Bulletin of the American Physical Society
49th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 63, Number 5
Monday–Friday, May 28–June 1 2018; Ft. Lauderdale, Florida
Session S07: Focus Session: Recent Developments in Electron-Atom Collisions |
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Chair: Don Madison, Missouri University of Science and Technology Room: Grand D |
Thursday, May 31, 2018 2:00PM - 2:30PM |
S07.00001: Inelastic electron-argon scattering in a laser field Invited Speaker: B.A. deHarak Laser-assisted electron scattering (LAES) experiments examine the effects of an electro-magnetic field on the collision of charged particles with atoms and molecules. Most LAES experiments involving elastic scattering have been well described by a simple theory (the Kroll-Watson Approximation\footnote{N. M. Kroll and K. M. Watson Phys. Rev. A 8, 804 (1973).}) that assumes that laser-target interactions can be ignored. In 2015, Morimoto, Kanya, and Yamanouchi reported on LAES experiments in xenon\footnote{Y. Morimoto, R. Kanya and K. Yamanouchi Phys. Rev. Lett. 115, 123201 (2015)} that, for the first time, showed the unambiguous breakdown of the KWA. Those experiments were extremely challenging due to (among other things) the fact that the laser-field dressing of the target xenon atoms was slight, and only apparent at scattering angles less than 0.5$^\circ$. Here we will present an overview of LAES experiments and describe our recent work to observe laser-target interactions during \textit{inelastic} electron-argon scattering. The polarizability of the excited argon is relatively large, making dressing effects easier to observe. [Preview Abstract] |
Thursday, May 31, 2018 2:30PM - 3:00PM |
S07.00002: Integrated Stokes Parameter Measurements of Zn Polarized-Electron Impact Excitation Invited Speaker: Nathan Clayburn Integrated Stokes parameter measurements of the fluorescence resulting from transversely-spin-polarized electron impact excitation of the Zn (4s5s)5$^{\mathrm{3}}$S$_{\mathrm{1}}$ state have been made. The atomic Zn target was produced by a Zn oven and a heated effusive channel that directed an atomic beam at right angles to both the fluorescence observation direction and the electron beam axis. In the cascade-free range of excitation between threshold of the (4s5s)5$^{\mathrm{3}}$S$_{\mathrm{1}}$ excitation and the first cascading (4s5p)5$^{\mathrm{3}}$P$_{\mathrm{J}}$ threshold, integrated Stokes measurements are consistent with theoretical predictions and inconsistent with other previously reported experimental results [1], [2]. The discrepancy between theory and the aforementioned previous experimental study motivated our investigation. Observed optical excitation functions are also reported and are in good agreement with other independent experimental measurements. [1] N. B. Clayburn and T. J. Gay, Phys. Rev. Lett. \textbf{119}, 093401 (2017). [2] L. Pravica et al., Phys. Rev. A \textbf{83}, 040701R (2011). [Preview Abstract] |
Thursday, May 31, 2018 3:00PM - 3:12PM |
S07.00003: Electron-impact double ionization of helium at low incident energy James Colgan, M. S. Pindzola Electron-impact double ionization of helium is an example of the Coulomb four-body problem - that is, three electrons moving in the field of a charged nucleus. At low incident electron energy, ionization results in all three electrons move relatively slowly away from the nucleus, so that the electron-electron interactions between all three electrons can be expected to govern the fragmentation process. Measurements of this ionization process at such energies are exceedingly difficult due to the very low count rates (cross sections) at such energies. However, measurements of the fully differential cross sections for electron-impact double ionization of helium for an excess electron energy of 5 eV have been reported [1], and show that a `triangle' break up is produced at low energies. We present time-dependent close-coupling calculations of this process. Such calculations are extremely time consuming due to the requirement of large box sizes and long propagation times for such low-energy ionization processes. However, we find good agreement with the reported measurements and are able to confirm the triangle break up pattern. We present results of our calculations at the conference and compare and contrast our results with the related process of triple photoionization of lithium. [Preview Abstract] |
Thursday, May 31, 2018 3:12PM - 3:24PM |
