Neutralization behavior in medium energy ion scattering

A review is given of some of the experimental literature on the neutralization of ions having primary energies of 5 to 200 keV, as they are reflected from solid surfaces or pass through thin foils. Most of the data are presented in the form of ion-fractions (Y+/Y) as functions of energy of the scattered particles. A transition is observed: from the behavior of H and He at 30 to 200 keV in which there is no direct evidence of neutralization inside the solid, either because it does not occur or because equilibration lengths are too short to be resolved, to that found in the 5 to 30 keV range where H, He, Ne, and Ar have all shown evidence of enhanced neutralization of ions which penetrate the surface. Present evidence does not permit a confident choice between the theoretical models for protons above 10 keV which assert that neutralization inside the solid is prevented by electron screening, and another (proposed primarily for protons at higher velocities) which accepts the older concept of alternate electron capture and loss inside. Neutralization and scattering results demonstrate that the sharply peaked ion spectra of low energy ion scattering result from a combination of two factors: large scattering cross sections which cause depletion of the ion beam as it enters and leaves the solid and more efficient neutralization of ions which penetrate beyond the surface