Bulletin of the American Physical Society
72nd Annual Gaseous Electronics Conference
Volume 64, Number 10
Monday–Friday, October 28–November 1 2019; College Station, Texas
Session LW1: Electron and Positron Collisions with Atoms |
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Chair: Dmitry Fursa, Curtin University Room: Century I |
Wednesday, October 30, 2019 1:45PM - 2:15PM |
LW1.00001: Electron Scattering from Complex Atoms and Ions: Current Status and Future Prospects. Invited Speaker: Klaus Bartschat Accurate data for electron collisions with atoms and ions are required for many modelling applications in a variety of fields, including astrophysics, atmospheric physics, as well as plasma physics over a wide range of electron temperatures [1]. Since it is virtually impossible to measure all the data needed for state-of-the-art collisional radiative models (CRMs), much of the responsibility for generating sufficiently comprehensive datasets has been put on theory. Consequently, a variety of methods have been developed and applied since the early days of quantum mechanics [2]. They include special-purpose approaches that are suitable but also limited to particular processes (e.g., elastic scattering), perturbative techniques (e.g., first- and second-order plane-wave or distorted-wave methods, which are usually limited to sufficiently high energies), and the non-perturbative close-coupling (CC) approach that is based on an (in principle complete) expansion of the projectile + target scattering wave function. CC methods were originally designed for low energies and near-threshold resonances, but the inclusion of so-called ``pseudo-states'' has extended the regime of applicability tremendously, even enabling the calculation of ionization cross sections. While the problem for electron collisions with light (quasi-)one- and (quasi-)two-electron targets (H, He, light alkalis and alkaline-earth elements, and the corresponding iso-electronic ions) is considered to be essentially solved, this is by no means the case for heavy, complex, open-shell targets. Examples include Fe and its lowly charged ions, which are of tremendous importance for astrophysics, the heavy noble gases (Ne$-$Xe) for modelling of various plasmas, and targets like W and its ions where data are needed to model fusion reactors. In this talk, I will introduce the basic ideas behind a selection of methods, discuss their strengths and weaknesses, and concentrate on how to assess the quality of the data [3] that are available from a number of databases maintained worldwide. [1] K. Bartschat, Journal of Physics B {\bf 51} (2018) 132001. [2] K. Bartschat, J. Tennyson, and O. Zatsarinny, Plasma Processes and Polymers {\bf 49} (2017) 1600093. [3] H.K. Chung {\it et al.}, Journal of Physics D {\bf 49} (2016) 363002. [Preview Abstract] |
Wednesday, October 30, 2019 2:15PM - 2:30PM |
LW1.00002: Modelling low energy laser assisted free-free scattering B.A. deHarak, K.C. North, R. Zhang, N.L.S. Martin Free-free scattering, particularly when it involves electrons with incident energies below the ionization potential of the target atom, can be a significant contributor to the opacity of a medium. Here we present calculations and Monte Carlo simulations of laser assisted free-free scattering for incident energies from 1 eV to 20 eV using the Kroll Watson Approximation (KWA) \footnote{N. M. Kroll and K. M. Watson Phys. Rev. A 8, 804 (1973)}. According to the KWA, laser assisted free-free scattering is target independent. Our results are therefore generic for all neutral atomic and molecular species. [Preview Abstract] |
Wednesday, October 30, 2019 2:30PM - 2:45PM |
LW1.00003: EUV emission from Sn plasmas James Colgan, Joris Scheers, Francesco Torretti, Ruben Schupp, Amanda Neukirch, Joe Abdallah, Manolo Sherrill, Christopher Fontes, Peter Hakel, Oscar Versolato, Dave Kilcrease We report on our continuing efforts to understand the EUV emission from tin plasmas of interest to nanolithography. The intense and narrow emission of EUV radiation centered at 13.5~nm is of interest to the micro-electronics industry where it has much potential for etching smaller features on micro-processors. The tin plasma of relevance (at a temperature around 30 eV and electron density around 10$^{21}$ cm$^{-3}$) is interesting from an atomic physics perspective, since it requires highly accurate atomic structure and transition information from complex open $4p$ and $4d$ subshells of tin ions from Sn$^{8+}$ to Sn$^{15+}$. Such calculations involve large-scale structure calculations involving many thousands of energy levels that require inclusion of full configuration-interaction to obtain the required accuracy. We use the Los Alamos suite of atomic physics codes and plasma emission calculations from the ATOMIC code to model such tin plasmas and compare them to recent measurements of emission from molten tin droplets illuminated by a high-intensity Nd:YAG laser performed at the ARCNL. We find good agreement between the modeling predictions and the emission measurements at various laser intensities. Latest results will be discussed. [Preview Abstract] |
