Rotational diffusion, in a monolayer of CH4 that was adsorbed onto the (100) MgO surface, was studied by using quasi-elastic neutron scattering techniques at temperatures of between 20 and 50K. It was found that the CH4 molecules executed isotropic rotations at temperatures above 40K. At 20 and 30K, the rotations could be intermediate between the free-rotor case and rotation about the 2-fold axis which was perpendicular to the surface in the dipole-down configuration. The rotational activation energy that was deduced from the temperature dependence of the diffusion coefficient was equal to 0.0027eV. The results were consistent with previous experimental data on the dipole-down configuration of a methane molecule that was adsorbed onto MgO at low temperatures. Comparison with the predictions of a molecular dynamics simulation revealed that there was a discrepancy concerning the type of rotational motion, and thus the configuration of adsorbed methane molecules upon the MgO surface.

J.M.Gay, P.Stocker, D.Degenhardt, H.J.Lauter: Physical Review B, 1992, 46[2], 1195-7