STATISTICAL-ANALYSIS OF ADSORBATES

Now that it is possible to observe individual atoms and molecules adso rbed on a surface, a method is required to evaluate the correlation be tween the occupancy of neighbouring sites. To meet this need, a rigoro us theoretical analysis of the distribution of adsorbates on surface s ites has been developed, first for uncorrelated adsorption, and then f or cases where the occupation of a site affects the probability of occ upation of a neighbouring site. Statistical parameters can be measured from experimental images and related directly to the theoretical mode l, with allowance being made for defects and edge effects. By analysin g scanning tunnelling microscope images in this way, it is found that trimethylgallium on GaAs(001)-(2x4) exhibits enhanced nearest neighbou r occupation, whereas ethylene on Si(001)-(2x1) shows nearest neighbou r repulsion.