  The velocity distribution of atoms in a thermal gas is usually described
through a Maxwell-Boltzman distribution of energy, and assumes isotropy. As a
consequence, the probability for an atom to leave the surface under an azimuth
angle {\theta} should evolve as cos {\theta}, in spite of the fact that there
is no microscopic basis to justify such a law. The contribution of atoms moving
at a grazing incidence towards or from the surface, i.e. atoms with a small
normal velocity, here called "slow" atoms, reveals essential in the development
of spectroscopic methods probing a dilute atomic vapor in the vicinity of a
surface, enabling a sub-Doppler resolution under a normal incidence
irradiation. The probability for such "slow" atoms may be reduced by surface
roughness and atom-surface interaction. Here, we describe a method to observe
and to count these slow atoms relying on a mechanical discrimination, through
spatially separated pump and probe beams. We also report on our experimental
progresses toward such a goal.
