Low coverages of sulfur chemisorbed on the rhenium (0001) surface were studied by scanning tunneling microscopy (STM). At one-quarter monolayer coverage, sulfur forms a p(2X2) ordered overlayer, consistent with low-energy electron-difFraction results. At lower coverages, some of the sulfur forms small islands of the p (2 X 2) structure. Between the islands, sulfur atoms diffuse over the surface as a lattice gas. In our conditions, the residence time of the sulfur atoms in each site is comparable to the STM scan rate, which gives rise to an apparently noisy image. However, a spatial corelation function was used to determine that this apparent noise is due to diffusing sulfur that maintains a local p (2 X 2) order. This order is due to a weak attractive interaction between the diffusing atoms at twice the Re lattice spacing and a repulsive interaction at closer 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 sulfur atoms on the surface. The energy barrier to diffusion was calculated from the sulfur residence time, and compares well with an extended Hiickel calculation.

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The coverage-dependent effects of alkali additives on hydrogenated Si(111)-7×7 surface are monitored using high-resolution electron energy-loss spectroscopy. A ∼0.2-monolayer (ML) coverage of alkali causes a large broadening in the quasielastically scattered 4.8-eV electrons from the surface. No appreciable broadening is observed either at high alkali coverages (∼1 ML) or when the deposition of ∼0.2 ML of alkali is followed by hydrogen saturation. Quasielastic broadening is explained in terms of single-particle excitations of low-density conduction electrons. Within the first ML of alkali coverage the H–Si stretch loss energy decreases almost linearly with coverage by ∼20 meV. The increase in the width of the stretch loss peak is correlated with the inhomogeneous distribution of H–Si dipoles with respect to the surface layer, and the increase in the intensity of the stretch loss excitation is correlated with the formation of the image dipoles.