Microdynamics of Methane Adsorbed on Magnesium Oxide

Abstract

The nuclear magnetic relaxation times of protons in methane and some of its deuterated modifications adsorbed on magnesium oxide have been measured as a function of temperature between 5 and 80°K. Coverages were 0.5 and 1.0 times a statistical monolayer. Two spin-lattice relaxation rate maxima are observed, a low temperature maximum due to anisotropic reorientation of the adsorbed methane on the magnesium oxide surface and a high temperature spin-lattice relaxation rate maximum due to both dipolar interactions between protons in neighboring methane molecules and between the protons in the adsorbed methane molecules and intrinsic surface paramagnetic centers. Correlation times and activation energies associated with both molecular reorientation and translational diffusion were measured, and an estimate of the magnitude of the electric field at the adsorption sites on the surface of MgO was obtained.