Buffer-gas induced absorption resonances in Rb vapor

We observe the transformation of the electromagnetically induced transparency (EIT) resonance to absorption in a Lambda configuration in a cell filled by Rb and buffer gas. This transformation occurs as the one-photon detuning of the coupling field is varied from resonance. No such absorption resonance is found in the absence of buffer gas. The width of the absorption resonance is several times smaller than the width of the EIT resonance, and its amplitude is about the same. Similar absorption resonances are detected in Hanle configuration in a buffered cell, although the reasons for these effects are quite different