Ab initiocalculation of electron-capture cross sections inH++BeHcollisions

Abstract

We present calculations of electron-capture cross sections in collisions of ${\mathrm{H}}^{+}$ with BeH molecules in the energy range $25\phantom{\rule{0.28em}{0ex}}\mathrm{eV}lEl10\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$. We discuss the validity of the models employed to describe nonadiabatic ion-molecule collisions, specifically the eikonal approximation, the Franck-Condon approximation, and the isotropic approximation to obtain orientation-averaged cross sections, which is based on the infinite-order sudden approximation. The calculation using the Franck-Condon approximation leads to a total electron-capture cross section that is practically independent of the collision energy with a value of around ${25\phantom{\rule{0.28em}{0ex}}\AA{}}^{2}$. The calculations using the more accurate sudden vibrational approximation indicate that the Franck-Condon approximation overestimates the electron-capture cross section by at most 20%. At $El1$ keV, the main product of the electron-capture process is the formation of ${\mathrm{BeH}}^{+}(2{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+})+\mathrm{H}(1s)$. At higher energies, the cross sections for formation of ${\mathrm{BeH}}^{+}(2{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+})+\mathrm{H}(1s)$ and ${\mathrm{BeH}}^{+}(1{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}^{+})+\mathrm{H}(1s)$ are practically identical. The Coriolis couplings are particularly relevant to the mechanism of this reaction, which precludes the merging of semiclassical (including Coriolis couplings) and quantal results (neglecting Coriolis couplings) in the energy range of the present calculation.