Cross sections for antiproton capture by helium ions

Abstract

Accurate quantum-mechanical calculations are carried out for antiproton capture by singly ionized He ions, that is, $\mathrm{p\ifmmode \bar{}\else \={}\fi{}}+{\mathrm{He}}^{+}\ensuremath{\rightarrow}\mathrm{p\ifmmode \bar{}\else \={}\fi{}}{\text{He}}^{2+}+e$, by the combined use of an $R$-matrix method and direct numerical solution. The total capture cross sections (or collision strengths) and the state distributions of the capture products, $\mathrm{p\ifmmode \bar{}\else \={}\fi{}}{\mathrm{He}}^{2+}$, are calculated at collision energies ranging from 0 to $4$ eV. The present system is rich in resonances, which appear as a Rydberg series and are characterized as an electron attached to an ion core, $\mathrm{p\ifmmode \bar{}\else \={}\fi{}}{\mathrm{He}}^{2+}$. Some resonance levels can be reproduced by a reasonable model based on molecular-quantum-defect theory. The capture into the highest energetically possible state of the products, $\mathrm{p\ifmmode \bar{}\else \={}\fi{}}{\mathrm{He}}^{2+}$, always takes place overwhelmingly in the presence or absence of the resonances. However, owing to the resonances, the angular momentum distribution of the products varies considerably depending on the collision energy. The energy-averaged capture cross sections and the capture rate coefficients are also presented for the convenience of future experiments.