LATERAL INTERACTIONS AND CORRUGATION IN PHYSISORPTION SYSTEMS - CH4 CU(100)/

We present and analyze isothermal adsorption and desorption measuremen ts for methane physisorbed on a Cu(100) surface to characterize the in terplay between molecule-surface and molecule-molecule interactions. A 2D gas-liquid phase boundary for the methane layer is observed and pa rtially mapped in coverage and temperature. Comparing our measured cri tical temperature of 66 K +/-2 K to previous work on related systems i mplies an attractive well between physisorbed molecules of similar to 130 K. This reduction compared to the gas phase well can be adequately understood in terms of dielectric screening of the dispersion interac tion, By contrast, analyzing the low-coverage adsorption and desorptio n isotherms using a quasiequilibrium model and imperfect gas statistic s, we deduce a lateral attractive intermolecular potential well of onl y 30-50 K. This drastic reduction of the attractive well from the gas phase value is not consistent with image charge screening, and is diff icult to reconcile with the observed critical temperature. This discre pancy is discussed in terms of surface corrugation which dictates diff erent positions for molecule-surface and molecule-molecule potential e nergy minima. (C) 1996 American Institute of Physics.