Electron Emission Cross Sections for Collisions of Dressed Ions with Atomic and Molecular Targets

Abstract

As ions pass through matter at sufficiently low impact energies, less than a few hundred keV.u-1, electrons are captured by the ion. Subsequent collisions can remove these, or other, projectile electrons from the ion. Although the actual energy lost by the projectile during charge changing collisions may be quite small, captured electrons may have a profound affect on the ionisation efficiency of the moving particle. For distant collisions involving high energy ions, the primary effect of bound projectile electrons is to screen the projectile nuclear charge and therby reduce the probability for energy loss. As the ion slows to below a few hundred keV.u-1, however, rather than simply screen the projectile nuclear charge, the 'incident' bound electrons tend to contribute to the target ionisation and the probability for target ionisation may actually be enhanced with respect to that expected for an incident fully stripped ion. This can be particularly important for neutral particle impact because of the larger number of bound electrons involved. Studies with hydrogen, helium and carbon ions and neutrals show that the cross sections for ejection of low-energy electrons (1-20 eV) by neutral atoms is comparable to, or greater than, those for equal velocity proton impact and that the cross sections for ejecting fast electrons (binary encounter electrons) are also increased by the presence of bound projectile electrons.