Velocity selective optical pumping and dark resonances in selective reflection spectroscopy

We report an experimental investigation of selective reflection spectra using a bichromatic field with a resonant \ensuremath{\Lambda}-type coupling to the ${\mathrm{D}}_{1}$ transition of Rb vapor atoms. The reflection spectra are found to have a more complex structure than equivalent spectra in transmission experiments. This structure is traced back to the effect of wall collisions on velocity selective optical pumping and saturation processes. The spectral line shapes are well described by a theoretical model developed in the text. In addition we report the observation of dark resonances in reflection spectroscopy.