Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session V6: Surface Collisions, Molecule-Molucule Collisions and Recombination followed by Electron and Positron Scattering - Experiment |
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Chair: Thomas Miller, Air Force Research Laboratory Room: Knoxville Convention Center 301C |
Friday, May 19, 2006 1:30PM - 1:42PM |
V6.00001: Neutralization of H- at Nanostructured Surfaces Boyan Obreshkov, Uwe Thumm The charge transfer rates and the neutralization probabilities for hydrogen anions colliding with nanostructured (vicinal) surfaces are obtained by direct numerical integration of the time-dependent Schroedinger equation for the motion of the active electron in the field of the projectile-surface compound. The electronic structure of the surface is calculated from a Thomas-Fermi - von Weizsaecker statistical model with local density approximation for the exchange-correlation energy. In fixed-ion approximation, the decay rate of the electronic state of the anion in front of the surface is obtained by projecting the density of states of the collision system onto the unperturbed projectile level. The ion neutralization probability is calculated from this static width within a rate equation approach for a set of broken-straight-line collision trajectories for kinetic energies of 1 keV. The dependence of decay rates and neutralization probabilities on the surface morphology and the scattering trajectories, and a comparison of our numerical results with the experiments will be discussed. [Preview Abstract] |
Friday, May 19, 2006 1:42PM - 1:54PM |
V6.00002: State-to-state rotational transition in H$_2$+H$_2$ collisions at low temperatures Teck-Ghee Lee, N. Balakrishnan, R.C. Forrey, P.C. Stancil, David R. Schultz, Gary J. Ferland In previous meeting, we reported a quantum mechanical close- coupling study of He+H$_2$. This meeting, we present a quantum mechanical close-coupling study involving collisions between two hydrogen molecules in the energy range from ultracold to thermal regime. The two most recently published potential energy surfaces (PESs) for the H$_2$-H$_2$ complex, the so-called DJ (Diep and Johnson 2000) and BMKP (Boothroyd et al 2002) surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions for the scattering of the two molecules. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the PES compared to the rest of the older versions of H$_2$-H$_2$ potential surfaces. We found significant differences in inelastic rotational cross sections computed on the two surfaces for rotational excitations in ortho- and para-H$_2$. The discrepancy persists over a large range of energies from the ultracold to thermal and occurs for other low-lying initial rotational levels. [Preview Abstract] |
Friday, May 19, 2006 1:54PM - 2:06PM |
V6.00003: Effects of a Coulomb Barrier on the Dissociation of Sodium Chloride Dianions Shaun Ard, Nasrin Mirsale Kohan, Al Tuinman, Robert Compton, Manfred Kappes, Patrick Weis Calculations show that doubly-charged sodium chloride anions of the stoiciometry (Na$_{7}$Cl$_{9})^{2- }$and (Na$_{9}$Cl$_{11})^{2- }$are metastable toward dissociation as a result of a coulomb barrier, similar to that which also inhibits electron detachment. Measurements of the energy threshold for collisional dissociation of sodium chloride dianions are used to examine this barrier to dissociation. As one example, cross section for dissociation of (Na$_{7}$Cl$_{9})^{2-}$ into ion-pairs of the type NaCl$_{2}^{-}$(NaCl)$_{n}$ + NaCl$_{2}$(NaCl)$_{5-n}^{-}$ for n = 0,1,2 are observed to appear at very low ( $\sim $1eV for n=0 ) collision energies and maximize a few eV above threshold. The observed threshold for n=0 ions agree reasonably well with the calculated coulomb barrier to dissociation. Interestingly, collisional electron detachment or loss of the Cl$^{-}$ ion from the these dianions is not observed over the energy range studied [0 to 50 eV, Lab]. [Preview Abstract] |
Friday, May 19, 2006 2:06PM - 2:18PM |
V6.00004: Ar$^{+}$ recombination with negative ions in a flowing afterglow: A new approach. Thomas M. Miller, A. A. Viggiano Ion-ion recombination (mutual neutralization) has been previously studied in a flowing-afterglow Langmuir-probe apparatus, using the probe to measure the positive-ion and negative-ion densities as a function of distance (time) along the flow tube axis.$^{1}$ A different approach has been taken in the present work, applicable to Ar$^{+}$ (or Kr$^{+}$ and Xe$^{+})$ recombination reactions. A flowing electron-Ar$^{+}$ afterglow plasma is first established, and the ambipolar diffusion frequency is measured. Then, an electron attaching gas is added to the afterglow, and the electron attachment rate constant and product ion branching fractions are measured in the usual manner.