Low-Energy Electron Diffraction Study of Molecular Oxygen Physisorbed on Graphite

Monolayers and multilayers of molecular oxygen physically adsorbed on single-crystal graphite have been studied using low-energy electron diffraction. At low coverages and temperatures O(,2) order in the (delta) phase, which has an incommensurate centered-parallelogram (oblique) structure with the molecules parallel to each other and to the graphite surface. The rotational epitaxy of the (delta) phase has been determined and changes with both temperature and coverage. Sometimes, another phase (the (delta)' phase) is also observed at the highest coverages that the (delta) phase is observed. Although the unit mesh of the (delta)' phase is the same as that of the (delta) phase, its rotational epitaxy is very different. The (delta) phase melts by a first-order transition into a well-correlated incommensurate liquid that is well-oriented with respect to the graphite. The melting and properties of the liquid phase are discussed. With increasing coverage, the liquid undergoes a first-order transition into the incommensurate (theta) phase, a previously unobserved phase that is approximately triangular. The exact nature of the (theta) phase (solid or fluid) is not known and several alternative interpretations are presented and discussed. Although the nearest-neighbor spacing in the liquid and the (theta) phases are very close, their rotational epitaxies are very different. With increasing coverage, the (theta) phase apparently goes through a transition into the fluid phase. As for the liquid, the diffraction from the fluid is readily observed, which indicates that it is also well-correlated. With increasing coverage at temperatures between 12 and 32 K, the (delta) phase undergoes a first-order transition into the zeta phase in which the molecules are perpendicular to the surface. Between 12 and 18 K, the unit mesh of this phase is a slightly distorted triangle (oblique). Near 18 K the zeta phase apparently goes through a phase transition where this distortion decreases to zero; however, the nature of the higher-temperature zeta phase is not completely clear. Another phase (the zeta' phase) with a structure similar to that of the zeta phase, but a very different rotational epitaxy, is also observed at high coverage. The anti-ferromagnetically ordered (epsilon) phase was not observed in this study.