Rotational diffusion of methane molecules adsorbed on MgO(100)
Phys. Rev. B 46, 1195 – Published 1 July, 1992
DOI: https://doi.org/10.1103/PhysRevB.46.1195
Abstract
Rotational diffusion in the methane monolayer adsorbed on MgO(100) is studied using the quasielastic neutron-scattering technique, between 20 and 50 K. The molecules execute isotropic rotations above 40 K. At 20 and 30 K, the rotations might be intermediate between the free-rotor case and rotations around the twofold axis perpendicular to the surface in the dipod-down configuration. The rotational activation energy deduced from the temperature dependence of the diffusion coefficient is 2.7±0.3 meV. These results are consistent with other experimental evidence of the dipod-down configuration of the methane molecule adsorbed on MgO at low temperature. The experimental results are also compared with the predictions of a recent molecular-dynamics simulation [A. Alavi, Phys. Rev. Lett. 64, 2289 (1990); Mol. Phys. 71, 1173 (1990)]. There is a disagreement concerning the type of rotational motion and consequently the configuration of adsorption of the methane molecules of the MgO surface.