Methane adsorbed on graphite. IV. Multilayer growth at low temperature

The structure of methane films on a smooth graphite substrate has been simulated along an isotherm (T=25 K) by use of canonical ensembles. The coverage was sequentially increased in small increments from 0.90 to 4.2 (√3 × √3 ) monolayers. Comparisons are made with simulations of argon adsorbates, recent laboratory experiments, and analytical calculations. The results indicate the first and second methane layers are not mutually commensurate for all but the highest coverage in this range, but, for the same conditions, argon layers are mutually commensurate. The incommensurate first and second layers modulate each other to form an interesting new structure where the unit cells in each of the layers have become oblique. The structural distribution functions suggest the compressive stresses in multilayer films are better described by a pressure tensor with radial and vertical components which are decreasing functions of the height above the substrate. Comparisons are made to results from Ar/graphite and ${\mathrm{CH}}_4$/Au(111) systems.