Double excitation of He by fast ions

Autoionization of He atoms following double excitation by electrons, protons, ${\mathrm{C}}^{\mathit{Q}+}$ (Q=4–6), and ${\mathrm{F}}^{\mathit{Q}+}$ (Q=7–9) ions has been studied. The electron-emission yields from the doubly excited 2${\mathit{s}}^2$ ${(}^1$S), 2s2p${(}^1$P), and 2${\mathit{p}}^2$ ${(}^1$P) 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 2${\mathit{s}}^2$ ${(}^1$S) and 2${\mathit{p}}^2$ ${(}^1$D) states increases as approximately ${\mathit{Q}}^3$, while excitation to the 2s2p${(}^1$P) state varies as approximately ${\mathit{Q}}^2$, where Q is the charge of the projectile. These charge dependences are significantly less than the ${\mathit{Q}}^4$ dependence expected in the independent-electron model, suggesting the interaction between the two target electrons is important in creating the doubly excited states.