Buffer-gas induced absorption resonances in Rb vapor

We observe transformation of the electromagnetically induced transparency (EIT) resonance into the absorption resonance in a $\Lambda$ interaction configuration in a cell filled with $^{87}$Rb and a buffer gas. This transformation occurs as a one-photon detuning of the coupling fields is varied from the atomic transition. No such absorption resonance is found in the absence of a buffer gas. The width of the absorption resonance is several times smaller than the width of the EIT resonance, and the changes of absorption near these resonances are about the same. Similar absorption resonances are detected in the Hanle configuration in a buffered cell.