Optical pumping in dense atomic media: Limitations due to reabsorption of spontaneously emitted photons

Resonant optical pumping in dense atomic media is discussed, where the abso rption length is less than the smallest characteristic dimension of the sam ple. It is shown that reabsorption and multiple scattering of spontaneous p hotons (radiation trapping) can substantially slow down the rate of optical pumping. A very slow relaxation out of the target state of the pump proces s is then sufficient to make optical pumping impossible. As model systems a n inhomogeneously and a radiatively broadened 3-level system resonantly dri ven with a strong broad-band pump field are considered.