A computer simulation investigation of surface disordering in adsorbed multilayers

Multilayers of methane on a magnesium oxide substrate have been simulated using a model potential. Layer-by-layer disordering was found below the bulk melting point. This disordering is the result of the formation of vacancies by promotion of molecules to overlayers. Both static and dynamic properties show that there is a continuous change in the nature of the disordered layer from solid-like through a lattice liquid to a random liquid as the temperature is raised rather than successive roughening and pre-melting transitions. There is a marked difference in the sharpness of the disordering in the outer layers of multilayers with different exposed faces. In the close packed (111) face the outermost layer disorders over a temperature range of less than 10 K, while the corresponding layers of a (100) or (110) face disorders gradually over a range of 50 K.