Two-dimensional gas phases of Ar, Kr, and Xe on Ag(111)

Abstract Observations and calculations are reported for the 2D gas phase of Ar, Kr, and Xe adsorbed on the (111) face of Ag. The measurement of the coverage of 2D gas is sensitive to the gas adsorbed on the coherently diffracting regions of the Ag; appreciable contributions to the measured coverage from gas adsorbed at extrinsic sites can be excluded. The gas density at the condensation of the solid monolayer is remarkably high, of the order of 15% of the 2D solid density. Statistical mechanical calculations, using Monte Carlo simulations, show that the unusually dense 2D gas is stabilized against condensation because substrate-mediated interactions raise the energy of the 2D solid relative to the value which would be calculated using the bare 3D pair potentials. The simulations show that the gas is very nonideal, with large specific heat and large density fluctuations. The only comparable states of a 3D gas occur near the gas-liquid critical point.