Ionization processes in He-He collisions

Differential scattering measurements of ionization in He-He collisions are made in the keV energy range using the energy loss technique. Combining the results with those obtained from the ejected electron spectra measurements of Gerber et al., the ionization process is discussed in the framework of the quasimolecular picture of the collision. Two-step mechanisms are proposed involving a primary radial or rotational coupling either to a molecular state which autoionizes during the collision or to a state which leads on dissociation to the formation of He- in an auto-ionizing state. Calculations taking account of rotational coupling effects between the 1 Sigma g, 1 Pi g and 1 Delta g states of He2 are in satisfactory agreement with experiment for energy E and scattering angle theta when E theta exceeds 3 keV deg.