ADSORBATE-INDUCED RECONSTRUCTION OF O AND S ON NI(100) - A LOCAL MODEL

The p(2 x 2) as well as the c(2 x 2) structures of sulphur and oxygen on Ni(100) at coverages of theta = 0.25 and theta = 0.5, respectively, were investigated with high precision LEED structure analyses. A uniform expansion of the topmost nickel layer with an underlying buckling reconstruction of the second layer was found in all cases. The total amplitude of adsorbate-induced changes of the substrate is roughly proportional to coverage and depends on the chemical species adsorbed. However, the geometry of the local adsorption cluster (adsorbate plus nearest neighbour atoms in the first and second substrate layers) is almost coverage-independent for both oxygen and sulphur. Nickel atom positions more distant from the adsorbate atom tend to be near that of the clean surface. These results indicate that a model of local reconstruction may be the key for an understanding of the observed structures: the adatom creates a local reconstruction which is largely independent of coverage but depends on the reactivity of the adsorbed chemical species. Additionally, we show that this simple model provides a possible explanation of the experimental observation that-in contrast to the well-ordered oxygen p(2 x 2)-structure at theta = 0.25-the adsorption of sulphur at the same coverage leads to a streaky p(2 x 2) LEED pattern, indicating partial disorder.