Xe and Kr adsorption on the Si(111) 7 × 7 surface

A study of adsorption of Xe and Kr on the Si(111) 7 × 7 surface is presented. Low energy electron diffraction indicates the substrate structure is not appreciably modified by the adsorption. The data consist primarily of isobars, i.e. the amount adsorbed as a function of temperature at fixed pressure. The stepwise adsorption of each of the first several adatoms per surface unit mesh is resolved. This indicates a sequence of distinguishable adsorption sites in the mesh each of which can be occupied by a single atom. There are appreciable differences in the measured binding energies between the various sites. Pairwise sums of 6–12 potentials have been used to estimate binding energies expected for sites presented by proposed structural models of the surface. The observed sequence of sites seems to exclude buckled models and those models which involve only arrays of identical entities. No presently published model is consistent with these data in detail. The triangular island models with some modifications might present sufficiently large binding energies and rich sequences of sites. Whatever the actual structure, it must be more complex than those which have usually been considered.