interactions on the ultrafast molecular dynamics. This technique may be applied to a variety of artificial and biological systems and complex molecular mixtures and has a potential for nanophotonic sensing applications. [Preview Abstract] |
Friday, June 8, 2012 9:24AM - 9:36AM |
T3.00005: Ultrafast Electron Diffraction of Laser-Aligned CF$_{3}$I Molecules Christopher Hensley, Jie Yang, Martin Centurion We present first experimental results of electron diffraction from non-adiabatically, laser-aligned molecules in the gas phase. Previous gas-phase diffraction studies have been successful in determining the structure of small molecules by comparing the data to theoretical models of the molecules. The random orientation of the molecules provides only 1D information (the interatomic distances), which makes it difficult to recover the structure of large molecules, or during conformational changes in the molecule where theoretical models cannot provide sufficient information. Using diffraction patterns from multiple projections of the aligned molecules it is possible to reconstruct the complete 3D structure of the symmetric top molecule (CF$_{3}$I). The alignment angle is adjusted by rotating the direction of the laser polarization. An aligned distribution is created using a femtosecond laser pulse that excites a rotational wave packet causing the molecules to align along the direction of laser polarization around 2 ps after interaction with the laser pulse. Both the electron packet and the intense alignment field are generated using a 300-fs pulse centered at 800 nm. Our results are in good agreement with the previous findings and theoretical models for CF$_{3}$I. [Preview Abstract] |
Friday, June 8, 2012 9:36AM - 9:48AM |
T3.00006: Imaging Polyatomic Molecules in Three Dimensions using Molecular Frame Photoelectron Angular Distributions J.B. Williams, A.L. Landers, C. Trevisan, T. Jahnke, M.S. Schoeffler, R. Doerner, I. Bocharova, F. Sturm, C.W. McCurdy, A. Belkacem, Th. Weber We demonstrate a method for determining the full three-dimensional molecular frame photoelectron angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay pathway, [H$^+$, H$^+$, CH$_2^+$]. We find the striking result that at low photoelectron energies the molecule is effectively imaged by the focusing of photoelectrons along bond directions. [Preview Abstract] |
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