Percolative aspects of nonequilibrium adlayer structure

For any adsorption process where all binding sites eventually fill, there exists a coverage θ_c at which a filled cluster (defined by linking neighboring filled sites) first spans the substrate. Such percolation features have been studied extensively for random distributions of filled sites. Here θ_c =0.59 monolayers for ‘‘p(1×1) ordering’’ on an infinite square lattice. Cooperative island‐forming adsorption involves competition between nucleation, growth, and coalescence or linkage of individual islands. Here clusters of linked islands eventually span the substrate. We use correlated percolation theory to provide a quantitative description of corresponding θ_c behavior, and of the fractal structure of the clusters of linked islands and their perimeters. Modified grain growth models, which correspond to continuum percolation problems, are also useful here. We show how percolation theoretic ideas can be extended to analyze nonpercolating c(2×2) ordering. Even for the essentially random adsorption mechanisms of H₂O on Fe(001), and oxygen on Pd(100), such nonequilibrium c(2×2) ordering is significant. We also discuss how island‐forming cooperativity here affects the c(2×2) ordering.