Electron loss by atomic and molecular hydrogen in collisions withHe++3andHe+4

Total cross sections for single-electron capture by ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ incident on atomic and molecular hydrogen have been measured in the velocity range (1-6)×${10}^8$ cm/sec. A classical-trajectory Monte Carlo method was also used to calculate the electron-capture and impact-ionization cross sections for ${\mathrm{He}}^{++}$ on H in this velocity range. The theoretical electron-capture cross sections are found to be in good agreement with the present experimental values at velocities above 2.5×${10}^8$ cm/sec, but tend to underestimate the experimental values by roughly 25% at the lowest velocities. The experimental ${\mathrm{He}}^{++}$ + H electron-capture cross sections are in excellent agreement with the measurements of Shah and Gilbody in the velocity region where the two experiments overlap. No significant isotope effect (${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ versus ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$) could be discerned on the theoretical cross sections as a function of relative velocity.