Unusually narrow spectral features are seen in fluorescence peaks from 85 Rb atoms under the action of two driving laser fields that are in a three-dimensional molasses configuration. One of the lasers, L1, is held at a fixed detuning from the "cooling" transition, while the other, L2, is scanned across the "repumping" transitions. The fluorescence peaks are split into symmetric pairs, with the separation within a pair increasing with the detuning of the L1 laser. For large detunings, additional small peaks are seen. A simple model is proposed to explain these experimental observations.

The unusually narrow features in the fluorescence from 85 Rb driven by the cooling and repumper laser fields, reported in [1], are explained on the basis of a four-level density matrix calculation. Quantum effects alter the efficiency of atom transfer by the probe (repumper) laser to the levels connected by the pump (cooling) laser. This, combined with the double resonance condition [1], leads to velocity selection from co-propagating and counter propagating pump and probe beams resulting in narrow fluorescence peaks from a thermal gas at room temperature.( 1 )The cooling and repumper beams are used in the sense as defined in [1].( 2 )Hyperfine levels of 5S 1/2 are denoted unprimed while that of 5P 3/2 are denoted primed.c EDP Sciences