SURFACE MELTING OF THIN-FILMS OF METHANE

Quasi-elastic neutron scattering is used to measure the temperature dependence of the thickness and of the translational mobility of the quasi-liquid phase stable below the bulk melting temperature, at the solid-vapor (111) and (100) interfaces of thin CH4 films absorbed on graphite and MgO, respectively. It is shown that both the close-packed (111) and the more open (100) surfaces undergo a premelting transition. Both faces have an average mobility of their quasi-liquid layer similar to that of bulk liquid (diffusion coefficient in the 10-5 cm2s-1 range). At the same temperature the quasi-liquid layer is slightly thinner on the (111) surface than on the (100) plane. This nearly complete isotropy with crystallographic orientation is expected from molecular dynamics simulation of van der Waals solids. © 1990 Pergamon Press plc.