The mechanism of an observed ionization channel which produces He+ at an energy loss of 18.7±0.5 eV in 100-200-eV (c.m.) collisions of metastable He atoms with ground state He atoms was investigated. It is postulated that flux follows the diabatic singly excited g1+ state associated with He*(2S1) projectiles and then transfers to the doubly excited g1+ state corresponding asymptotically to 2He*(2S3); autoionization follows at large internuclear separations. Ab initio calculations were performed on four He2 states of g1+ symmetry for internuclear separations of 0.72a0 to 20.0a0. The classical deflection function for the mechanism stated above, generated from the calculated potentials, is consistent with the experimental angular distribution of the product ions. © 1977 The American Physical Society.