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 Angstrom and 350 Angstrom 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 Angstrom on the pore walls. A quantitative analysis is performed with a statistical model based on the pocket state formalism. [S0163-1829(99)00413-0].