Broadening and intensity redistribution in the atomic hyperfine excitation spectra due to optical pumping in the weak excitation limit

Full text: We analyze spectral line broadening and variations in relative intensities of hyperfine spectral components due to optical pumping at exciting laser intensities below the saturation limit. The study was motivated by the lack of availability of detailed theoretical models describing such effects in partially open level systems. In experiment, the hyper-fine laser-excitation spectra of the Na(3p) state were measured in a supersonic beam as a function of laser intensity under the conditions when optical pumping time is shorter than transit time of atoms through the laser beam. The theoretical excitation spectra were calculated numerically by solving density matrix equations of motion using the split propagation technique. The following results will be reported: it will be shown that spectral lines can be significantly broadened at laser intensities well below the saturation intensity, which is usually regarded as the threshold for onset of broadening effects; it will be shown that the presence of dark mF sublevels can vary the effective branching coefficients of the transitions, and this variation depends on laser intensity. Changes in the effective branching coefficients lead to irregular changes of peak ratios, like minimum in the intensity dependence of the peak ratio, which deviate from those expected from the given original branching coefficients; analytical expressions will be presented, which allow the calculation of critical values for laser intensity and Rabi frequency, above which linewidths and peak ratios are notably affected by optical pumping. The critical laser intensity can be expressed via the saturation intensity Isat, the branching coefficient Π of the transition, and the ratio of natural lifetime and transit time of atoms through the laser beam τnat / τtr: Icr (8π2 ℎc)/√π3λ3) x 1/(τtrΠ(1-Π)) = (4τnat)/√πτtr(1 - Π) x Isat. Importantly, the critical laser intensity depends on the branching coefficient Π and has a a minimum at Π = 1/2, and it can be much smaller than the saturation intensity. (author)