Orientational effects in adsorption III: In-plane ordering in localized monolayers of diatomic molecules on crystalline solids

Orientational effects in monolayer films of diatomic molecules adsorbed on a single (100) face of a cubic crystal are discussed. The adsorbed molecules are assumed to take up an orientation parallel to the solid surface with their centres of mass located directly above the centres of the adsorption sites.It is shown that various orientationally ordered structures can appear and that their properties depend on the elongation of the adsorbed molecules, the size of the solid lattice, the density of the adsorbed film and the temperature. The heat capacities and the one-particle orientational distribution functions calculated for various systems are used to determine orientational properties of adsorbed films and to locate rotational transitions.