ELECTRON LOSS MECHANISMS IN COLLISIONS OF HE+ IONS WITH VARIOUS TARGETS

The electron loss of high-velocity ions by neutral atoms is due to two different and competing mechanisms. In the screening mode, the electr on loss is basically due to the nucleus-electron interaction, with the target electrons assuming the passive role of decreasing the Coulomb field of the target nucleus in the vicinity of the projectile active e lectron. For a fixed projectile velocity, this contribution is expecte d to give a non-linear dependence with the target atomic number Z(2) d ue to the incomplete screening at the impact parameter region where th e projectile ionization is more likely to occur. Within first-order th eories, if the screening is completely absent, the expected dependence would be Z(2)(2); With screening, it should scale between Z(2) and Z( 2)(2). On the other hand, in the antiscreening mode, where the loss is due to the action of the target electrons and the target nucleus play s no active role, the expected dependence would be approximately linea r with Z(2) Thus, for first-order theories, the expected overall depen dence with Z(2) would be dominated by the screening mode as Z(2) incre ases. We have measured total electron-loss cross sections of He+ ions impinging upon He, Ne, Ar, Kr and Xe targets in the energy range from 1.0 to 4.0 MeV to complement previous measurements and the results poi nt towards a much smaller contribution from the screening mode than ex pected from first-order theories, possibly due to a saturation effect manifested only in the screening channel.