Thermodynamics of freezing in two dimensions: The compressibility of monolayer xenon on graphite

We give a detailed account of vapor-pressure isotherm measurements of the freezing of monolayer xenon adsorbed on graphite. The carefully equilibrated isotherm data are numerically differentiated to yield the isothermal compressibility. We express the compressibility in terms of the thermodynamic variables of the film: the temperature, T, and the chemical potential, μ. The compressibility anomaly near the freezing transition is found to contain two components: (1) a sharp, Δμ∼2 K wide peak with a temperature-dependent amplitude which goes to zero near T=147 K and (2) a weak, broad peak with a temperature-independent shape and amplitude. We interpret the sharp peak as indicating a weak first-order transition for 116<T<147 K. In this temperature region, the broad component symmetrically underlies the sharp peak. For T>147 K, where only the broad component persists, the transition appears to be continuous. We compare our results to x-ray scattering and other thermodynamic measurements.