The diffusion and low frequency vibrational motions of atoms and molecules on surfaces could be measured by means of quasi-elastic He atom scattering. An investigation was made of various analytical approximations, based upon the theory of stochastic processes, to the dynamic structure factor, considering the 2 motions on an equal level. Particular emphasis was place upon the nature of the corresponding lineshapes, explained in terms of the motional narrowing effect. Also considered was the influence of the diffusional and vibrational coupling and several ways of experimentally separating the 2 types of contribution to the dynamic structure factor. In particular, it was proposed that the so-called inelastic focusing singularity from atom–surface scattering controlled the lineshapes of quasi-elastic and vibrational peaks.

Quasielastic and Low Vibrational Lineshapes in Atom–Surface Diffusion. J.L.Vega, R.Guantes, S.Miret-Artés: Journal of Physics - Condensed Matter, 2004, 16[29], S2879-94