He-atom scattering study of Xe coadsorbed with alkali metal atoms on graphite

The coadsorption of alkali atoms with Xe on a graphite substrate has been studied using He-atom scattering and low-energy electron diffraction (LEED). When Xe is adsorbed on a graphite surface which has a low density of alkali metal adatoms, ordered Xe monolayers and bilayers are formed. Differences in adsorption rate and desorption temperatures of the Xe indicate that the presence of alkali metal atoms on the surface increase the sticking coefficient and the desorption temperatures of Xe adatoms. As deduced from specular He-atom scattering desorption curves, this stronger adsorption behavior appears to be a local effect, causing the largest changes for Xe atoms next to the alkali metal adatoms. An increase in the Xe–graphite stretching frequency is also observed with inelastic He-atom scattering, consistent with the stronger adsorption of Xe. We observed no evidence for Xe adsorption onto full alkali metal overlayers down to 40 K, consistent with theoretical studies which indicate small physisorption well depths for rare gases on alkali metal surfaces.