SURFACE MELTING ON THE CLOSE-PACKED (111) FACE OF METHANE THIN-FILMS CONDENSED ON GRAPHITE

Quasi-elastic neutron scattering was used to measure the temperature dependence of the thickness and of the translational mobility of the liquid-like phase, stable below the bulk melting temperature, at the solid-vapor interface of thin CH4(111) films adsorbed on graphite. It is shown that on the close-packed (111) face of CH4 surface melting occurs. The average mobility of the liquid-like layer is similar to that of bulk liquid, as well as for the more open (100) face (diffusion coefficient in the 10-5 cm-2 s-1 range). At the same temperature, the liquid-like layer is slightly thinner on the (111) surface than on the (100) plane. This near isotropy with crystallographic orientation was expected from the molecular dynamics simulation of van der Waals solids. © 1990.