Surface melting of the (111) face of methane films adsorbed on graphit
e is investigated by quasielastic neutron scattering with special atte
ntion paid to translational diffusion within the liquid-like layer par
allel and normal to the surface. The comparison of the experimental sp
ectra measured in different geometries clearly shows diffusion anisotr
opy. Simulations of the data provide an estimation of the anisotropy a
s a function of temperature.