Neutron scattering study on a benzene layer on the basal planes of graphite

The structure and dynamics of absorbed layers of benzene on the basal planes of graphite (Grafoil, Carbopack B) in the monolayer regime has been investigated by thermal neutron scattering. The structure has been determined by diffraction from a C6D6 layer. A phase transition from a solid √7 x √7 registered phase with the benzene ring parallel to the surface at low temperatures to a compressible fluid phase above 150 K has been observed. The dynamics has been studied mainly at 100 K and 200 K by inelastic incoherent scattering on a C6H6 layer. The phononlike scattering intensity of the low temperature phase has been compared to results from a lattice dynamical model of a 2-D molecular crystal with external potential. The fluid phase shows quasielastic scattering similar to that of liquid benzene at 288 K, which can be accounted for by a model for a dense gas with a collision time in the order of 3 ps. (orig.)