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

Testing the limits of the Maxwell distribution of velocities for atoms flying nearly parallel to the walls of a thin cell