Positronium formation and the phases of two-dimensional oxygen physisorbed on graphite

The connection between positronium formation at a graphite surface and the presence of physisorbed oxygen has been investigated. Unique measurements reveal the sensitivity of the positronium signal to the coverage and phase of the adsorbed layers. Below 38 K, the solid zeta(2) phase strongly promotes the creation of positronium and the temperature dependence of the intensity may be related to the tilt of the molecules on the substrate. In contrast, a fluid bilayer denies the observation of positronium probably due to the rapid formation of PsO(2).