We have investigated certain aspects of the dynamics of methane monolayers physisorbed on graphite. In particular, phonon frequencies have been calculated by applying the selfhyphen;consistent harmonic approximation to a twohyphen;dimensional array of methane molecules arranged as stable tripods on a triangular lattice. The methanendash;graphite interaction was not considered explicitly, and only three degrees of freedom per molecule were taken into account: two translations parallel to the surface and rotation about the axis normal to the surface. A new intermolecular potential proposed by Righini and present authors was employed. It was found that the translationndash;rotation coupling plays an important role in determining the anharmonic frequency shift. The results are in good agreement with a recent neutron incoherent inelastic scattering experiment in which the momentum transfer is parallel to the graphite surface. The two main peaks observed around 9 and 5 meV are assigned to librational and translational modes, respectively.
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