Pore-size dependence of rotational tunneling in confined methyl iodide

High resolution inelastic incoherent neutron scattering measurements have been performed on methyl iodide, a quantum rigid rotor, confined to porous silica glasses with varying pore size distributions. A simple geometrical model composed of weakly and strongly disordered molecules is applied to describe the disordered tunneling molecules, and it is found that the strongly disordered rotors are constrained to three molecular layers. No simple relationship is found between the pore size distribution and the barrier height distribution, thus suggesting that the tunneling line shape cannot be expressed as a simple integral over the pore size distribution.