EPITAXIAL ROTATION OF 2-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 latt ices 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 prea dsorbate. These lattice rotation angles are not those predicted by the Novaco-McTague (NM) theory for epitaxial rotation, nor those predicte d by any other model for epitaxial rotation. Molecular statics calcula tions for Xe/Ag(111) produce results in agreement with the NM predicti on, not the experimental results. We speculate that atoms or molecules preadsorbed at step sites play an active role in the equilibrium rota tion angles, possibly by changing the corrugation of the charge densit y in their vicinity.