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Physical Review A

Systematic theoretical investigation of charge exchange in He+ -alkali-atom collisions

Victor Sidis, C. Kubach, J. Pommier

Published Jan 1, 1981• 27 citations
Publisher

Abstract

A comprehensive systematic study of near-resonant charge exchange into ${\mathrm{He}}^{*}(n=2)$ levels in ${\mathrm{He}}^{+}$ -alkali-atom collisions is reported. The relevant quasimolecular properties of the ${(\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i})}^{+}$ systems are obtained within the one-electron projected-valence-bond method of Kubach and Sidis. The detailed investigation of the considered systems enables one to assess the relevance of the two-state Demkov model for near-resonant charge exchange and the role of rotational coupling. The present multistate close-coupling treatment considerably improves upon the previous simplified theory of Olson and Smith and nicely reproduces the measured relative abundance of the specific ${\mathrm{He}}^{*}(2^{3,1}S,2^{3,1}P)$ states formed by charge exchange. The calculated summed total charge-exchange cross sections generally agree with experiment for ${\mathrm{He}}^{+}$-Cs, -Rb, and -K, whereas problems remain for ${\mathrm{He}}^{+}$-Na.

DOI
10.1103/physreva.23.119

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A comprehensive systematic study of near-resonant charge exchange into He∗(n=2){\mathrm{He}}^{*}(n=2)He∗(n=2) levels in He+{\mathrm{He}}^{+}He+ -alkali-atom collisions is reported. The relevant quasimolecular properties of the (He\ensuremath−alkali)+{(\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i})}^{+}(He\ensuremath−alkali)+ systems are obtained within the one-electron projected-valence-bond method of Kubach and Sidis. The detailed investigation of the considered systems enables one to assess the relevance of the two-state Demkov model for near-resonant charge exchange and the role of rotational coupling. The present multistate close-coupling treatment considerably improves upon the previous simplified theory of Olson and Smith and nicely reproduces the measured relative abundance of the specific He∗(23,1S,23,1P){\mathrm{He}}^{*}(2^{3,1}S,2^{3,1}P)He∗(23,1S,23,1P) states formed by charge exchange. The calculated summed total charge-exchange cross sections generally agree with experiment for He+{\mathrm{He}}^{+}He+-Cs, -Rb, and -K, whereas problems remain for He+{\mathrm{He}}^{+}He+-Na.
The ratio of the charge exchange cross sections for Ba + (5d) and Ba + (6s) on Na, Rb and Cs, for Sr + (4d) and Sr + (5s) on Na and Rb, and for Ca + (3d) and Ca - (4s) on Na have been measured for ion impact energies between 0.5 keV and 30 keV. For the collisions with Na, absolute charge exchange cross section values are derived using data for the resonant process Na+Na + as a reference. The results are compared to theoretical near-resonance predictions. A method is proposed to study the polarization dependence of the neutralization cross sections for reactions involving ionic D states.
Four different spin-dependent scattering cross sections are calculated by semiclassical and by partial-wave analyses for each of the five alkali atoms, Li, Na, K, Rb, and Cs, for 124 different relative kinetic energies. These cross sections are finally averaged thermally. The results agree very well with experiments.In the calculation, the interaction potential between two alkali atoms is calculated by individually considering the interaction due to the two valence sss electrons and to the closed electronic shells of the two atoms. The two-sss-electron interaction is generated ad hoc phenomenologically from the known hydrogen-hydrogen interatomic potential and the closed-shell interaction from interatomic potentials of two inert atoms corresponding to the cores. The potentials so obtained are in agreement with those constructed from band spectra.
Collisions of alkali-metal atoms in the ground state with the electron spin S = 1/2 are accompanied by exchange of electron coordinates between colliding particles, which leads to polarization transfer between them (i.e., to the well-known phenomenon of spin exchange). In addition, along with the polarization transfer from one partner to another, the magnetic resonance lines of colliding atoms are broadened and shifted in spin-exchange collisions. In particular the last two processes depend on the complex spin-exchange cross section. The real part of the cross section determines the so-called "spin-exchange cross section," which is responsible for the broadening of magnetic resonance lines, while the imaginary part determines the shift cross section, which governs the frequency shift of magnetic resonance. In order to describe the spin-exchange process one have to know complex spin-exchange cross sections. Complex spin-exchange cross sections are calculated on the basis of the data on the singlet (X 1 Σ + ) and triplet (a 3 Σ + ) potentials describing the interaction alkali-metal atoms in the ground state. Collisions of alkali-metal atoms Cs, Rb, K, Na in the ground state are considered in the energy interval of 10 −4 −10 −2 au.
The pseudopotential molecular-structure method has been used to calculate the X1{X}^{1}X1\ensuremath{\Sigma} and a3{a}^{3}a3\ensuremath{\Sigma} interaction potentials for the alkali-metal-atom--hydrogen-atom systems. These potentials were then used in a quantum-mechanical calculation to determine the spin-exchange cross sections in the energy range from 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ eV to 2.5 eV. The cross sections follow the general form Q1/2{Q}^{1/2}Q1/2=a-b lnv. However, abundant structure on the cross sections is present due to orbiting resonances induced by the deeply bound well of the X1{X}^{1}X1\ensuremath{\Sigma} molecular state. The spin-exchange cross sections range from 17\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ to 25\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ cm2{\mathrm{cm}}^{2}cm2 at room temperature for H(F=1) transferring to H(F=0) in collision with unpolarized lithium and cesium, respectively.