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 C5: Ion Collisions |
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Chair: David Schultz, Oak Ridge National Laboratory Room: Clark Hall 107 |
Wednesday, May 20, 2009 2:00PM - 2:12PM |
C5.00001: Merkuriev cutoff below the three-body threshold: practical considerations for bound state and scattering calculations Vladimir Roudnev Merkuriev regularization is necessary for correct formulation of scattering theory for three charged particles above the three-body threshold [1]. The cutoff procedure of Mercuriev is useful, however, even for studying states below the three-body threshold, for instance three-body bound states and $2\rightarrow2$ scattering processes. Even though physical quantities can not depend on the parameters of regularization, we demonstrate that the appropriate choice of the parameters leads to significant acceleration of numerical convergence. It can also be used for constructing simple but accurate models of three-body bound states (such as vibrational states of different isotopes of the $H_{2}^{+}$) and low energy scattering of three charged particles. [1] Merkuriev S P Ann. Phys. {\bf 130} 395 (1980) [Preview Abstract] |
Wednesday, May 20, 2009 2:12PM - 2:24PM |
C5.00002: Theoretical Fully Differential Cross Sections for Transfer-Ionization Collisions A.L. Harris, J.L. Peacher, D.H. Madison Theoretical fully differential cross sections (FDCS) will be compared with experimental results for transfer-ionization occurring in proton-helium collisions. In the experiments, the incident proton captures one electron from a helium atom, and the remaining electron is ionized into the continuum. The theoretical approach we use is a full four-body approach, taking each particle and interaction into account. The calculations will address the effects of the projectile-target atom interaction, projectile-residual ion interaction, electron correlation, and post-collision interaction. [Preview Abstract] |
Wednesday, May 20, 2009 2:24PM - 2:36PM |
C5.00003: Experimental and Theoretical Fully Differential Cross Sections for Ionization of H2 by 75 keV Proton Impact Uttam Chowdhury, Jason Alexander, Aaron Laforge, Ahmad Hasan, Michael Schulz, Don Madison We have performed fully differential experimental and theoretical studies of single ionization of H2 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 have been interpreted as an interference resulting from the two-center potential of the molecule. Previously, we have developed the three-body distorted-wave (3DW) model for electron-impact ionization of molecules and have used this model to investigate interference effects for electron collisions. We have now generalized the 3DW model to proton-impact ionization and we will use the model to investigate possible interference effects for heavy particle ionization. [Preview Abstract] |
Wednesday, May 20, 2009 2:36PM - 2:48PM |
C5.00004: Electron continua and ECC in strongly perturbing ion-atom collisions Siegbert Hagmann We have measured doubly differential cross sections(DDCS) d$^{2}\sigma $/dv$_{e}$d$\Omega _{e}$ for single and double electron emission for the collision systems F$^{8+,9+}$ and I$^{23+,25+}$ + He via electron recoil coincidences. For the first time the entire relevant phase space 0$^{0}\le \theta _{e}\le $360$^{0}$ and 0$<$v$_{e}$/v$_{proj}\le $2 was covered. We observe that for a large perturbation strength q/v$_{proj}$ the v$_{e}$=v$_{proj}$ cusp is originating dominantly from double ionization of the target and not single ionization as is commonly believed. It is to be noted that the projectile centered continuum overwhelmingly prevails over the target centered continuum. We discuss the relevant structures observed in the coincident DDCS. [Preview Abstract] |
Wednesday, May 20, 2009 2:48PM - 3:00PM |
C5.00005: Investigation of Charge Transfer in Low Energy $D_2^+$ + H Collisions using Merged Beams V.M. Andrianarijaona, J.J. Rada, R. Rejoub, C.C. Havener The hydrogen - hydrogen(deuterium) molecular ion is the most fundamental ion-molecule two-electron system. Charge transfer proceeds through dynamically coupled electronic, vibrational and rotational degress of freedom. Using the ion-atom merged- beams apparatus at Oak Ridge National Lab we have measured absolute charge transfer cross sections for D$_2^+$ + H from keV/u collision energies where the collision is considered ``ro-vibrationally frozen'' to meV/u energies where collision times are long enough to sample vibrational and rotational modes. The molecular ions are extracted from an ECR ion source with a distribution of ro-vibrational states. The measurments benchmark high energy theory (Errea et al., NIMB 235, 362 (2005) and vibrationally specific adiabatic theory (Krstic PRA 66, 042717 (2002)). [Preview Abstract] |
Wednesday, May 20, 2009 3:00PM - 3:12PM |
