It was noted that the diffusion and low-frequency vibrational motion of atoms and molecules on surfaces could be measured by means of quasi-elastic He-atom scattering. Various analytical approximations were investigated here which considered the two motions equally. Particular emphasis was placed on the nature of the corresponding lineshapes; explained in terms of the motional narrowing effect. The influence of the diffusional and vibrational coupling, and several means for experimentally separating the 2 types of contribution to the dynamic structure factor, were also considered. It was proposed that the so-called inelastic focusing singularity from atom–surface scattering controlled the lineshapes of quasi-elastic and vibrational peaks.

Quasi-Elastic 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