Doppler broadening and collisional relaxation effects in a lasing-without-inversion experiment

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.