Ionization in collisions of metastable He with He

Detailed differential cross-section measurements have been performed for production of ${\mathrm{He}}^{+}$ in collisions of metastable ${\mathrm{He}}^{*}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S \mathrm{and} 2{}_{}{}^1{}_{}{}^{}S\right)$ with He at center-of-mass energies of 50, 99, and 199 eV. The product ion distribution contains several distinct features dominated by an intense peak at small angles and an energy-loss value near threshold. The major features of the ionization are shown to be consistent with various calculated processes, each of which follows the ${\Sigma }_g^{+}$ diabatic repulsive potential on the incoming trajectory. Ionization occurs at the crossing into the continuum, at the classical inner turning point, at crossings with higher continua, and through molecular autoionization of doubly excited neutral states. Possible contributions from ${\mathrm{He}}^{+}$ + ${\mathrm{He}}^{-}$ autoionizing states are considered. Evidence is found for significant contributions to the ionization from the small amount of ${\mathrm{He}}^{*}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ in the beam.