Electron transfer in keV-energyHe++4collisions. II. Formation ofHe+4He+4(2s)in collisions with He, Ar,H2, andN2

Cross sections for single-electron transfer forming the metastable state ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}\mathrm{}\left(2s\right)\mathrm{}$ have been measured for a ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ beam incident on thin He, Ar, ${\mathrm{H}}_2$, and ${\mathrm{N}}_2$ gas targets. The ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ energy range was 7-70 keV. The experiments utilized detection of the $2s$ state via a delayed time coincidence between a field-induced Lyman-$\alpha$ photon and the ion itself. While the measured cross sections are believed accurate to ±35%, they are three times larger than the recent cross sections of Shah and Gilbody using incident ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ ions. The present time-coincidence technique directly measured the fraction of the total electron-transfer cross section resulting in metastable production with results ranging between 5% and 17%.