Double excitation of He by fast ions

Autoionization of He atoms following double excitation by electrons, protons, CQ+ (Q=4--6), and FQ+ (Q=7--9) ions has been studied. The electron-emission yields from the doubly excited 2s²{(}¹$S), 2s2p${(}¹$P), and 2${p}²(¹P) states were measured at the reduced projectile energy of 1.5 MeV/nucleon for observation angles between 10^∘{} and 60^∘{}. The results indicate excitation to the 2s²{(}¹$S) and 2${p}²(¹D) states increases as approximately Q³, while excitation to the 2s2p(¹P) state varies as approximately Q², where Q is the charge of the projectile. These charge dependences are significantly less than the Q⁴ dependence expected in the independent-electron model, suggesting the interaction between the two target electrons is important in creating the doubly excited states.