Electron capture in collisions of [Formula Presented] ions with S atoms and its reverse process below kilo-electron-volt energies

Electron capture in [Formula Presented] collisions is studied theoretically by using a semiclassical molecular representation with five molecular channels from the initial ground and excited states at collision energies above 10 eV. Electron capture in [Formula Presented] [Formula Presented] collisions is also investigated by using three molecular channels in order to assess the earlier estimation which gave the rate constant of [Formula Presented] at [Formula Presented] for the process. The ab initio potential curves and nonadiabatic coupling matrix elements for the [Formula Presented] system are obtained from multireference single- and double-excitation configuration-interaction calculations employing relatively large basis sets. Dominant capture channels corresponding to the [Formula Presented] and [Formula Presented] states lie lower by 0.2 and 1.4 eV, and 1.34 and 2.5 eV from the initial ground [Formula Presented] and excited [Formula Presented] states, respectively. The present results show that electron capture from the excited species is found to be rather weak at lower energies. But it rapidly becomes comparable to that from the ground state. Electron capture in [Formula Presented] [Formula Presented] collisions proceeds through the two-step mechanism at lower energies, and therefore, the cross section is found to be small with the value less than [Formula Presented] below 1 keV, thus supporting the earlier estimation.