Heat capacity ofO2films adsorbed on Grafoil

Measurements of the heat capacity of adsorbed ${\mathrm{O}}_2$ films are reported. Coverages extend from a small fraction of a monolayer up to multilayers and bulk formation in the temperature range between 7 and 58 K. We show melting of the first-layer solid with an apparent triple line, the appearance and disappearance of the two-dimensional $\alpha -\beta$ film transition, the $\alpha -\delta$ and $\beta -\delta$ coexisting regions, and the appearance of the bulk melting and the three-dimensional $\alpha -\beta$ transformation at a reduced temperature. While most of our low-coverage features qualitatively agree with neutron and susceptibility studies, our results suggest that a more complex layer structure may exist.