  For a gas at thermal equilibrium, it is usually assumed that the velocity
distribution follows an isotropic 3-dimensional Maxwell-Boltzmann (M-B) law.
This assumption classically implies the assumption of a "cos theta" law for the
flux of atoms leaving the surface, although such a law has no grounds in
surface physics. In a variety of recently developed sub-Doppler laser
spectroscopy techniques for gases one-dimensionally confined in a thin cell,
the specific contribution of atoms moving nearly parallel to the boundary of
the vapor container becomes essential. We report here on the implementation of
an experiment to probe effectively the distribution of atomic velocities
parallel to the windows for a thin (60 microns) Cs vapor cell. The principle of
the set-up relies on a spatially separated pump-probe experiment, where the
variations of the signal amplitude with the pump-probe separation provide the
information on the velocity distribution. The experiment is performed in a
sapphire cell on the Cs resonance line, which benefits of a long-lived
hyperfine optical pumping. Presently, we can analyze specifically the density
of atoms with slow normal velocities ~ 5-20 m/s, already corresponding to
unusual grazing flight theta at ~85{\deg}-88.5{\deg} from the normal to the
surface and no deviation from the M-B law is found within the limits of our
elementary set-up. Finally we suggest tracks to explore more parallel
velocities, when surface details -roughness or structure- and the atom-surface
interaction should play a key role to restrict the applicability of a M-B-type
distribution.
