Electron transfer in keV-energyHe++4atomic collisions. I. Single and double electron transfer with He, Ar,H2, andN2

Single- and double-electron-transfer cross sections have been measured for a ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ beam incident on thin He, Ar, ${\mathrm{H}}_2$, and ${\mathrm{N}}_2$ gas targets over the energy range 15-125 keV. Comparison with previous ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ experimental results is made at the same incident particle velocity. For ${\mathrm{He}}^{++}$ collisions with He, we find good agreement with the theory for double electron transfer, but conclude that the single-electron-transfer cross section is far from being understood. Interpretation of the data in terms of simple theoretical ideas indicates that capture into excited states of the ${\mathrm{He}}^{+}$ ion can be the dominant single-electron-transfer process in some keV-energy ${\mathrm{He}}^{++}$ atomic collisions.