Adsorption geometry of Cu(111)-Cs studied by scanning tunneling microscopy

Using scanning tunneling microscopy at low temperatures we investigated cesium adsorbed on Cu(111). At low coverages we observe a hexagonally ordered Cs adsorption layer with a mutual adsorbate distance of 1.1 nm. This distance is discussed in terms of a commensurate adsorption superstructure which is stabilized by long-range adsorbate interactions mediated by Cu(111) surface state electrons. Intermediate coverages are characterized by incommensurate superstructures which are rotated with respect to the substrate lattice. The rotation angle varies with coverage and follows a trend which is consistent with models of epitaxial rotation. With increasing coverage the adsorption layers are found to rotate toward alignment with the substrate.