Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session SR2: Ion Atom/Molecule Collisions II |
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Chair: T.J. Gay, University of Nebraska Room: Salon A-D |
Thursday, October 16, 2008 4:00PM - 4:30PM |
SR2.00001: Ion-Impact Induced Ionization of Atoms and Molecules in Single-Pass Experiments and Storage Rings Invited Speaker: In the last decade cold target recoil ion momentum spectroscopy (COLTRIMS) and so-called Reaction Microscopes became standard tools to study the dynamics of atomic and molecular break-up processes. They enable to obtain a complete momentum analysis of all final state fragments produced in single collisions with electrons, ions, single photons, or with strong laser fields. In combination with ion storage rings, which provide excellent experimental conditions w.r.t beam intensities and emittances, they represent the ideal tool to obtain highly differential information on fundamental processes like ionization or charge transfer in ion-atom collisions. An overview on recent advances in collision studies in single-pass experiments at the tandem accelerator of the MPI in Heidelberg, at the ion storage ring CRYRING at MSL in Stockholm and the experimental storage ring ESR at the GSI in Darmstadt will be given. With the COLTRIMS setup implemented in CRYRING, single and double electron capture from hydrogen molecules in collisions with protons and He$^{2+}$-projectiles has been investigated. A variation in the total electron-transfer cross section as function of the molecular orientation has been observed which is attributed to interference in analogy to a Young-type double-slit scenario. Recently, for the first time a fully-equipped Reaction Microscope allowing the momentum-resolved detection of electrons in coincidence with recoil ions was operated in a storage ring, the ESR at GSI in Darmstadt. The results of the first experiments on target ionization and charge transfer in collisions between He, Ne, and Ar targets and highly charged projectiles ranging from 13 AMeV U$^{92+}$ to 400 AMeV Ni$^{28+}$ will be presented. [Preview Abstract] |
Thursday, October 16, 2008 4:30PM - 4:45PM |
SR2.00002: Fully Differential Cross Sections for Ionization of H$_{2}$ by 75 keV Proton Impact Jason Alexander, Aaron Laforge, Michael Schulz, Zaal Machavariani, Ryan Hupe, Uttam Chowdhury, Don Madison, Marcelo Ciappina We have performed fully differential experimental and theoretical studies of single ionization of H$_{2}$ by 75 keV proton impact. In the scattering angle dependence of the measured cross sections for fixed electron energies pronounced structures were observed at relatively large angles. These structures are interpreted as an interference resulting from the two-center potential of the molecule. In our theoretical calculations such structures are only found if the interaction between the projectile and the nuclei of the molecule is incorporated. This suggests that the interference pattern originates mostly from indistinguishable diffraction of the projectile wave from the two centers rather than from the ejected electron wave. Furthermore, we found that the interference pattern is consistent with ionization being most likely for an orientation of the molecular axis perpendicular to the projectile beam axis at small scattering angles and parallel to the projectile beam axis at large angles. However, the preferred molecular orientation does not depend sensitively on the ejected electron energy, which may explain that the observed structures are more pronounced in the scattering angle dependence than in the electron energy dependence. [Preview Abstract] |
Thursday, October 16, 2008 4:45PM - 5:00PM |
SR2.00003: Four-Body Interactions in Excitation-Ionization A.L. Harris, D.H. Madison, S. Bellm, J. Lower, E. Weigold, I. Bray, D.V. Fursa, K. Bartschat, J. Colgan The process of electron impact excitation-ionization of helium has recently been of interest in order to better understand four body processes. Calculations of the 4 body distorted wave model (4DW) and the 2$^{nd}$ order R-matrix with pseudostates (DWB2-RMPS) model for this process show discrepancies when compared with absolute experiment. The original experimental measurements were presented as cross section ratios for ionization plus excitation relative to ionization without excitation. New cross-normalized experimental data now allow for a comparison of the individual triple-differential cross sections (TDCS) used in the ratios. These experimental results are compared with the 4DW and DWB2-RMPS models, allowing for more accurate investigation into the physics involved in the four body process. [Preview Abstract] |
Thursday, October 16, 2008 5:00PM - 5:15PM |
SR2.00004: Double ionization of helium by ion impact assessed using four-body Dalitz plots Michael Schulz, Marcelo Ciappina, Robert Moshammer, Daniel Fischer, Joachim Ullrich, Tom Kirchner We have performed experimental and theoretical studies of double ionization (DI) of helium by 6 MeV proton impact using a recently developed tool: four-particle Dalitz plots.\footnote{M. Schulz \textit{et al}, J. Phys. \underline {B22}, 3091 (2007)} These plots are basically an extension of conventional Dalitz plots originally introduced in particle physics to analyze three-body fragmentation processes. DI represents a \textit{four-body} fragmentation process and consequently an extra dimension needs to be incorporated. Four-particle Dalitz plots are very powerful because they enable the representation of multiple differential cross sections as a function of all four fragments in a single spectrum without loss of any part of the total cross section. As a result, the relative importance of the various interactions between the fragments can be studied in great detail. Our results suggest that an uncorrelated DI mechanism, involving two independent interactions of the projectile with both electrons, is significantly more important than previously expected for such fast collisions. [Preview Abstract] |
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