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 X5: Exotic Particle and Molecular Ion Collisions |
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Chair: Barry Dunning, Rice University Room: Arboretum I-III |
Saturday, May 29, 2010 10:30AM - 10:42AM |
X5.00001: Charge exchange in C$^{5+}$ + H collisions J.L. Nolte, P.C. Stancil, H.-P. Liebermann, R.J. Buenker, Y. Hui, D.R. Schultz, I.N. Draganic, C.C. Havener Charge exchange between the solar wind ion C$^{5+}$ and neutral interstellar hydrogen is thought to contribute to the heliospheric component of the soft x-ray background, as the highly charged resultant C$^{4+}$ ion emits an x-ray photon through the electron's cascade to the ground state. Accurate state-specific electron capture cross sections are therefore a vital ingredient in the modeling of this diffuse emission. In this study, we calculate $n-$, $l-$, $S$-resolved cross sections for capture into the dominant $n=3$, 4, 5 manifolds for both singlet and triplet states, over a range of collision energies 0.01-50 keV/u, using the fully quantal molecular orbital close coupling method. We include a comparison with results obtained through classical trajectory Monte Carlo, multichannel Landau-Zener, atomic orbital close, coupling, and merged-beams total cross section measurements. [Preview Abstract] |
Saturday, May 29, 2010 10:42AM - 10:54AM |
X5.00002: Polarization of the X-ray emission induced by charge-exchange collisions of solar wind ions and neutral heliospheric gas Marko Gacesa, Hans-Reinhard M\"uller, Robin C\^ot\'e, Vasili Kharchenko Charge-exchange collisions between highly charged solar wind ions and neutral hydrogen and helium present in heliosphere contribute a significant part of the diffuse X-ray background emissions detected by telescopes in space. The same process is responsible for X-ray emissions observed from comets, moons and planets in the solar system. Underlying physical mechanism is a two-species charge-exchange collision of type $A^{q+} + B \rightarrow A^{(q-1)+*} + B^{+}$, followed by a cascading radiative decay with emission of energetic photons. We present our calculations of intensity distribution and polarization of heliospheric charge-exchange X-rays and discuss their implications on current models of the heliosphere and the boundary with the local interstellar medium. [Preview Abstract] |
Saturday, May 29, 2010 10:54AM - 11:06AM |
X5.00003: Simplified Theoretical Model to treat the Dissociative Recombination of polyatomic ions by strong Jahn-Teller effects: Study of H$_3$O$^+$ and H$_3^+$ ions Nicolas Douguet, Chris H. Greene, Ivan Mikhailov, Viatcheslav Kokoouline Recent progress in theoretical treatment of dissociative recombination (DR) in closed-shell triatomic ions (H$_3^+$, HCO$^+$ or LiH$_2^+$) have demonstrated reasonable agreement with available experimental data. These studies take into account vibrational, rotational, electronic, and nuclear spin degrees of freedom, and for this reason, the numerical methods are rather demanding. In more recent studies, Mikhailov {\it et al.} and Pratt \& Jungen proposed simplified theoretical methods for DR, considering that once the electron is captured by the ion into a vibrationally excited state, the system will dissociate. The probability to capture the electron is mainly determined by non-Born-Oppenheimer effects in HCO$^+$ (Renner-Teller effect) and H$_3^+$ (Jahn-Teller effect). The thermally averaged rate constants obtained with the simplified methods are in good agreement with the fully-quantum treatment. In this study we made a step further in the simplified theory of DR in small polyatomic ions and applied it to H$_3$O$^+$. [Preview Abstract] |
Saturday, May 29, 2010 11:06AM - 11:18AM |
X5.00004: Formation of the simplest stable negative molecular ion H$_3^-$ in interstellar medium Viatcheslav Kokoouline, Mehdi Ayouz, Maurice Raoult, Jacques Robert, Olivier Dulieu We present the theory of radiative association of atoms and molecules, and we apply it to the (H$_2$-H$^-$) van der Waals complex. We discuss the possibility for the H$_3^-$ ion to be formed in the interstellar medium in an environment with abundant electrons. The observation of H$_3^-$ would also be a probe for the presence of H$^-$ in the interstellar medium. By computing the electronic structure of the H$_3^-$ ion, we determine its dipole moment, bound states, rotational constants, predissociated vibrational resonances and their lifetimes, and suggest a way to detect the ion in the interstellar medium. [Preview Abstract] |
Saturday, May 29, 2010 11:18AM - 11:30AM |
X5.00005: Measurement of Few Body Interactions in Tri-Atomic Molecular Dissociation into Three Charged Particles Dennis Mueller, Brandon Jordon-Thaden, Lisa Weise, Duane Jaecks Threshold ionization of atoms by electron impact remains an area of interest. Near threshold, where the total energy of the system is approximately zero, the motion of charged particles is highly correlated. Similarly, near threshold dissociation into three or more charges particles is involve highly correlated motion as the particles slowly move apart under the influence of the long-range Coulomb interaction. We will present a novel approach to gain insight into these interactions, where no simplifying approximations such as placing one of the particles near the center-of-mass, is theoretically viable. In these triple coincidence experiments, the final state momenta of all particles are measured with sub-meV resolution sufficient to resolve rovibrational levels. This allows us to determine the initial state of the tri-atomic molecular ion. [Preview Abstract] |
