Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session LW2: Positron and Electron Collisions |
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Chair: Stephen Buckman, Australian National University Room: 255E |
Wednesday, November 16, 2011 2:00PM - 2:30PM |
LW2.00001: Absolute measurements of differential cross sections in low energy positron scattering Invited Speaker: The measurement of positron scattering cross sections has been transformed by the development of the Surko trap and beam system\footnote{T. Murphy and C. M. Surko, Phys. Rev. A \textbf {46}, 5696 (1992)}, which provides a high energy resolution source of positrons\footnote{S. J. Gilbert et al., Appl. Phys. Lett. \textbf{70}, 1944 (1997)} with which to make measurements of low energy scattering processes \footnote{J. P. Sullivan et al., Phys. Rev. A \textbf{66}, 042708 (2002)}. By taking advantage of the fact that the resulting positron beam is magnetised, new experimental techniques have been developed to enable the determination of absolute cross sections for a range of scattering processes that have previously been unable to be measured quantitatively. One of the many scattering cross sections that can now be accurately measured is the elastic differential scattering cross section. This is provides a sensitive window into the scattering process, being a good test of the quality of any theoretical calculation. In addition, the differential information can be used in simulations of positron transport in biological systems, allowing a more complete and accurate picture of the underlying processes in positron thermalisation. Recent measurements have also shed light on some longstanding disagreements between previous positron scattering measurements \footnote{J. P. Sullivan et al., J. Phys. B: At. Mol. Opt. Phys. \textbf{44} (2011) 035201}. This talk will give an overview of the experimental techniques used to measure positron scattering cross sections and then present a variety of DCS measurements, from targets relevant to fundamental and applied studies. The importance of these measurements will be discussed with regards to understanding positron scattering data and application to biomedical processes. [Preview Abstract] |
Wednesday, November 16, 2011 2:30PM - 2:45PM |
LW2.00002: Numerical modeling of buffer gas positron traps Srdjan Marjanovic, Milovan Suvakov, Zoran Lj. Petrovic Buffer gas positron traps are the prime tool for study of low energy antimatter matter interaction. Our Monte Carlo based model includes all three stages of trap operation, loading, cooling and dumping, allowing us to examine the properties of exiting beam. At the same time we can study the processes inside the trap, that govern its efficiency in number of trapped particles, energy resolution and operation time. The model trap is a classic three stage potential well design using N$_{2}$ as a buffer gas in the first two stages and a N$_{2}$/CF$_{4}$ mixture in the third. It was found that including cross sections for rotational e$^{+}$-N$_{2}$ excitation is essential to achieve final stages of thermalization. Temporal and spatial evolution of the energy distribution of particles allows us to show gradual transition of a beam into a swarm of particles. We will give an overview of various loss processes inside and offer ways to mitigate the loss. Various trap implementations use different sources and moderators, and depending on the properties of the incoming beam, the trap itself can be optimized by changing its attributes (buffer gas pressures, dimensions of the chamber, electric potential shape, duration of different operation stages, etc.). [Preview Abstract] |
Wednesday, November 16, 2011 2:45PM - 3:00PM |
LW2.00003: Positron Annihilation on Molecules: Effects Beyond the Gribakin-Lee Model A.C.L. Jones, J.R. Danielson, M.R. Natisin, C.M. Surko Annihilation at positron energies in the range of the molecular vibrational modes is dominated by large-amplitude vibrational Feshbach resonances (VFR) in which the positron attaches to the molecule.\footnote{G. F. Gribakin, J. A. Young, and C. M. Surko, {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010). } In small molecules, there is a quantitative description of the annihilation rates, $Z_{\rm eff}$.\footnote{G. F. Gribakin and C. M. R. Lee, {\it Phys. Rev. Lett.} {\bf 97}, 193201 (2006).} This talk focuses on other resonant annihilation phenomena that are less well understood. A broad spectrum of enhanced annihilation will be described that is observed in the annihilation spectra of many, if not most molecules. This spectral component, for example, dominates the spectrum in small molecules with relatively large binding energies such as CCl$_4$ and CBr$_4$. The relationship of this seemingly ubiquitous feature to a model of statistically complete resonant, multi-mode annihilation\footnote{G. F. Gribakin and C. M. R. Lee, {\it Eur. Phys. J. D} {\bf 51}, 51 (2009).} will be discussed. [Preview Abstract] |
