  Grazing incidence fast atom diffraction (GIFAD or FAD) is a sensitive tool
for surface analysis, which strongly relies on the quantum coherence of the
incident beam. In this article the influence of the incidence conditions and
the projectile mass on the visibility of the FAD patterns is addressed. Both
parameters determine the transverse coherence length of the impinging
particles, which governs the general features of FAD distributions. We show
that by varying the impact energy, while keeping the same collimating setup and
normal energy, it is possible to control the interference mechanism that
prevails in FAD patterns. Furthermore, we demonstrate that the contribution
coming from different positions of the focus point of the incident particles,
which gives rise to the spot-beam effect, allows projectiles to explore
different zones of a single crystallographic channel when a narrow surface area
is coherently lighted. In this case the spot-beam effect gives also rise to a
non-coherent background, which contributes to the gradual quantum-classical
transition of FAD spectra. Present results are compared with available
experimental data, making evident that the inclusion of focusing effects is
necessary for the proper theoretical description of the experimental
distributions.
