Bulletin of the American Physical Society
67th Annual Gaseous Electronics Conference
Volume 59, Number 16
Sunday–Friday, November 2–7, 2014; Raleigh, North Carolina
Session QR3: Collisions Involving Antimatter Particles and Atoms |
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Chair: Ugo Ancarani, Universite de Lorraine Room: State D |
Thursday, November 6, 2014 3:30PM - 4:00PM |
QR3.00001: Positron Annihilation as a Probe of Intramolecular Vibrational Energy Redistribution (IVR) Invited Speaker: J.R. Danielson Experiments at incident energies in the range of the molecular vibrations show that positrons can attach to molecules via vibrational Feshbach resonances.\footnote{G. F. Gribakin, et al., {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010).} While attached, the positron has an increased probability of annihilation, leading to an enhancement of the measured annihilation rate. This enhancement is limited because the vibrational auto-detachment rate is typically much faster than the annihilation time, meaning that most positrons escape before annihilating. However, in many molecules, intramolecular vibrational energy redistribution (IVR) couples the entrance mode energy into nearly isoenergetic multimode states. This process leads to either suppression or enhancement of the annihilation depending on whether the auto-detachment rate of the coupled vibrations is faster or slower than that of the entrance mode. These effects have recently been combined into a simplified rate-equation model which describes the effect of IVR on the measured annihilation rates.\footnote{J. R. Danielson, et. al., {\it Phys. Rev. A} {\bf 88}, 062702 (2013).} With certain approximations, the primary unknown in the model is the IVR coupling rate. This model will be described and used to show how observations of annihilation enhancement or suppression can be used to extract an estimate of the IVR coupling rate for selected modes in several small molecules. [Preview Abstract] |
Thursday, November 6, 2014 4:00PM - 4:15PM |
QR3.00002: Total cross sections for positron scattering from the noble gases Robert McEachran, Allan Stauffer Our complex, relativistic optical potential method for the elastic scattering of electrons and positrons from atoms includes the effects of excitation and ionization of the target and thus produces elastic cross sections more accurately than using a purely real potential. We have used this method to calculate differential and integrated cross sections for scattering of electrons and positrons from the noble gases. Recently, we have included a simplified form of positronium formation in our formulation, resulting in very good agreement with experimental cross sections for positron scattering from the heavy noble gases at energies where positronium formation is important [1]. Since our method now produces results for total scattering cross sections (i.e. including contributions from elastic, excitation and ionization scattering as well as positronium formation) we can compare the results from our calculations with recent measurements of this quantity. Detailed comparisons will be made at energies above the positronium formation threshold which is the inelastic channel with the lowest energy threshold in positron scattering from the noble gases.\\[4pt] [1] R. P. McEachran and A. D. Stauffer, J. Phys. B \textbf{46} 075203 (2013). [Preview Abstract] |
Thursday, November 6, 2014 4:15PM - 4:45PM |
QR3.00003: Collisions and Transport in Antihydrogen Physics Invited Speaker: Michael Charlton It has been possible for more than a decade to form antihydrogen atoms by the controlled mixing of antiprotons and positrons held in arrangements of charged particle traps [1]. More recently, magnetic minimum neutral atom traps have been superimposed upon the anti-atom production region, promoting the trapping of a small quantity of the antihydrogen yield [2-4] and first facilitating experiments [5]. We will describe some of the collision and plasma/transposrt physics that underpin these achievements, including a discussion of topical issues. \\[4pt] [1] see e.g., M.H. Holzscheiter, M. Charlton and M.M. Nieto, Phys. Rep. \textbf{401} (2004) 1 for a review \\[0pt] [2] G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature \textbf{468} (2010) 673 \\[0pt] [3] G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature Phys. \textbf{7} (2011) 558 \\[0pt] [4] G. Gabrielse \textit{et al.} (ATRAP Collaboration), Phys. Rev. Lett. \textbf{108} (2012) 113002 \\[0pt] [5] C. Amole \textit{et al}. (ALPHA Collaboration) Nature \textbf{483} (2012) 439 [Preview Abstract] |
Thursday, November 6, 2014 4:45PM - 5:00PM |
QR3.00004: B-spline R-matrix with pseudostates calculations for electron collisions with atomic nitrogen Yang Wang, Oleg Zatsarinny, Klaus Bartschat The \hbox{{\it B}-spline} \hbox{{\it R}-matrix} (BSR) with pseudo\-states method~[1] is employed to treat electron collisions with nitrogen atoms. Predictions for elastic scattering, excitation, and ionization are presented for incident energies between threshold and about 100~eV. The largest scattering model included 690 coupled states, most of which were pseudostates that simulate the effect of the high-lying Rydberg spectrum and, most importantly, the ionization continuum on the results for transitions between the discrete physical states of interest. Similar to our recent work on e-C collisions~[2], this effect is particularly strong at ``intermediate'' incident energies of a few times the ionization threshold. Predictions from a number of collision models will be compared with each other and the very limited information currently available in the literature. Estimates for ionization cross sections will also be provided.\\[4pt] [1] O. Zatsarinny and K. Bartschat, J. Phys. B~{\bf 46} (2013) 112001.\\[0pt] [2] Y. Wang, O. Zatsarinny, and K. Bartschat, Phys. Rev. A~{\bf 87} (2013) 012704. [Preview Abstract] |
Thursday, November 6, 2014 5:00PM - 5:15PM |
QR3.00005: Calculation of the polarization fraction and electron-impact excitation cross section for the Cd$^+$(5p)$^2$P$_{3/2}$ state Christopher J. Bostock, Dmitry V. Fursa, Igor Bray, Klaus Bartschat We present fully relativistic convergent close-coupling and semi\-relativistic Breit-Pauli $R$-matrix calculations of the integrated cross section and the polarization fraction for electron-impact excitation of the $(5s)\,^2S_{1/2}\rightarrow (5p)\,^2P_{3/2}$ transition in ${\rm {Cd}}^+$. Above 30 eV, the polarization fraction results are in agreement with earlier RDW calculations~[1], but in disagreement with measurements~[2], particularly above 60~eV. Cascade contributions and hyperfine depolarization are found to have a negligible affect on the polarization fraction but have a significant effect on the predicted cross section. We also find that the cross section over the entire energy range scales in proportion to the optical oscillator strength of the target model. This is an important generalization of Kim's $f$-scaling idea~[3], since it does not require an ad-hoc shift of plane-wave Born results in the intermediate energy regime.\\[4pt] [1] Sharma {\it et al.}, Phys. Rev. A~{\bf 83} (2011) 062701.\\[0pt] [2] Goto {\it et al.}, Phys. Rev. A~{\bf 27} (1983) 1844.\\[0pt] [3] Kim {\it et al.}, Phys. Rev. A~{\bf 64} (2001) 032713. [Preview Abstract] |
Thursday, November 6, 2014 5:15PM - 5:30PM |
QR3.00006: Effect of Charge Distribution in Out-of-Plane Structure for Excitation-Ionization Collisions A.L. Harris, T.P. Esposito We present fully differential cross sections (FDCS) for electron-impact excitation-ionization of helium when the ionized electron is found outside of the scattering plane. Using our 4-Body Distorted Wave and First Born Approximation models, we show that the shape of the FDCS is largely due to the charge distribution of the He$+$ ion in the final state. We also examine the effects of electron correlation in the target helium atom, and the effects of the projectile interactions with the target. [Preview Abstract] |
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