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

Abstract High-resolution INS spectra of adsorbed monolayers of CH 4 , a CH 4 /CD 4 mixture and CH 3 D on exfoliated graphite have been obtained at 4.5 K to study rotational tunneling. For CH 4 it is necessary to introduce an extra parameter representing the interaction between orientations connected by two-fold rotation. For a 1:2 CH 4 /CD 4 mixture, the tunneling spectra show that the mixture must be ordered and that the barriers to rotation of the CH 4 are slightly higher when the molecules are surrounded by the heavier CD 4 . For CH 3 D the higher-resolution spectra allow an unambiguous assignment of the tunneling spectra. Conversion between the spin species of CH 4 (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.