ROTATIONAL AND TRANSLATIONAL MOTIONS OF TRAPPED METHANE - INCOHERENT INELASTIC NEUTRON SCATTERING OF METHANE HYDRATE

The temperature dependence of the rotational and translational motion of methane encaged in a deuterated clathrate hydrate has been studied with incoherent inelastic neutron scattering. The methane molecules we re found to behave almost like free rotors even at 6 K. The most inter esting observation is that, unlike methane. deposited in rare-gas matr ices, the rotational modes do not converge to the classical rotation-d iffusion limit at high temperature. The spectral features indicate tha t the methane interacts strongly with the host lattice vibrations and its motion is damped at high temperature. The methane vibrational dens ity of states obtained from the experiment using a self-consistent ite rative procedure to correct for multiexcitation and multiple scatterin g effect is found to be in good agreement with molecular dynamics calc ulations.