Mobility of two-dimensional ethane adsorbed on graphite. A quasielastic neutron-scattering study

Abstract

The diffusive mobility of a rod-like molecule, ethane, adsorbed on the (0001) surface of graphite has been studied by incoherent quasielastic neutron scattering as a function of temperature. The measurements have been limited to the submonolayer domain (θ < 0.7 monolayer) and to the 10–122 K temperature range. At 10 K ethane molecules do not display any diffusive motion; at 53.5 K the herringbone solid structure exhibits reorientational motion of the methyl groups about the threefold carbon–carbon axis. Above the two-dimensional melting point (65 K) ethane molecules perform an isotropic rotational motion plus a diffusive translation which can be described, at 87 K, as a jump diffusion between equivalent surface sites (lattice liquid). At 122 K the adsorbed molecules display brownian motion (isotropic liquid).