We have studied the structure of Xe adsorbed on a highly oriented pyrolytic
graphite surface in the submonolayer regime using scanning tunneling micro
scopy (STM) at T = 5 K. The Xe adlayer forms hexagonal domains in a honeyco
mblike superstructure. We observe domain walls tilted by roughly 12 degrees
with respect to the atomic rows of the Xe layer, which explains the previo
usly found rotation of the diffraction pattern of about 0.5 degrees relativ
e 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.