Double excitation of He by fast ions

Abstract

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}$P) states were measured at the reduced projectile energy of 1.5 MeV/nucleon for observation angles between 10\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{}. The results indicate excitation to the 2${\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.