Low-energy electron diffraction study of molecular oxygen physisorbed on graphite

Monolayers of molecular oxygen physically adsorbed on single-crystal graphite have been studied using low-energy electron diffraction (LEED). The lattice parameters for the low-coverage oblique {δ} phase and the higher-coverage centered-rectangular δ* phase are presented as a function of density and compared with x-ray studies. New information about the {δ}' phase (which is oriented differently on the substrate) indicates that it may be an equilibrium phase. Three different phases are observed at temperatures above those where the {δ}, δ*, or {δ}' phases exist: a low-coverage liquid, an intermediate-coverage {θ} phase, and a high-coverage fluid phase. The liquid and fluid phases have molecular-axis disorder, sufficient short-range order to produce LEED patterns, and bond-orientational order. We discuss the properties of these phases and the nature of the {θ} phase, which we believe is a two-dimensional solid with molecular-axis disorder. Our data provide a detailed monolayer density-temperature phase diagram.