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 were found to behave almost like free rotors even at 6 K. The most interesting observation is that, unlike methane deposited in rare-gas matrices, the rotational modes do not converge to the classical rotation−diffusion limit at high temperature. The spectral features indicate that the methane interacts strongly with the host lattice vibrations and its motion is damped at high temperature. The methane vibrational density of states obtained from the experiment using a self-consistent iterative procedure to correct for multiexcitation and multiple scattering effect is found to be in good agreement with molecular dynamics calculations.