Two dimensional band-gap restricted diffusion of negatively charged excitons in rare gas solids

We present measurements of electron stimulated desorption (ESD) yields of D⁻, obtained from sub-monolayer amounts of C₂D₆ physisorbed on rare gas substrates; the latter consisting of alternating layers of Xe and Kr adsorbed at 20 K on Pt. Negatively charged excitons (NCE), for example, Xe*⁻ or Kr*⁻, which are produced by electron impact, may couple to dissociative anion states of C₂D₆, resulting in sharp resonance enhancements in the D⁻ ESD yields. Our measurements suggest that resonant diffusion of the Xe NCEs can be restricted to a single Xe layer, by placing a Kr multilayer spacer between the Xe and the metal substrate. This is attributed to the 1.85 eV insulating band-gap difference at the Xe---Kr interface.