Rotational tunneling and spin conversion in methane monolayers adsorbed on the surface of graphite

High-resolution INS spectra of adsorbed monolayers of CH4, a CH4/CD4 mixture and CH3D on exfoliated graphite have been obtained at 4.5 K to study rotational tunneling. For CH4 it is necessary to introduce an extra parameter representing the interaction between orientations connected by two-fold rotation. For a 1:2 CH4/CD4 mixture, the tunneling spectra show that the mixture must be ordered and that the barriers to rotation of the CH4 are slightly higher when the molecules are surrounded by the heavier CD4. For CH3D the higher-resolution spectra allow an unambiguous assignment of the tunneling spectra. Conversion between the spin species of CH4 (A, T and E) has been observed at 0.35 K, with trace amounts of oxygen. The rate of spin conversion depends upon the concentration of the species present as well as temperature and the amount of oxygen.