  Diffraction patterns produced by fast He atoms grazingly impinging on a
LiF(001) surface are investigated focusing on the influence of the beam
collimation. Single- and double- slit collimating devices situated in front of
the beam source are considered. To describe the scattering process we use the
Surface Initial Value Representation (SIVR) approximation, which is a
semi-quantum approach that incorporates a realistic description of the initial
wave packet in terms of the collimating parameters. Our initial wave-packet
model is based on the Van Cittert-Zernike theorem. For a single-slit
collimation the width of the collimating aperture controls the shape of the
azimuthal angle distribution, making different interference mechanisms visible,
while the length of the slit affects the polar angle distribution.
Additionally, we found that by means of a double-slit collimation it might be
possible to obtain a wide polar angle distribution, which is associated with a
large spread of the initial momentum perpendicular to the surface, derived from
the uncertainty principle. It might be used as a simple way to probe the
surface potential for different normal distances.
