Influence of ion diffusion in velocity space on the saturation effect

A saturation curve, representing the dependence of the power of the radiation absorbed or emitted by ions on the incident radiation intensity, is obtained. The frequency of stimulated transitions is found for ions in a low-temperature plasma in which collisions result in a diffusive wandering (random walk) in velocity space. It is shown theoretically that the distribution of the transition rates on the ion velocity scale has a sharp dip in the region of a resonance. The profile of this dip is in agreement with experiments. The saturation curve is compared with experimental data on the resonant fluorescence generated in a hollow-cathode discharge and on the power of single-frequency radiation emitted by an argon laser. The theory is shown to be in satisfactory agreement with these experimental results.