C5.00006: Collision Induced Dissociation for 1.5 keV/amu HeH$^{+}$ Impact on Argon Kevin Carnes, Nora G. Johnson, A. Max Sayler, Dag Hathiramani, Itzik Ben-Itzhak Collision induced dissociation [CID, e.g. HeH$^{+}$ + Ar -$>$ H$^{+}$ + He] is measured for 1.5 keV/amu HeH$^{+}$ on argon using 3D momentum imaging techniques and compared to similar measurements with H$_{2}^{+}$. Unlike H$_{2}^{+}$, HeH$^{+}$ dissociation is dominated by vibrationally excited rather than electronically excited CID. This difference is explained in terms of the larger energy gap between the ground and excited states in HeH$^{+}$. An asymmetry in dissociation channels between H$^{+}$ and He$^{+}$ products is also observed and explained in terms of the potential energy curves. [Preview Abstract] |
Wednesday, May 20, 2009 3:12PM - 3:24PM |
C5.00007: High accuracy potential calculations for atom-ion chambers Janine Shertzer, Jacob Golde, Paul Oxley Although commercial packages are available for calculating the electric potential in atom-ion chambers, it is often difficult to input the details of the specific geometry and configuration of various components. One can obtain highly accurate solutions to Laplace's equation for any type of chamber using the finite element method (FEM). FEM is sufficiently straightforward that it can be implemented by undergraduate students. Our interest is in designing a chamber for laser excitation of atoms which evolve into coherent elliptical states (CES); the CES atoms subsequently undergo charge transfer collisions with an ion beam. We present FEM results for a grounded cylindrical chamber with five concentric rings. By a careful choice of geometry and potentials applied to the rings we have optimized the electric field to be uniform to within 0.1{\%} over approximately 70{\%} of the 2mm-diameter laser excitation region. The required field homogeneity is attained without the use of fine metal meshes over the rings, which in real experiments can introduce un-calculable perturbations to the potential. [Preview Abstract] |
Wednesday, May 20, 2009 3:24PM - 3:36PM |
C5.00008: Isotope Effects in Low Energy Ion-Atom Collisions D.G. Seely, C.C. Havener, R. Rejoub Isotope effects for charge transfer processes have recently received increased attention Stolterfoht et al., PRL 99 (2007) 10301. The ion-atom merged-beams apparatus at Oak Ridge National Laboratory is used to measure charge transfer for low energy collisions of multi-charged atomic and molecular ions with H and D and is therefore well suited to investigate isotope effects. When charge transfer occurs at relatively large inter- nuclear distances (via radial couplings) the ion-induced dipole attraction can lead to trajectory effects, causing differences in the charge transfer cross sections for H and D. A strong isotope effect (nearly a factor of two) has been observed in the cross section for Si$^{4+}$ + H(D) below 0.1 eV/u. However, little or no difference is observed for N$^{2+}$ + H(D). Recently, strong effects have been predicted by Stolterfoht et al. for the fundamental system He$^{2+}$ + H(D,T) at collision energies below 200 eV/u where charge transfer occurs primarily through united-atom rotational coupling. We are currently exploring systems where rotational coupling is important and isotopic differences in the cross section can be observed. [Preview Abstract] |
Wednesday, May 20, 2009 3:36PM - 3:48PM |
C5.00009: Measurement of Ion-Collisional Frequency through Optical Pumping in Ultracold Neutral Plasmas Jose Castro, Thomas Killian Ion-collisional frequency measurements were performed in Strontium Ultracold Neutral Plasmas (UNP) through optical pumping between the two ground level spin states. The spin states of the ground and excited levels of a Sr ion form a $\Lambda$ energy configuration when coupled with the appropriate circularly-polarized light. Optical pumping from one ground level spin state to the other is strongly affected by collisions between ions. Fluorescence measurements of this process show that velocity changing collisions between ions slow down the pumping rate into the ``dark'' ground level spin state. Fluorescence measurements were modeled by a set of coupled rate equations to calculate the ion- collisional frequency. [Preview Abstract] |
Wednesday, May 20, 2009 3:48PM - 4:00PM |
C5.00010: Close-encounter collisions between few keV H$_{2}^{+}$ and Ar: Can the H$_{2}^{+}$ survive? Nora G. Johnson, A.M. Sayler, Ben Berry, Wania Wolff, B. Gaire, M. Zohrabi, J. McKenna, K.D. Carnes, I. Ben-Itzhak Collisions between 3 keV H$_{2}^{+}$ and Ar atoms lead predominantly to dissociative capture (DC) and collision-induced dissociation (CID). The large momentum transfer in close encounters between the nuclei of these collision partners can result in dissociation driven by vibrational excitation. One interesting question is, \textit{can the} H$_{2}^{+}$ \textit{molecule remain bound after absorbing the large momentum transfer typical to a trajectory that passes through the atom's electronic shells}? Our recent experimental evidence suggests that this may be the case and gives indications for what specific conditions make it possible. Explicitly, this insight is gained by studying the non-dissociating direct ionization process, H$_{2}^{+}$ + Ar $\to $ H$_{2}^{+}$ + Ar$^{+}$ + e$^{-}$, and the complimentary collision induced dissociation process, H$_{2}^{+}$ + Ar $\to $ H$^{+}$ + H + Ar, for the same momentum transfer. [Preview Abstract] |
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