$^{2}$ Finally, the reactant gas concentration is reduced enough that the attachment reaction has not gone to completion by the end of the flow tube. Modeling of the diffusion, attachment, and recombination processes allows us to determine rate constants for each negative ion type recombining with Ar$^{+}$. For example, Ar$^{+}$ neutralized by Cl$^{-}$, Cl$_{2}^{-}$, and CCl$_{2}$O$^{-}$ produced in attachment to oxalyl chloride,$^{2}$ shows that Cl$_{2}^{-}$ recombines at about half the rate constant of CCl$_{2}$O$^{-}$, and Cl$^{-}$ + Ar$^{+}$ recombination is negligible. $^{1}$D. Smith and N. G. Adams, in \textit{Physics of Ion-Ion and Electron-Ion Collisions}, Ed. F. Brouillard and J. W. McGowan (Plenum, New York, 1983). $^{2}$J. M. Van Doren, T. M. Miller, and A. A. Viggiano, J. Chem. Phys. (submitted). [Preview Abstract] |
Friday, May 19, 2006 2:18PM - 2:30PM |
V6.00005: On the rate of dissociative recombination of H$_3^+$ in recent afterglow plasma experiments Viatcheslav Kokoouline, Ivan Mikhaylov Determination of the rate of the dissociative recombination (DR) in H$_3^+$ has been a cotroversal issue for several decades. At present, the experimental rate about $7-9\times 10^{-8}$ cm$^3$/s at 300~K obtained from several independent storage ring experiments seems to be the most reliable for the ground state of H$_3^+$. There is only one major persistant issue: Two recent experiments in flowing and stationary afterglow plasma with H$_3^+$ made by Glosik and collaborators demonstrated a significant dependence of the DR rate as a function of density of molecular hydrogen, which is present in the plasma. In this study we are suggesting a model that explains the observed dependence of the DR rate on the density of H$_2$. The model is based on the long-living metastable states of H$_3^*$ created in the decaying stationary or flowing plasma. [Preview Abstract] |
Friday, May 19, 2006 2:30PM - 2:42PM |
V6.00006: Renner-Teller effects in HCO$^{+}$ dissociative recombination Viatcheslav Kokoouline, Ivan Mikhaylov, Asa Larson, Stefano Tonzani, Chris Greene A theoretical description of the dissociative recombination process for the HCO$^{+}$ ion suggests that the Renner-Teller coupling between electronic and vibrational degrees of freedom plays a vital role. This finding is consistent with a recent study of this process for another closed-shell molecule, the H$_{3}^{+}$ ion, where Jahn-Teller coupling was shown to generate a relatively high rate. The theoretical cross section obtained here for the dissociative recombination of HCO$^{+}$ exhibits encouraging agreement with a merged-beam experiment. [Preview Abstract] |
Friday, May 19, 2006 2:42PM - 2:54PM |
V6.00007: Benchmark Experiment and Theory for Near-Threshold Excitation of Helium by Electron Impact. Michael Lange, Jun Matsumoto, Julian Lower, Stephen Buckman, Klaus Bartschat, Oleg Zatsarinny, Igor Bray, Dmitry Fursa Experimental measurements of absolute angle-differential scattering cross sections for electron-impact excitation of the n=2,3 states of helium, obtained with a new time-of-flight technique for incident energies below 24 eV, are presented. The results are compared with state-of-the-art calculations employing the R-matrix and convergent close-coupling approaches. The agreement between experiment and theory is generally very good. A detailed analysis of the theoretical predictions highlights the need for multi-configuration expansions of the target states, as well as coupling to the ionization continuum, in order to achieve a highly accurate theoretical description of the problem. [Preview Abstract] |
Friday, May 19, 2006 2:54PM - 3:06PM |
V6.00008: Simultaneous (e,2e) Ionization-Excitation of Helium. Susan Bellm, Julian Lower, Klaus Bartschat Although it is the simplest example of a process, in which an atom or molecule is ionized and a second electron in the target is excited to a higher level, simultaneous ionization-excitation of helium poses significant challenges to both experimentalists and theorists. We present recent experimental and theoretical results for this process, leaving the He$^+$ ion in the $n=2, 3$, and 4 excited states. These (e,2e) experiments were performed for scattering geometries involving both asymmetric and symmetric energy sharing between the two final-state electrons. For the asymmetric geometry, the scattered and ejected electron energies were 200 eV and 44 eV, respectively, while both final-state electrons had 44 eV in the equal-energy sharing case. Comparison of the measurements to theoretical predictions obtained with a hybrid approach, employing a perturbative treatment for the projectile and a convergent $R$-matrix with pseudo-states (close-coupling) description for the initial bound state and the ejected-electron$-$residual-ion interaction, shows that accounting for second-order effects in the projectile$-$target interaction is critical for the theoretical description of the process. [Preview Abstract] |
