A statistical model for oxygen adsorption on W (211)

A series of previous experimental investigations revealed that oxygen adsorption on a W (211) surface causes, at $\text{θ =}\text{1}\text{2}$, a 2 × 1 LEED pattern whose half order spots are streaked at lower coverages and become continuously weaker at $\text{θ >}\text{1}\text{2}\text{until at}\text{θ = 1}$ a 1 × 1 structure is formed. A model is proposed wherein the configuration of the adsorbed atoms with respect to each other is governed by a set of anisotropic interactions between neighbouring particles. These (three) interaction energies have magnitudes between kT and 2kT. The equilibrium arrangements for various coverages were simulated by means of the Monte Carlo technique, and the corresponding LEED patterns (angular profiles) were evaluated by using the kinematic approximation. Good agreement with experimental data is obtained for the variation of the relative intensity as well as of the breadth of the half order LEED spots, without the need for any further assumptions on the mechanism of the formation of the ordered adsorbed layers.