Melting Transition of Submonolayer Ar Adsorbed on Graphite

Precision ac heat-capacity scans of submonolayer Ar on Graphite Foam exhibit a liquidvapor anomaly near 55 K, a small, sharp, coverage-independent peak at 47.2 ±{} 0.2 K, and a broad anomaly near 49.5 K. The 47.2-K peak suggests that the Ar overlayer melts via a "weak" first-order (rather than continuous) transition. The 49.5-K anomaly appears to be related to the gradual decrease of correlation (possibly substrate-induced orientational order) that exists in the liquid phase.