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 T2: Positron Interactions with Atoms and Molecules |
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Chair: Stephen Buckman, Australian National University Room: Knoxville Convention Center Ballroom EFG |
Friday, May 19, 2006 10:30AM - 11:06AM |
T2.00001: Positronic Atoms --- Understanding a Sticky (Few-Body) Situation Invited Speaker: It has been known for about a decade that positrons definitely bind to neutral atoms and, at last count, 11 different atoms are theoretically known to bind positrons, of which none have yet been demonstrated experimentally [1]. We have revisited the application of the configuration interaction (CI) method to the study of various positronic atoms [2]. The accurate representation of electron-positron clustering using only single particle orbitals centered on the nucleus requires the inclusion of orbitals with much higher angular momenta than a roughly equivalent electron-only calculation. Given that helium is described as slowly convergent [3], one struggles to find an adjective that could characterize the CI convergence properties of positronic systems! And this says nothing of the additional radial convergence induced horrors involved in calculating annihilation rates to even within $5\%$. However, armed with convergence patterns, we have obtained reliable estimates of various positronic atom structures and annihilation properties. Our CI calculations, for example, have shown that positronic calcium is the strongest positron binding system yet found, binding a positron much more strongly than it does an extra electron [4]. Recent progress on understanding the role of the $p$-wave in low-energy positron-atom interactions using a hybrid CI-Kohn scattering method will also be discussed. \\ $[1]$ J.Mitroy, M.W.J.Bromley and G.G.Ryzhikh J.Phys.B \textbf{35} R81 (2002)\\ $[2]$ J.Mitroy and M.W.J.Bromley Phys.~Rev.~A (under review)\\ $[3]$ C.Schwartz Phys.~Rev. \textbf{126} 1015 (1962)\\ $[4]$ M.W.J.Bromley and J.Mitroy Phys.~Rev.~A (under review)\\ [Preview Abstract] |
Friday, May 19, 2006 11:06AM - 11:42AM |
T2.00002: Resonances in Positron-molecule Interactions Invited Speaker: The development of cold, trap-based beams has enabled high-resolution, energy-resolved studies of positron scattering and annihilation processes [1]. This talk focuses on three topics in this area. For hydrocarbon molecules such as alkanes (C$_n$H$_{2n+2}$), giant enhancements in annihilation rates are observed due to vibrational Feshbach resonances. The dependence of the rates on positron energy provides evidence that positrons bind to these molecules and a measure of the binding energies [1]. Recent results include evidence for a second, ``positronically excited'' bound state and new data for the methane series, CH$_3$X, where X is a halogen. Other ``resonance-like features'' are sharp increases in the near-threshold electronic excitation cross sections for CO and N$_2$ [2], and in the vibrational excitation cross sections for CO, CO$_2$ and CH$_4$ [3, 4]. Outstanding questions and the relationship of these observations to available theoretical predictions will be discussed.\\ 1. C. M. Surko, G. F. Gribakin, and S. J. Buckman, \textit{J. Phys. B} \textbf{38}, R57 (2005).\\ 2. J. P. Marler and C. M. Surko, \textit{Phys. Rev. A} \textbf{72}, 062713 (2005).\\ 3. J. P. Marler and C. M. Surko, \textit{Phys. Rev. A } \textbf{72}, 062702 (2005).\\ 4. J. P. Marler, G. F. Gribakin and C. M. Surko, \textit{Nuclear Instrum. and Meth. B}, in press (2006). [Preview Abstract] |
Friday, May 19, 2006 11:42AM - 12:18PM |
T2.00003: Ionisation by Positron and Positronium Impact Invited Speaker: Gaetana Laricchia For positron impact, unexpected asymmetries in the energy sharing between the scattered projectile and ejected electron have been observed for the first time: the electron spectrum is shifted to significantly lower (and the scattered positron to correspondingly higher) energies than predicted by theory. The observations are not fully understood but it has been suggested that the correlation between the electron/positron and the remaining ion and/or the competition from the positronium might be responsible. In the case of Ps projectiles, their fragmentation in atomic collisions has been investigated and a predicted asymmetry between the differential energy spectra of the residual particles has now been experimentally confirmed. The support of the Engineering and Physical Science Research Council UK (grant No GR/S16041/01), the Royal Society and the European Union (HPRN-CT-2002-00179 EPIC) is gratefully acknowledged. [Preview Abstract] |
Friday, May 19, 2006 12:18PM - 12:54PM |
T2.00004: Many-body theory of positron-atom interactions Invited Speaker: The interaction of low-energy positrons with atoms is characterised by strong electron-positron correlation effects. For example, they increase the positron annihilation rate in collisions with Xe atoms by three orders of magnitude! We have developed a many-body theory approach [1] which accounts for all major correlation effects, i.e, polarization of the atom by the positron, virtual positronium formation, and short-range enhancement of the electron-positron contact density. The first two give rise to strong positron-atom attraction. It affects positron elastic scattering and promotes positron-atom binding. The third effect determines positron-atom annihilation rates and spectra of annihilation gamma quanta [2]. Thus, a good quantitative understanding of positron-atom interactions has been achieved. This gives hope of understanding a much more difficult problem of positron annihilation with polyatomic molecules, where molecular vibrational degrees of freedom play an important role [3,4]. \begin{enumerate}\setlength{\itemsep}{-3pt}\setlength{\itemindent}{-12pt} \item G. F. Gribakin and J. Ludlow, Phys. Rev. A {\bf 70}, 032720 (2004). \item L. J. M. Dunlop and G. F. Gribakin, submitted to J. Phys. B (2005). \item C. M. Surko, G. F. Gribakin and S. J. Buckman, J. Phys. B {\bf 38}, R57 (2005). \item G. F. Gribakin and P. M. W. Gill, Nucl. Instrum. and Methods B {\bf 221}, 30 (2004). \end{enumerate} [Preview Abstract] |
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