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 t o 1.7 a.u.) by highly charged ions of C6+, N7+, O8+, Ar16+, and Xe30+. The measured single ionization cross sections increase rapidly with i ncreasing velocity. The cross sections, divided by the charge of the i ncident projectile, are found to lie on a universal curve when plotted versus the ratio of the projectile velocity to the classical Bohr-Lin dhard velocity limit for ionization. Impact parameter information was determined for O8+ on He collisions and suggests that single target io nization happens over an impact parameter range very similar to that f or single capture.