The librational ground state of monodeuteromethane adsorbed on the surface of graphite

Abstract

The heat capacity of CH3D on graphite has been measured at low temperatures (0.3–7 K) and shown to have a Schottky anomaly resulting from the difference in energy between states with the single deuterium atom pointing away from the surface (D up) and states where the deuterium atom is part of the tripod of atoms pointing towards the surface (D down). The energy difference is 380±20 μeV with the D-down configuration being the more stable. Incoherent neutron scattering spectra of CH3D adsorbed on exfoliated graphite have been used to resolve the finer detail of the tunneling splittings in the system. The energy levels can be described quantitatively in terms of five parameters, the splitting between D-up and D-down configurations and two pairs of parameters, one pair related to the rotational potential about the unique C–H(D) bond normal to the surface and the other pair to rotation about one of the three equivalent C–H(D) bonds pointing towards the surface. Comparison of the results from CH3D and earlier results from CH4 show that both potentials contain two Fourier components, approximately 75% of a threefold and 25% of a sixfold component in phase with one another. The values originally obtained from the CH4 tunneling spectrum for the two barrier heights 202 and 172 cm−1 for rotation about the unique and nonunique axes, respectively, are shown to have an uncertainty of about 5% resulting from the approximations used for the shape of the potential in the interpretation of the earlier experiment.