Low-energy electron diffraction determination of the structure of the ζ phase of oxygen physisorbed on graphite

Low-energy electron diffraction measurements on the ζ phase of diatomic oxygen physisorbed on the basal plane of graphite single crystals are reported for $12<T<28$ K. Two different monolayer phases are found with a phase transition between them near 18 K. The ζ 1 phase is stable below 18 K and has a slightly distorted triangular (oblique) unit mesh; the ζ 2 phase is stable above 18 K and has a simple triangular unit mesh. On imperfect crystals, another structure (ζ^') is observed that is similar to ζ 2 except for a different rotational epitaxy; this structure is apparently stabilized by imperfections on the surface such as steps. These low-energy electron diffraction results are compared with those of other studies of the ζ phase.