Low coverages of S which were chemisorbed onto the (001) surface were studied by means of scanning tunnelling microscopy. At a coverage of 0.25 of a monolayer, the S formed a p(2 x 2) ordered overlayer which was consistent with low-energy electron diffraction data. At lower coverages, some of the S formed small islands with the p(2 x 2) structure. Between the islands, the S atoms diffused over the surface as a lattice gas. Under the present conditions, the residence time of the S atoms in each site was comparable to the scanning tunnelling microscope scan rate, and gave rise to an apparently noisy image. However, a spatial correlation function showed that the apparent noise was due to diffusing S which maintained local p(2 x 2) order. This order was due to a weak attractive interaction between the diffusing atoms at twice the lattice spacing, and to a repulsive interaction at lower distances. The strength of the attractive interaction was measured by fitting the results of the correlation function to an Ising model of the interaction of S atoms on the surface. The energy barrier to diffusion was deduced from the S residence time, and compared well with an extended Hückel calculation.

J.C.Dunphy, P.Sautet, D.F.Ogletree, O.Dabbousi, M.B.Salmeron: Physical Review B, 1993, 47[4], 2320-8