Fluorescence from doubly driven four-level atoms - A density matrix   approach
  The unusually narrow features in the fluorescence from Rubidium-85 driven by cooling and repumper laser fields, reported in an earlier experiment [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], results in velocity selection from co-propagating and counter propagating probe and pump beams resulting in narrow fluorescence peaks from a thermal gas at room temperature. 
Andal Narayanan, R. Srinivasan, Ashok Vudayagiri, Uday Kumar Khan and   Hema Ramachandran