The structure and dynamics of methane adsorbed on graphite

Abstract

The application of neutron scattering techniques to the study of methane physisorbed on graphite is described. Two types of adsorbent have been used, graphitized carbon blacks of high and low surface area, and an exfoliated graphite. Neutron diffraction has been used to determine the structure of the adsorbed layer at low temperatures, part of the phase diagram, and the distance of the methane molecule from the surface. Below a monolayer the methane molecules occupy a triangular lattice with a √3 x √3 structure in register with the underlying basal plane that forms the surface of graphite. Just above a monolayer, the lattice contracts out of register with the surface. The molecule-surface distance (carbon to surface) is found to be 3.30 ± 0.05 Å. Incoherent neutron elastic scattering spectra give the frequencies of the vibrational modes of the adsorbed layer. The frequencies of both whole molecule displacements and torsional motions are found to be similar for directions perpendicular and parallel to the surface, at about 100 and 70 cm -1 , respectively. Rotational tunnelling transitions have been observed in the range 0-200 μeV (0-1.6 cm -1 ) corresponding to hindered rotation in a potential field of trigonal symmetry. The two types of barrier to rotation are estimated to be 150-200 cm -1 high. All of the experimental parameters are compared with values calculated from atom-atom potentials by using different empirical parameters.