The role of exciton diffusion in the electron induced sputtering of alkali halides

Exciton diffusion allows excitons created some distance beneath the surface to contribute to sputtering and causes the sputtering efficiency to peak at a higher bombarding electron energy than would be expected if no diffusion occurred. Measurements of this peak energy for KI and KI containing 0.1% Tl indicate exciton diffusion ranges 21 and 13 nm. In KI:Tl a reduction in sputtering efficiency and a lower peak energy are consistent with the model of exciton diffusion, the Tl + ions acting as recombination centres. The exciton trapping cross section of thallium ions which can be estimated in this model is comparable with the ionic size of Tl + .