  The influence of the collimating conditions of the incident beam on
diffraction patterns produced by grazing scattering of fast atoms off crystal
surfaces is studied within a semi-quantum approach, named Surface Initial Value
Representation (SIVR) approximation. In this approach we incorporate a
realistic description of the incident particle in terms of the collimating
parameters, which determine the surface area that is coherently illuminated.
The model is applied to He atoms colliding with a LiF(001) surface after
passing through a rectangular aperture. As it was experimentally observed [1],
SIVR spectra as a function of the azimuthal angle are very sensitive to the
width of the collimating slit. We also found that the length of the collimating
aperture affects polar angle distributions, introducing additional interference
structures for the longer collimating slits.