Saturday, May 29, 2010 11:30AM - 11:42AM |
X5.00006: Cusp formation in positron scattering at the positronium formation threshold James Sullivan, Adric Jones, Peter Caradonna, Casten Makochekanwa, Dan Slaughter, Stephen Buckman The extent to which channel coupling plays a role in positron scattering cross sections has long been debated, and recent work has suggested that a step-like feature in the elastic cross section is due to the rapid rise of the positronium formation cross section from threshold [1]. The low energy, high resolution beamline at the Australin Positron Beamline Facility provides a useful tool with which to study such effects. It provides a means to measure the grand total, positronium formation and elastic scattering cross sections in the region of interest, as well as having the capacity to make measurements of other inelastic scattering processes [2]. This talk will present measurements of positron scattering from noble gas atoms in the region of the positronium formation threshold showing cusp-like features in the elastic scattering channel for each target measured. Comparison with recent work on this phenomenon will be made and discrepancies in the two measurements discussed. \\[4pt] [1] Coleman et al., Phys. Rev. Lett. \textbf{102}, (2009) 173201 \\[0pt] [2] Sullivan et al., Rev. Sci. Instr. \textbf{79} (2008) 113105 [Preview Abstract] |
Saturday, May 29, 2010 11:42AM - 11:54AM |
X5.00007: Positron transport in water vapor Zoran Petrovic, Ana Bankovic, Sasa Dujko, Gordana Malovic, Stephen Buckman, Casten Makochekanwa, Joan Marler, Ronald White As a first step towards a better understanding of Positron emission Tomography we have undertaken a program to investigate the basic features of positron transport in gases and soft condensed matter. First, a set of cross sections for positron scattering on water molecules is presented. A compilation has been made using the best experimentally measured and theoretically calculated cross sections available in the literature. Second, this set of cross sections is applied to perform Monte Carlo calculations of the positron transport properties in water vapor. Basic trends in positron transport properties as a function of the reduced electric and magnetic fields are addressed using physical arguments. In addition, the thermalization time for different initial conditions of positron swarms is considered. The results presented in this work may be employed for a future optimization of medical diagnostics based on a detailed knowledge of positron interactions with matter. [Preview Abstract] |
Saturday, May 29, 2010 11:54AM - 12:06PM |
X5.00008: Ionization of atoms by axion and search for dark matter Vladimir Dzuba, Victor Flambaum, Maxim Pospelov Cross section of the ionization of Ar, Ge and Xe by a weakly interacting pseudoscalar particle such as axion is calculated in the relativistic Hartree-Fock approximation for the particle mass range from 10 to 50 keV where it can be considered as a candidate for dark matter. An analytical formula is presented which fits numerical calculations with about 10\% accuracy and can be used for any atom. The results are to be used in laboratory search for dark matter. [Preview Abstract] |
Saturday, May 29, 2010 12:06PM - 12:18PM |
X5.00009: Collision-Induced Infrared Absorption by Molecular Hydrogen Pairs at Thousands of Kelvin Martin Abel, Katharine L.C. Hunt, Xiaoping Li, Lothar Frommhold The interaction-induced absorption by collisional pairs of H2 molecules is an important opacity source in the atmospheres of the outer planets and cool stars [1]. The emission spectra of cool white dwarf stars differ significantly in the infrared from the expected blackbody spectra of their cores, which is largely due to absorption by collisional H2---H2, H2---He, and H2---H complexes in the stellar atmospheres. Using quantum-chemical methods we compute the atmospheric absorption from hundreds to thousands of kelvin. Laboratory measurements of interaction-induced absorption spectra by H2 pairs exist only at temperatures below 300 K. We show that our results reproduce these measurements closely so that our computational data permit reliable modeling of stellar atmosphere opacities even for the higher temperatures. This work has been supported in part by the National Science Foundation through Grants AST-0709106 and AST-0708496.\\[4pt] [1] L. Frommhold, Collision-Induced Absorption in Gases, Cambridge University Press, Cambridge, New York, 1993 and 2006 [Preview Abstract] |
Saturday, May 29, 2010 12:18PM - 12:30PM |
X5.00010: The Structure and Stability of Aqueous Electrons and Protons Edward Chen, Edward Chen The fundamental properties of the electron and proton were determined a century ago and ion gas phase clusters with water were observed. However, the structure and stability of aqueous electrons and protons remain uncertain. Before the 1960's the ``hydrated'' electron was unknown and the hydration enthalpy of the proton at 298K was between 258 to 283 kcal/mol. We report hydration free energies -dG$_{aq}$[298]: H(+), 266(2); e(-), 46(1) (kcal/mol) and ionic radius, r: H(+), 0.443(5); e(-), 2.56(5) angstroms using the ionization potential of the hydrogen atom, the bond dissociation energy of molecular hydrogen, the standard hydrogen electrode potential, and the Born equation. Also reported are simple structures of the mono hydrated proton and hydroxide ion and the radii for the halides from precise Morse potentials for X$_{2}$(-) constructed in 2003 following Herschbach and the -dG$_{aq}$[X(-)] from reduction potentials and electron affinities. [Preview Abstract] |
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