Dynamics of methane molecules in the mesopores of controlled-pore glass at low temperatures

Rotational tunneling of CH₄ molecules in the mesopores of controlled-pore glass with mean diameters of 120 {} and 350 {} has been studied by means of inelastic neutron scattering. The rotational excitation energies of the spherical quantum rotor CH₄ are a sensitive probe of the strength and symmetry of local potentials. The spectra show a superposition of contributions from the partially ordered low temperature phase II of solid CH₄ and from another phase without long range order. This latter phase is attributed to a thin layer with thickness of about 20 {} on the pore walls. A quantitative analysis is performed with a statistical model based on the pocket state formalism.