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.