Cross sections of O^q+(q=1 - 4) electron loss in collisions with He, Ne and Ar

Electron-loss cross sections of O^q+ (q=1-4) colliding with He, Ne and Ar atoms are measured in the intermediate velocity regime. The ratios of the cross sections of two-electron loss to that of one-electron loss R₂₁ are presented. It is shown that single-channel analysis is not sufficient to explain the results, but that projectile electron loss, electron capture by the projectile and target ionization must be considered together to interpret the experimental data. The screening and antiscreening effects can account for the threshold velocity results, but cannot explain the dependence of the ratio R₂₁ on velocity quantitatively. In general, the effective charge of the target atom increases with velocity increasing because the high-speed projectile ion can penetrate into the inner electronic shell of target atom. Ne and Ar atoms have similar effective charges in this velocity regime, but He atoms have smaller ones at the same velocities due to its smaller nuclear charge.