Saturation and optical pumping effects in the fluorescence that follows the excitation of the D2 transition in atomic rubidium

In this article we study saturation effects in the fluorescence spectra at the D2 transition in 87Rb excited by two counter-propagating laser beams of the same frequency. The effects appear as dips in the Doppler broadened fluorescence emission lines at the atomic transitions and in all but one of the cross-over resonances. The 5SF=1→5P3/2F=0, 1 cross-over presents a strong dependence in the laser intensity. For low intensities it appears as a weak dip, but at higher laser intensities an increase in fluorescence is clearly present. A rate equation calculation which includes the velocity profile is used to interpret the experimental data. For the 5SF=2→5P3/2F=1, 2 and 3 manifold, a five level model that takes into account the hyperfine structure of both initial and final states is in good agreement with the experiment. For the 5SF=1→5P3/2F=0, 1 and 2 manifold, it is necessary to include the magnetic sublevel structure of all hyperfine levels involved. The calculation also shows that the onset of this magnetic pumping effect occurs near the 5S→5P3/2 saturation intensity. In both cases the comparison between theory and experiment allowed a direct determination of the ratio of the laser intensity to the atomic saturation intensity.