Wednesday, November 16, 2011 3:00PM - 3:15PM |
LW2.00004: Comparison of Positron- and Electron-Molecule Bound States J.R. Danielson, A.C.L. Jones, M.R. Natisin, C.M. Surko Positrons can attach to molecules via Feshbach resonances (VFR) in which a vibrational mode absorbs the excess energy. Using a high-resolution positron beam, this VFR process has been used to measure positron-molecule binding energies for many chemical species.\footnote{Danielson, et. al., {\it J. Phys. B}, {\bf 42}, 235203 (2009).}$^,$\footnote{Danielson, et. al., {\it Phys. Rev. Lett.}, {\bf 104}, 233201 (2010).} New measurements will be discussed of positron binding to relatively simple molecules and molecules with large permanent dipole moments ($\mu$). For example, the binding energy is 75 meV for CS$_2$ ($\mu = 0$) and 180 meV for acetonitrile (CH$_3$CN, $\mu = 3.9$ debye). Other species studied include aldehydes, ketones, and nitriles, which have $\mu$ in the range $2.5 - 4.0$ debye. These data will be compared to analogous, weakly bound electron-molecule (negative-ion) states.\footnote{Hammer, et. al., {\it J. Chem. Phys.} {\bf 119}, 3650 (2003).} The positron binding energies are surprisingly large (i.e., by a factor of 10 to 100) compared to those for the analogous negative ions, and origins of these differences will be discussed. [Preview Abstract] |
Wednesday, November 16, 2011 3:15PM - 3:45PM |
LW2.00005: Nanoscale models for energy deposition of photons, electrons and positrons in atomic and molecular gases Invited Speaker: Many radiation applications require detailed energy deposition maps in reduced volumes, typically at the nanoscale. In addition, information about the type of interaction processes taking place in these reduced areas is usually needed. In order to achieve this level of description, single particle tracks, both for primary radiation and secondary generated species, should be simulated upon reasonable physical descriptions of the interaction processes in terms of cross sections and energy loss. In this study we present a Low Energy Particle Track Simulation (LEPTS) Monte Carlo code which is based on experimental and theoretical cross section data we have previously derived as well as on the observed energy loss distribution functions. This model will be applied to the irradiation of atomic (Ar) and molecular (SF$_{6})$ gases with high energy electrons, positrons and photons by simulating single particle tracks until their final thermalization in the medium. Special attention will be paid to the low energy secondary electrons generated along the tracks. Detailed energy deposition pictures and local radiation effects will be derived from the simulated track structure and compared with direct observations in simple experiments.\\[4pt] In collaboration with Martina Fuss and Ana G. Sanz, IFF-CSIC; Francisco Blanco, Universidad Complutense de Madrid; and Antonio Mu\~noz, CIEMAT. [Preview Abstract] |
Wednesday, November 16, 2011 3:45PM - 4:00PM |
LW2.00006: Electron-impact total cross sections for plasma pro- cessing gases using magnetized beam formation method Dae-Chul Kim, Young-Rock Choi, Yonghyun Kim, Young-Woo Kim, Jung-Sik Yoon, Hyck Cho As interest has increased in the interaction between low- temperature plasmas and materials, the role of modeling and simulation of processing plasmas has become important in understanding the effects of charged particles and radicals in plasma applications. However, we suffer from lack of theoretical and experimental electron-impact cross section data for plasma processing gas, such as plasma etching and deposition processes. Thus, in this work, the total cross sections for electron scattering from plasma processing gases has been developed using magnetized beam formation in the low- and intermediate-energy region. Also, here the work conducted at the Data Center for Plasma Properties (DCPP) over last 5 years on the systematic synthesis and assessment of fundamental knowledge on low-energy electron interactions with plasma processing gases is briefly summarized and discussed. [Preview Abstract] |
Wednesday, November 16, 2011 4:00PM - 4:15PM |
