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 K. The Xe adlayer forms hexagonal domains in a honeycomblike superstructure. We observe domain walls tilted by roughly 12 degrees with respect to the atomic rows of the Xe layer, which explains the previously found rotation of the diffraction pattern of about 0.5 degrees relative to the ideal (root 3 x root 3)R30 degrees 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.