Saturation of the 0-0 hyperfine-transition linewidth enhancement factor in optically pumped alkali-metal vapors

In a previous publication [Phys. Rev. A 31, 1440 (1985)], we determined the linewidth enhancement factor (LEF) of the microwave-power-broadened linewidth of the 0-0 hyperfine transition in optically pumped alkali-metal vapors. This factor is essentially a measure of the ratio of the transition linewidth to the microwave Rabi frequency. In the previous study, optical pumping out of only one hyperfine multiplet was considered. In the present study, the expressions are generalized to include optical pumping out of both hyperfine multiplets simultaneously. Dramatic changes in the behavior of the LEF as a function of the optical-pumping rate are predicted.