Physisorbed Rare-Gas Monolayers: Evidence for Domain-Wall Tilting

We have studied the structure of Xe adsorbed on a highly oriented pyrolytic graphite surface in the submonolayer regime using scanning tunneling microscopy (STM) at $T=5\mathrm{}\mathrm{K}$. The Xe adlayer forms hexagonal domains in a honeycomblike superstructure. We observe domain walls tilted by roughly 12° with respect to the atomic rows of the Xe layer, which explains the previously found rotation of the diffraction pattern of about 0.5° relative to the ideal $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ layer. The STM signal exhibits a voltage-dependent contrast between Xe domains and domain walls, which can be understood by the different adsorption sites and the band structure of the Xe adlayer.