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

Rotational tunneling of CH4 molecules in the mesopores of controlled-pore g lass with mean diameters of 120 Angstrom and 350 Angstrom has been studied by means of inelastic neutron scattering. The rotational excitation energie s of the spherical quantum rotor CH4 are a sensitive probe of the strength and symmetry of local potentials. The spectra show a superposition of contr ibutions from the partially ordered low temperature phase II of solid CH4 a nd from another phase without long range order. This latter phase is attrib uted 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].