Epitaxial rotation of two-dimensional rare-gas lattices on Ag(111)

Low-energy electron-diffraction results from rare gases adsorbed on Ag(111) are presented. On the clean surface, the rare-gas overlayer lattices are always aligned with the substrate step direction, but when a small amount of a different adsorbate is put on the surface first, the overlayer grows in a rotated orientation at a nonsymmetry angle which depends on the rare gas and its exact lattice spacing and on thr preadsorbate. These lattice rotation angles are not those predicted by the Novaco-McTague (NM) theory for epitaxial rotation, nor those predicted by any other model for epitaxial rotation. Molecular statics calculations for Xe/Ag(111) produce results in agreement with the NM prediction, not the experimental results. We speculate that atoms or molecules preadsorbed at step sites play an active role in the equilibrium rotation angles, possibly by changing the corrugation of the charge density in their vicinity.