Wednesday, October 30, 2019 2:45PM - 3:00PM |
LW1.00004: A multipass laser system for free-free experiments C.M Weaver, B.N. Kim, N.L.S. Martin, B.A. DeHarak A multipass laser system is being developed for an electron scattering apparatus that will be used for laser-assisted free-free electron scattering experiments. The basic idea is that a gated Pockels cell is used to rotate horizontally polarized light, in an injection mode, to vertically polarized light which is then trapped in a repetitive path using mirrors and a polarizing beam-splitter cube. A test bed has proved the feasibility of this technique: 20 passes have been observed with a 20\% loss per round trip; this would correspond to in an increase of a factor of 5 in data collection. The system is now being installed on the scattering apparatus with similar initial results; we will present a progress report on this multipass system. We hope to increase the efficiency of the system so that there is only 10\% loss per round trip in order to achieve an order of magnitude improvement in data collection compared to that of a single pass set up. [Preview Abstract] |
Wednesday, October 30, 2019 3:00PM - 3:15PM |
LW1.00005: Calculation of positron scattering on the hydrogen negative ion Dmitry Fursa, Ravshanbek Utamuratov, Alisher Kadyrov, Igor Bray Positron collisions with the hydrogen negative ion have been a subject of interest due to exotic nature of the collision system and its role in understanding positron propagation through the interstellar media. This collision system is directly related to the Ps-H scattering problem which provides a testing ground for theoretical studies of Ps interactions with media. Recent development of a coordinate-space method (Utamuratov {\em{et al}}, Comput. Phys. Commun. {\bf 239} (2019) 64) to calculate Ps-formation matrix elements has allowed application of the two-center CCC approach to e$^+$-H$^-$ scattering. Accurate results have been obtained for Ps-formation, electron detachment and electron-loss cross sections for impact energy range from 0.1~eV to 1~keV. [Preview Abstract] |
Wednesday, October 30, 2019 3:15PM - 3:45PM |
LW1.00006: Vortices for positron ionization and positronium formation Invited Speaker: S. J. Ward Recently, vortices in the velocity field associated with the ionization amplitude provided an explanation [1] for a deep minimum in experimental measurements of the triply differential cross section (TDCS) for electron-helium ionization [2]. We applied the Coulomb-Born approximation to this process [3] and obtained a deep minimum in the TDCS whose position was in reasonable agreement with a time-dependent close-coupling calculation [4]. Recently, we applied the Coulomb-Born approximation to positron-helium ionization in the double symmetric in-plane geometry and obtained a deep minimum [3]. We computed the velocity field associated with the transition matrix element and noticed that for positron ionization it rotates in the opposite direction to that for electron ionization for the same target helium. Recently, using the Kohn and inverse Kohn variational methods we found two zeros in the scattering amplitude for Ps-formation in positron-hydrogen collisions in the Ore gap [5]. We introduced an extended velocity field associated with the Ps-formation scattering amplitude in which both the incident energy and angle of the outgoing Ps atom are allowed, and we noticed that this field rotates around the two zeros and in opposite directions [5]. \begin{thebibliography}{References} \bibitem{[1]} J. H. Macek, J. B. Sternberg and S. Yu. Ovchinnikov, Phys. Rev. Lett. {\bf 104}, 033201 (2010). \bibitem{[2]} A. J. Murray and F. H. Read, Phys. Rev. A {\bf 47}, 3724 (1993). \bibitem{[3]} C. M. DeMars, J. B. Kent and S. J. Ward, http://meetings.aps.org/Meeting/DAMOP19/Session/L01.13 \bibitem{[4]} J. Colgan, O Al-Hagan, D. H. Madison, A. J. Murray and M. S. Pindzola, J. Phys.~B: At.~Mol. Opt. {\bf 42}, 171001 (2009). \bibitem{[5]} A.~W.~Alrowaily, S.~J.~Ward, P.~Van Reeth, {\it submitted}. \end{thebibliography} [Preview Abstract] |
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