Low-energy electron diffraction determination of the structure of the $\zeta$ phase of oxygen physisorbed on graphite

Low-energy electron diffraction measurements on the $\zeta$ phase of diatomic oxygen physisorbed on the basal plane of graphite single crystals are reported for $12<T<\mathrm{}28\mathrm{}$ K. Two different monolayer phases are found with a phase transition between them near 18 K. The $\zeta$ 1 phase is stable below 18 K and has a slightly distorted triangular (oblique) unit mesh; the $\zeta$ 2 phase is stable above 18 K and has a simple triangular unit mesh. On imperfect crystals, another structure (${\zeta }^{\prime }$) is observed that is similar to $\zeta$ 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 $\zeta$ phase.