Target ionization by highly charged ions at low velocities

The authors have found the effective threshold for target single ionization by highly charged ions. Cross sections for single ionization in the collisions of C6+, N7+, O8+, Ar12+, Ar16+, and Xe30+ on He, and Xeq+ (q = 25, 28, 30, 35, 40) on Xe were measured as a function of collision velocity from 0.2 a.u. to 1.7 a.u. The measured single ionization cross sections for the He target increase rapidly with increasing velocity starting around, for example, 1.4 a.u. for O8+. This effective threshold depends strongly on the projectile charge, and is in good agreement with the classical Bohr-Lindhard limit for ionization, vmin = q1/4 I1/2. Here I is the ionization potential of the target electron. For Xeq+ - Xe collisions, ionization was observed to be below 0.5% of single capture. For all charge states, the ionization cross section increases very slowly as the velocity increases up to 1.2 a.u., which is still below vmin for these collisions. No statistically significant double ionization was found in any case. Angular differential cross sections for the collisions of O8+ on He were also measured. The angular distribution for single ionization is very similar to that for single capture, suggesting that single ionization happens at a relatively large impact parameter