Quasi-elastic peak lineshapes in adsorbate diffusion on nearly flat   surfaces at low coverages: the motional narrowing effect in Xe on Pt(111)
  Quasi-elastic helium atom scattering measurements have provided clear evidence for a two-dimensional free gas of Xe atoms on Pt(111) at low coverages. Increasing the friction due to the surface, a gradual change of the shape of the quasi-elastic peak is predicted and analyzed for this system in terms of the so-called motional narrowing effect. The type of analysis presented here for the quasi-elastic peak should be prior to any deconvolution procedure carried out in order to better extract information from the process, e.g. diffusion coefficients and jump distributions. Moreover, this analysis also provides conditions for the free gas regime different than those reported earlier. 
R. Martinez-Casado, J. L. Vega, A. S. Sanz, and S. Miret-Artes