Mm. Santanna et al., ELECTRON LOSS MECHANISMS IN COLLISIONS OF HE+ IONS WITH VARIOUS TARGETS, Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms, 99(1-4), 1995, pp. 46-49
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.