Friday, May 19, 2006 3:06PM - 3:18PM |
V6.00009: Absolute angle-differential cross sections for electron-impact excitation of neon atoms from threshold to 19.5 eV. Michael Allan, Kai Franz, Hartmut Hotop, Oleg Zatsarinny, Klaus Bartschat Absolute angle-differential cross sections for electron-impact excitation of neon atoms to the four levels with the (2p$^{5}$3s) configuration have been determined both experimentally and theoretically for incident energies from threshold up to 19.5$\,$eV at scattering angles of $45^\circ$, $90^\circ$, $135^\circ$, and $180^\circ$. The latter measurements were possible through the use of a magnetic angle changer of special design. Excellent agreement between the experimental data and theoretical predictions, obtained by a Breit-Pauli $B$-spline $R$-matrix method with non-orthogonal orbitals, has been found in terms of both absolute values and the energies and widths of the numerous resonant features. [Preview Abstract] |
Friday, May 19, 2006 3:18PM - 3:30PM |
V6.00010: An investigation of electron helicity density in bromocamphor and dibromocamphor as a source of electron circular dichroism Adam Scheer, Gordon Gallup, Timothy Gay Using electron transmission spectroscopy (ETS), we have measured the vertical attachment energies (VAEs) of bromocamphor and dibromocamphor. We have also performed quantum chemical calculations to obtain the virtual orbital energies (VOEs) of camphor, bromocamphor and dibromocamphor. With scaling based on the trend between VAEs and VOEs of previously studied chloroalkanes, we have assigned the resonances observed in ETS to temporary occupation of specific normally empty orbitals. Further calculations were conducted to determine the helicity densities of bromocamphor and dibromocamphor. We find large helicity densities in the regions of large wave function amplitude of the normally unoccupied orbitals responsible for resonances in the scattering cross sections. Our ETS assignments and helicity density results are compared with the chiral asymmetry data observed in electron-circular dichroism experiments by the M\"{u}nster group [1]. Evidence is seen to support helicity density as a source for chiral asymmetry at certain resonance positions in bromocamphor and dibromocamphor. [1] C. Nolting, S. Mayer, and J. Kessler, J. Phys. B \textbf{30}, 5491 (1997). [Preview Abstract] |
Friday, May 19, 2006 3:30PM - 3:42PM |
V6.00011: Electron Collisions with Formic Acid Thomas Rescigno, Cynthia Trevisan, Ann Orel Low-energy electrons ($<$2 eV) can vibrationally excite as well as fragment gas phase formic acid (HCOOH) molecules through resonant collision processes. Recent experiments have shown that the principal reaction products of dissociative electron attachment are formate ions (HCOO$^-$) and hydrogen atoms. Using first-principles electron scattering calculations, we have identified the responsible negative ion state as a transient $\pi^*$ anion. Symmetry considerations dictate that the associated dissociation dynamics are intrinsically polyatomic: the anion must first deform to non-planer geometries before dissociating on a second anion surface that can be reached through a conical intersection. The calculations we have performed that support these conclusions will be described. We will also present differential and integrated elastic scattering cross sections and compare our results with recent experiment. [Preview Abstract] |
Friday, May 19, 2006 3:42PM - 3:54PM |
V6.00012: Investigation of Positron-CO scattering using Positronium Annihilation Ratio Spectroscopy W.E. Kauppila, J.J. Edwards, E.G. Miller, T.S. Stein, E. Surdutovich We are using the technique [1] of positronium annihilation ratio spectroscopy (PsARS) to study the annihilation of 6 - 13 eV positrons colliding with CO. In this method we detect in coincidence (a) two 511 keV annihilation gamma rays, and simultaneously (b) two 300 - 460 keV gamma rays from the three gamma decay of ortho-Ps. The ratio of these signals R$_{3\gamma /2\gamma }$ versus positron impact energy reveals anomalous behavior for CO when compared with other gases [1] suggesting that some other mechanism than Ps formation is contributing to positron annihilation at an energy about 1 eV above the Ps formation threshold of 7.2 eV. Recognizing that the threshold for electronic excitation by positron impact is located where the anomalous R$_{3\gamma /2\gamma }$ behavior occurs suggests that we may be observing an effect where the positron is electronically exciting CO and temporarily binding to the molecule in a resonance-like state from which the positron can annihilate with an electron producing an enhanced 511 keV coincidence signal consistent with our measurements. [1] W.E. Kauppila, E.G. Miller, H.F.M. Mohamed, K. Pipinos, T.S. Stein and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004). [Preview Abstract] |
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