S07.00004: B-spline R-matrix with pseudo-states calculations for electron-impact excitation and ionization of calcium. Oleg Zatsarinny, Klaus Bartschat The \hbox{B-spline} R-matrix with Pseudo-States method [1,2] was employed to treat electron collisions with calcium atoms. Predictions for elastic scattering, excitation, ionization, and ionization-excitation were obtained for all transitions between the lowest 39 states of Ca in the energy range from threshold to 100~eV. The accuracy of the results was checked by comparing them with available experimental data and testing different approximations with increasing number of coupled states. The largest scattering model included 483 states, most of which were pseudo-states that simulate the effect of the high-lying Rydberg continuum and, most importantly, the ionization continuum on the results for transitions between the discrete states of interest. This effect is particularly strong at ``intermediate'' incident energies of a few times the ionization threshold. The dataset generated from the largest model is estimated to be accurate to within a few percent for the cross sections of relevance for plasma modelling. [1] O.~Zatsarinny, Comp.\ Phys.\ Commun.~{\bf 174} (2006) 273. [2] O.~Zatsarinny and K.~Bartschat, J.~Phys.\ B~{\bf 46} (2013) 112001. [Preview Abstract] |
Thursday, May 31, 2018 3:24PM - 3:36PM |
S07.00005: Time-Resolved Interatomic Coulomb Elec\-tron Capture by Ba\textsuperscript{+} through Rb proximity Axel Molle, Oriol Vendrell, Annika Bande A time-resolved numerical investigation of the Interatomic Coulomb Electron Capture (ICEC) is presented. In the ICEC process, a species $A$ captures a free electron by long-range energy transfer through Coulomb interaction to a bound electron in a neighbouring species $B$. From a theoretical perspective, ICEC was first predicted for atoms and molecules through asymptotic approximation [1], and then successfully modelled by time-dependent techniques in low-dimensional systems [2]. From the experimental side, on the other hand, techniques for trapping ultracold ions and atom clouds are advancing. This may enable time-resolved ICEC experiments in the near future. We thus numerically study the dynamics of such an exemplary experiment at ultracold temperatures, with a barium cation trapped in a cloud of rubidium atoms, in order to predict and pave the way for time-resolved ICEC experiments.\\ ~[1] Gokhberg / Cederbaum, Phys.~Rev.~A \textbf{82} (2010).\\ ~[2] Pont \textit{et al.}, J.~Phys.~Condens.~Matter \textbf{28} (2016). [Preview Abstract] |
Thursday, May 31, 2018 3:36PM - 3:48PM |
S07.00006: Improved Collision Strength and Line Emissivity Ratios of Astrophysical Importance for Cl III Rahla Naghma, Sultana Nahar, Anil Pradhan The relativistic Breit-Pauli-R-Matrix method has been used to carry out computation for the electron impact excitation collision strengths for all the fine structure transitions within the $^4S^o$, $^2D^o$ and $^2P^o$ levels in the $3s^23p^3$ ground configuration of astrophysically important phosphorous-like ion Cl III. The effective collision strengths, obtained by averaging the electron impact excitation collision strengths over a Maxwellian distribution of electron velocities are presented over a wide temperature domain of astrophysical interest (Te=$10^3$ to $10^6$ K) for all the 10 transitions. We aim to develop an atomic model for forbidden optical and near-IR transitions among low-lying levels of the ground configuration, as well as UV lines among dipole transitions between the ground and excited configurations. Density and temperature dependent line emissivity ratios for various lines, including several observed lines, are computed and are compared with existing values. We find higher values than those available. Results will be presented and explanations for the difference will be discussed. [Preview Abstract] |
Thursday, May 31, 2018 3:48PM - 4:00PM |
S07.00007: Exchange and correlations in Ps-atom and Ps-molecule scattering Ilya Fabrikant, Robyn Wilde Exchange and correlations play a particularly important role in positronium (Ps) collisions with atom and molecules, since the static potential for Ps interaction with a neutral system is zero. Theoretical description of both effects is a very challenging task. In the present work we use the free-electron-gas model to describe exchange and correlations in Ps collisions with molecules similar to the approach widely used in the theory of electron-molecule collisions [1,2]. The results for exchange and correlation energies are presented as functions of the Fermi momentum of the electron gas and the Ps incident energy. Using the Thomas-Fermi model, we can convert these functions into exchange and correlation potentials for Ps interaction with molecules as functions of the distance between the projectile and the target. These potentials are used then for calculation of cross sections for Ps collisions with N$_2$ molecules. $^1$ S. Hara, J. Phys. Soc. Japan {\bf 22}, 710 (1967). $^2$ J. K. O'Connel and N. F. Lane, Phys. Rev. A {\bf 27}, 1893 (1983). [Preview Abstract] |
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