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Journal of Physics B Atomic Molecular and Optical Physics

Charge exchange collisions between alkaline-earth ions and alkali atoms

Ludo Vermeeren, Peter Lievens, Anita Buekenhoudt, R. E. Silverans

Published Mar 14, 1992• 5 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/25/5/014

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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.
A combined optical-pumping--laser-induced-fluorescence method was developed to determine the fraction of metastable-state ions in a Ba+{\mathrm{Ba}}^{+}Ba+ beam. This allowed us to measure the ratio of the charge-exchange cross sections for collisions of Ba+{\mathrm{Ba}}^{+}Ba+(6s) and Ba+{\mathrm{Ba}}^{+}Ba+(5d) ions with a sodium vapor for impact energies ranging from 3 to 29 keV. For a collision energy of 5 keV, the ratio of the cross sections for Ba+{\mathrm{Ba}}^{+}Ba+(5d 2^{2}2D3/2{\mathit{D}}_{3/2}D3/2​) and Ba+{\mathrm{Ba}}^{+}Ba+(5d 2^{2}2D5/2{\mathit{D}}_{5/2}D5/2​) was measured. All results are compared to theoretical near-resonance predictions.
We report the results of comparative studies of ion-pair formation and quenching processes in collisions of Rydberg Li( nl ) and Cs( nl ) atoms with Ca(4s 2 ), Sr(5s 2 ) and Ba(6s 2 ) atoms possessing small electron affinities. Our consideration includes both the cases of selectively excited Rydberg nl -states with small orbital angular momentum ( l ≪ n ) and nearly circular states with l ∼ n − 1. Calculations of the electron-transfer processes are based on the semiclassical theory of nonadiabatic transitions and exact expression for the Rydberg-covalent–ionic coupling terms. Calculations of nonresonant quenching processes are carried out within the framework of the available theory of inelastic and quasielastic transitions between Rydberg-covalent states. The ion-pair formation and resonant quenching cross sections are shown to be significantly dependent not only on the principal quantum number n but also on the orbital angular momentum l and the binding energy of the alkaline-earth anion. For each system under study we find the regions of n in which either the resonant quenching or the ion-pair formation processes are predominant. The relative role of the resonant and nonresonant mechanisms of depopulation of Rydberg states is investigated.
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 ${(\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i})}^{+}$ 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 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.
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.
Three mutually related problems concerning the interaction of an electron with an alkaline-earth atom are examined. (i) The scattering problem: the phaseshifts for p-electron scattering by Ca, Sr and Ba are calculated. (ii) The negative ion problem: the wavefunction and the binding energy are obtained for the outer electron in Ca - 4s 2 4p, Sr - 5s 2 5p and Ba - 6s 2 6p ions. (iii) The photodetachment problem: the photodetachment cross sections are calculated for the outer electron in each of these ions. The most intriguing feature of these problems is the strong correlational potential, attracting the electron to the atom due to the large dipole polarizabilities of Ca, Sr and Ba. The calculations are performed by means of the Dyson equation of the many-body theory.