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 ${\delta }^{\mathrm{*}}$ 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, ${\delta }^{\mathrm{*}}$, 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.