  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.