LW2.00007: Electron Impact Ionization of CH$_{4 }$and NH$_{3}$ Hari Chaluvadi, Don Madison, Kate Nixon, Andrew Murray, Chuangang Ning Experimental and theoretical Triply Differential Cross Sections (TDCS) will be presented for electron-impact ionization of Methane (CH$_{4}$) and Ammonia (NH$_{3})$ (same number of electrons and protons) for both the highest occupied molecular orbital (HOMO) and next highest occupied molecular orbital (NHOMO). M3DW (molecular 3-body distorted wave) results will be compared with experiment for coplanar geometry and for perpendicular plane geometry (a plane which is perpendicular to the incident beam direction). In both cases, the final state electron energies and observation angles are symmetric and the final state electron energies range from 1.5eV to 30eV. [Preview Abstract] |
Wednesday, November 16, 2011 4:15PM - 4:30PM |
LW2.00008: Double ionization of helium by energetic Au$^{33+}$ projectiles analyzed using the frozen-correlation approximation T. Kirchner, M.F. Ciappina, M. Schulz We present calculations for double ionization of helium by 158 MeV Au$^{33+}$ projectiles based on the frozen-correlation approximation (FCA) of the two-electron dynamics [1,2]. We have implemented the FCA within the framework of the Monte Carlo Event Generators that allow us to generate theoretical event files and to compare with experimental data in a direct way using the four-body Dalitz (4-D) plots [3]. The idea of the FCA is to separate electronic correlations in the asymptotic initial and final states from those which might operate during the collision. It was argued that the latter can be neglected when the collision time is short compared to a suitably defined correlation time of the system [1]. Following this suggestion we restrict the incorporation of correlation effects to the initial and final states for the energetic collision system studied in this work. Results for 4-D plots are compared with previous calculations and experimental data [3]. \\[4pt] [1] F. Martin and A. Salin, Phys. Rev. A \textbf{54},3990 (1996).\\[0pt] [2] L. Gulyas et al., Phys Rev. A \textbf{74}, 032713 (2006).\\[0pt] [3] D. Fischer et al., Phys. Rev. A \textbf{80}, 062703 (2009). [Preview Abstract] |
Wednesday, November 16, 2011 4:30PM - 4:45PM |
LW2.00009: A break-up model solved in hyperspherical coordinates Ancarani Lorenzo Ugo, Gasaneo Gustavo, Mitnik Dario One way to describe ion-atom break-up processes consists in splitting the total wave function as $\Psi^{+}=\Psi_{0}$+$\Psi_{sc}^{+}$, where $\Psi_{0}$ is an asymptotically prepared initial state and $\Psi_{sc}^{+}$ is the scattering solution. Adequate asymptotic behavior should be imposed, and hyperspherical coordinates ($\rho,\alpha$) are well adapted for this purpose. A Sturmian approach in these coordinates may be used to solve the scattering problem. To test numerically the proposed Sturmian-hyperspherical approach we make use of an analytically solvable model for three particles break up processes with a Coulomb interaction. Closed forms in hyperspherical coordinates are derived for the solution with outgoing wave behavior and for the scattering transition amplitude. They compare very well with numerical results validating the use of the proposed Sturmian hyperspherical approach. Moreover, as all the Sturmian basis functions possess the correct outgoing Coulombic asymptotic behavior and diagonalize not only the kinetic energy but also the Coulomb interaction, the convergence rate is strongly accelerated: only a few basis functions are necessary to reproduce the analytical solutions for both the scattering wave function and the transition amplitude. [Preview Abstract] |
Wednesday, November 16, 2011 4:45PM - 5:00PM |
LW2.00010: Interaction between LiH and Li based on electronic structure calculations Yiming Mi, Xingxing Zhao, Shuichi Iwata The potential energy surface for the LiH molecule interacting with the Li atom with adiabatic approximation was calculated in terms of ab initio methods. The interaction potential was obtained by using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations for the core-core and core- valence correlation and full configuration interaction for the valence-valence correlation. A global minimum has acquired for the potential energy surface under the condition of fixing the Li-H bond length at the monomer equilibrium distance. And a strongly anisotropic potential of the ground state of Li-LiH is obtained too. The interaction between the two potential energy surfaces and its possible impact on the collisional dynamics are also analyzed in this work. [Preview Abstract] |
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