Session PR1: Electron-Atom Collisions

Angular Momentum in Electron-Helium Collisions

The study of atomic ionization collisions provides crucial information about the interactions of charged particles, and helium has long been a target atom of interest due to its simple, but unique electronic structure. Recently, a growing body of work has focused on ionization collisions involving oriented targets, which can provide even greater detail about charged particle interactions. In particular, the use of oriented targets is ideal for the study of angular momentum in the collision process. Here we present theoretical results that examine the sources and role of angular momentum in ionizing collisions between electron projectiles and helium targets. In particular, we examine angular momentum transfer from the projectile electron to the ionized electron, as well as angular momentum within the target atom. We show that the amount of angular momentum transferred to the ionized electron can depend on the collision kinematics, as well as the collision process itself. We present both fully and double differential cross sections and show that both the projectile and target atom are sources of angular momentum.   

Light polarization after electron impact excitation of Zinc.

There is significant controversy regarding the polarization of the radiation emitted in the $\rm (4s 5s)^3S_1\to(4s 4p)^3P_{0,1,2}$ transitions in Zn after impact excitation by spin-polarized electrons. The recent experimental data of Clayburn and Gay~[1] agree well with those of Pravica {\it et al.}~[2] for incident energies above the thresholds for the $\rm (4s 5p)^3P_{0,1,2}$ states, which can populate the $\rm (4s 5s)^3S_1$ state by cascades. However, there is major disagreement for the linear light polarization $P_2$ ($45^\circ/135^\circ$) in the energy region where the state can only be excited directly (by electron exchange). For these energies, Bartschat and Blum~[3] predicted $P_2$ to be zero, in an $LS$-coupled model that is expected to be accurate for the problem at hand. While the data of~[1] support the prediction of~[3], the question arises how significant linear polarizations can occur even in the cascade regime, since they require the cascading radiation to be strongly polarized as well. We performed extensive numerical calculations to address this issue. [1]~N.~B.\ Clayburn and T.~J.~Gay, Phys.~Rev.\ Lett.~{\bf 119} (2017) 093401. [2] L.~Pravica {\it et al}., Phys.~Rev.~A~{\bf 83} (2011) 040701R. [3] K.~Bartschat and K.~Blum, Z.~Phys.\ A~{\bf 304} (1982) 85.   

Cross Sections and Spin Asymmetries for Electron Collisions with Lead Atoms

We present integrated and angle-differential cross sections as well as spin asymmetries for elastic and inelastic electron collisions with lead atoms. The results were obtained using the fully relativistic convergent close-coupling (RCCC)~[1] and the Dirac B-spline R-matrix (DBSR)~[2] methods. They will be compared with experimental data and predictions from previous calculations. In particular, we find substantial discrepancies regarding the normalization of the experimental data at a number of energies for elastic and excitation cross sections. The spin asymmetries for the optically forbidden inelastic transitions from the (6p$^2$)$^3$P$_0$ ground state to other states of the 6$p^2$ manifold, measured by Geesmann {\it et al.}~[3], are known to be very challenging for theory~[4]. We analyze the sensitivity of the predictions to the quality of the target description as well as the number of channels included in the close-coupling expansion. [1] D.\ V.\ Fursa and I.\ Bray, Phys.\ Rev.\ Lett.~{\bf 100} (2008) 113201. [2] O.\ Zatsarinny and K.\ Bartschat, Phys.\ Rev.\ A~{\bf 77} (2008) 062701. [3] H.\ Geesmann, M.\ Bartsch, G.\ F.\ Hanne, and J.\ Kessler, J.\ Phys.\ B~{\bf 24} (1991) 2817. [4] O.\ Zatsarinny, Y.\ Wang, and K.\ Bartschat, J.\ Phys.\ B~{\bf 46} (2013) 035202.   

Dressing effects in laser assisted inelastic electron-atom scattering

How does a laser field influence the scattering of an electron by an atom or molecule? That is the question investigated by laser-assisted electron scattering (LAES) experiments. The first, and most commonly performed LAES experiments involved elastic scattering. Most of those experiments were well described by the Kroll-Watson Approximation\footnote{N. M. Kroll and K. M. Watson Phys. Rev. A 8, 804 (1973).} which ignores laser-target interactions. However, if the cross sections predicted using the KWA are small, then laser-target interactions are not necessarily negligible. Morimoto, et al reported the first observation of laser-target interactions in an LAES experiment in 2015\footnote{Y. Morimoto, R. Kanya and K. Yamanouchi Phys. Rev. Lett. 115, 123201 (2015)}. Their experiments were extremely challenging largerly due to the fact that the polarization of the target xenon atoms by the laser-field dressing was small, 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-dressing effects during \textit{inelastic} electron-argon, and electron-helium scattering. The polarizability of excited argon, and helium is relatively large, making dressing effects easier to observe.   

