STM IMAGING AND LOCAL ORDER OF OXYGEN ADLAYERS ON NI(100)

High resolution STM data of the clean and oxygen-covered Ni(100) surface are presented, resolving individual atoms of both the substrate and the adsorbate. Adsorbed oxygen atoms are represented by local minima of 0.3 angstrom in depth, centered on fourfold hollow sites. This contrasts the known binding geometry, in which O(ad) resides 0.8 angstrom above the topmost Ni layer, and reflects electronic effects. Local structures in dilute adlayers, below formation of an ordered p(2 x 2) and in the almost filled c(2 x 2) phase, allow direct conclusions on the adsorbate-adsorbate interactions up to third-nearest neighbors to be made. There is also evidence of third- or higher-order interactions.