Observation of Direct Ionization of He by Highly Charged Ions at Low Velocity

We have observed direct target ionization of He at low velocity (0.2 to 1.7 a.u.) by highly charged ions of C⁶⁺, N⁷⁺, O⁸⁺, Ar¹⁶⁺, and Xe³⁰⁺. The measured single ionization cross sections increase rapidly with increasing velocity. The cross sections, divided by the charge of the incident projectile, are found to lie on a universal curve when plotted versus the ratio of the projectile velocity to the classical Bohr-Lindhard velocity limit for ionization. Impact parameter information was determined for O⁸⁺ on He collisions and suggests that single target ionization happens over an impact parameter range very similar to that for single capture.