Effect of optical pumping on alkali-atom Doppler-limited spectra

Using a three-level model and semiclassical density matrix formalism, we show that optical pumping influences both line positions and amplitudes in Doppler-broadened alkali-atom D-1 and D-2 spectra for intensities much lower than the two-level atom saturation intensity. The influence on the D-2 spectrum is particularly interesting due to the presence of closed hyperfine transitions unaffected by optical pumping. This effect is of importance for example when lasers are frequency stabilized using linear absorption or when the alkali-atom vapour pressure is determined using absorption measurements.