  Using scanning tunneling microscopy at low temperatures we investigated the
adsorbate system Cu(111)-Cs for various cesium coverages. 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 behavior at 9K follows a
general trend which is consistent with models of epitaxial rotation. With
increasing coverage the adsorption layers are found to rotate toward alignment
with the substrate.
