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Physical Review A

Comment on “Entropy lowering in ion-atom collisions”

Valentin N. Ostrovsky

Published Jan 18, 2006• 0 citations
Publisher

Abstract

The recent experimental result by Nguyen et al. [Phys. Rev. A 71, 062714 (2005)] on the ratio of cross sections for charge exchange processes ${\mathrm{Rb}}^{+}+\mathrm{Rb}(5s)\ensuremath{\rightarrow}\mathrm{Rb}(5p)+{\mathrm{Rb}}^{+}$ and ${\mathrm{Rb}}^{+}+\mathrm{Rb}(5p)\ensuremath{\rightarrow}\mathrm{Rb}(5s)+{\mathrm{Rb}}^{+}$ is quantitatively derived from simple considerations within the general framework of the quasimolecular theory. Contrary to the expectations, applicability of the Demkov model for charge exchange with small energy defect is not shattered.

DOI
10.1103/physreva.73.016702

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The recent experimental result by Nguyen et al. [Phys. Rev. A 71, 062714 (2005)] on the ratio of cross sections for charge exchange processes Rb++Rb(5s)\ensuremath→Rb(5p)+Rb+{\mathrm{Rb}}^{+}+\mathrm{Rb}(5s)\ensuremath{\rightarrow}\mathrm{Rb}(5p)+{\mathrm{Rb}}^{+}Rb++Rb(5s)\ensuremath→Rb(5p)+Rb+ and Rb++Rb(5p)\ensuremath→Rb(5s)+Rb+{\mathrm{Rb}}^{+}+\mathrm{Rb}(5p)\ensuremath{\rightarrow}\mathrm{Rb}(5s)+{\mathrm{Rb}}^{+}Rb++Rb(5p)\ensuremath→Rb(5s)+Rb+ is quantitatively derived from simple considerations within the general framework of the quasimolecular theory. Contrary to the expectations, applicability of the Demkov model for charge exchange with small energy defect is not shattered.
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.
The authors report on the observation of a laser-induced charge exchange process in a Sr + -Ba mixture. Two maxima occur in the cross section profile when the barium atoms are ionised to the final 5p 2 P 1/2 level. The photon energy for each maximum differs from the energy defect of the reaction partners at infinite separation. In the framework of the quasimolecular model, a complete analysis of the profile is derived. The first peak corresponds to the expected assisted signal while the second one is interpreted as a satellite line.
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.