Collisionally induced quantum interference in resonance fluorescence of two-level atoms

In the experiment with barium vapor (close approximation of a two-level system) we found spectra of resonance fluorescence having the form of a pressure-broadened line with a narrow, not collisionally broadened, dip. They are interpreted as the result of collisionally-induced quantum interference among possible spontaneous emission channels of a dressed atom subjected to stochastic perturbation. Also, the analogy between these effects and the coherent population trapping in three-level systems is pointed out.