Detection of slow atoms in laser spectroscopy of a thin vapor film

We report on high-resolution single-light-beam transmission spectroscopy through an ultra thin vapor cell (thickness 10-100 mu m) In addition to the expected Doppler-broadened absorption, a novel sub-Doppler structure is observed under normal incidence irradiation. This structure originates from the optical response of atoms with very small velocity components perpendicular to the cell walls, and is connected with the transient atom excitation regime during the wall-to-wall time of flight. In experiments performed in Cs vapor, the slow mechanism of optical 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 temperature in the sub-mK range.