We report on the first expeimental observation of recoil-induced resonances. These resonances, which can be considered as the atomic analog of the grain mechanism of the free-electron laser, are observed on the transmission spectrum of a probe beam making a small angle with a one-dimensional lin⊥lin cesium optical molasses. They are interpreted either as stimulated Raman transitions between differently populated velocity groups (the internal atomic state being unchanged), or as stimulated Rayleigh resonances originating from a bunching of the atomic density. Their shape provides information about the transverse velocity distribution of the atoms.

• Received 23 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.3017