Electron detachment in low-energy collisions ofH−andD−with He

Measurements and calculations have been made of elastic scattering and electron detachment in collisions of ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ with He at energies of 5-120 eV. The measurements show no other inelastic processes occurring in this energy range. The mechanism responsible for electron detachment is assumed to be the crossing of the ${\mathrm{H}}^{-}$ bound state with the continuum of free states; the bound state is then assigned a complex energy. The measured elastic scattering differential cross section shows no structure except at $E\theta \sim 200$ eV deg, where there is a region of downward curvature in the graph of $log\sigma$ vs $\theta$. This is interpreted as the threshold angle for electron detachment, and it is directly related to the crossing point. By empirically fitting the experimental differential cross section, the general features of the complex potential were obtained. By using the resulting potentials, the total detachment cross section was calculated and compared to the experimental results of Bailey, May, and Muschlitz. Only fair agreement is found at low energies, and poor agreement at higher energies (>100 eV). The theory predicts an isotope effect in the elastic differential cross section, and this effect provides a test of the theory. A careful series of experiments at 20 eV displayed the effect. A preliminary measurement of the electron energy spectrum was also made.