In the early experiments on optical pumping and coherent population trapping (‘‘dark resonances’’) increasing buffer gas pressure enhances the effect. In our recent lasing-without-inversion experiments, based on population trapping, we find the opposite behavior, namely, decreasing atomic coherence with increasing buffer gas pressure. We provide a theoretical explanation of this result in agreement with experiment.

• Received 12 January 1995

DOI:https://doi.org/10.1103/PhysRevA.51.4030