Rotational tunnelling of methane adsorbed on graphite

Transitions between energy levels split by rotational tunnelling have been observed for methane on an exfoliated graphite (papyex) using incoherent neutron scattering. Five lines are observed, falling in the energy range 18–112 μeV. From their frequencies, intensities, and polarizations, they are assigned to the allowed transitions of methane in a field of trigonal symmetry. A quantitative treatment of the observed transitions shows that they may be characterized by two parameters, h 4 and h, respectively related to the barriers to rotation of methane about a threefold axis perpendicular to the surface and three equivalent threefold axes pointing towards the surface in a tripod configuration. Both potentials are of the cos 3ϑ form and the barrier heights are 202 and 172 cm-1 respectively.