DETECTION OF SLOW ATOMS IN LASER SPECTROSCOPY OF A THIN VAPOR FILM

We report on high-resolution single-light-beam transmission spectrosco py through an ultra thin vapor cell (thickness 10-100 mu m) In additio n to the expected Doppler-broadened absorption, a novel sub-Doppler st ructure is observed under normal incidence irradiation. This structure originates from the optical response of atoms with very small velocit y components perpendicular to the cell walls, and is connected with th e transient atom excitation regime during the wall-to-wall time of fli ght. In experiments performed in Cs vapor, the slow mechanism of optic al pumping leads, for weak light intensities (10-100 mu W/cm(2)) and a 10 mu m cell, to a velocity selection equivalent to an effective 1D t emperature in the sub-mK range.