Non-perturbative calculations for electron-impact ionization of low charge states of tungsten

Tungsten is a leading material for plasma facing components in future burning plasma devices. Accurate atomic data for low charge states of tungsten will be necessary, for spectroscopic diagnosis of the divertor region and to gauge wall material erosion rates. A relativistic, non-perturbative R-matrix electron-impact excitation calculation has recently been completed for neutral tungsten. In conjunction with reliable electron-impact ionization data, this calculation enables the determination of S/XB ratios and by extension erosion rates. We present new R-matrix electron impact ionization calculations for ground and select metastable terms of W I, W II, and W III along with comparisons to recent configuration-averaged Time-Dependent-Close-Coupling calculations converted to LSJ-resolution using appropriate angular factors. When the ionization cross sections and rate coefficients are compared to the currently used results from Exchange Classical Impact Parameter and Coulomb Born methods, sizeable differences are found.   

Laser driven production of antimatter molecules

Recent years have seen marked progress in the production of, and experimentation with, atomic antimatter in the form of antihydrogen, $\overline{\rm H}$. Now we investigate the feasibility of producing the anti-molecular hydrogen anion, $\overline{{\rm H}}_2^-$ (consisting of two anti-protons and and a positron), in the laboratory. Utilizing reaction rates calculated here involving the interaction of antiprotons trapped with antihydrogen atoms, key processes are identified that could lead to anion production. These reactions are discussed in the context of present day and near future experimental capabilities.   

Dirac B-spline R-Matrix Calculations for e--Xe$^+$ collisions.

Xenon is the most important ingredient for electric propulsion systems, including Hall and ion thrusters. The performance of these devices depends critically on the kinetic processes involving Xe$^+$ ions. However, in numerical simulations of such thrusters, excited states of Xe$^+$ cannot be studied due to the lack of fundamental cross-section data. Also, ionic emission lines are absent in the non-invasive diagnostic approach of optical emission spectroscopy, due to the lack of collisional-radiative models based on a reliable set of atomic data. We applied a relativistic Dirac B-spline R-matrix (DBSR) method to calculate oscillator strengths and electron-impact excitation cross sections for the $\rm 5s^25p^5,~5s5p^6,~5p^46s,~5p^45d,~5p^46p,~5p^47s$, and $\rm 5p^46d$ states of Xe$^+$, which exhibits a complex open-shell structure with strongly term-dependent orbitals. The excitation cross sections from the ground, metastable, and quasi-metastable states of Xe$^+$ are analyzed. We will use the present e--Xe$^+$ cross sections together with those obtained for e--Xe~[1] to build a complete CR model and to compare its predictions with experimental data from Hall and ion thrusters. [1] O.~Zatsarinny and K.~Bartschat, Phys. Scr. T{\bf 134} (2009) 014020.   

Electron impact fine structure excitation cross sections of Xe$^{\mathrm{+}}$

Xenon is an ideal and preferred propellant for Hall thrusters......~and ion thrusters. The observed emitted spectra from xenon thruster plasma show also lines from Xe$^{\mathrm{+}}$ [1].$^{\mathrm{\thinspace }}$For the diagnostics of such plasma a collisional-radiative (CR) model is required which involve the dominant process of electron impact excitation (EIE). There are no experimental and theoretical results available for EIE of Xe$^{\mathrm{+}}$. We report detailed EIE results of Xe$^{\mathrm{+}}$ using our fully relativistic distorted wave theory [2] from its ground 5$p^{\mathrm{5}} (J=$3/2) state to the fine structure excited states of 5$p^{\mathrm{4}}$6$s$, \quad 5$p^{\mathrm{4}}$6$p$, \quad 5$p^{\mathrm{4}}$7$s$, \quad 5$p^{\mathrm{4}}$7$p$, \quad 5$p^{\mathrm{4}}$5$d $and \quad 5$p^{\mathrm{4}}$6$d $configurations. [1] Y. H. Chiu \textit{et al}., \textit{J. Appl. Phys.} \textbf{99}, 113304, 2006 [2] Dipti and R. Srivastava, JQRST$.$ \textbf{176}, 1223, 2016