DYNAMICS AND KINETICS OF MONOLAYER CH4 ON MGO(001) STUDIED BY HELIUM-ATOM SCATTERING

The structure, vibrational excitations, and adsorption and desorption kinetics of monolayer CH4 on MgO have been investigated using several techniques of helium scattering. Structural information is presented in the form of high-order diffraction-peak intensities. A vibrational excitation of 7.5 meV measured by time-of-flight methods shows no dispersion. This excitation energy is used in an analysis of the Debye-Waller effect for the [00] and [10BAR] beams. Studies of adsorption and desorption rates exploiting the He-methane diffuse-scattering cross section indicate an island-growth mode and allow determination of the desorption activation energy. Differences between the low-coverage adsorption rates for adsorption on fresh versus previously exposed surfaces suggest that higher-binding-energy sites are present after the desorption of a methane monolayer.