STRONG SATURATION OF DOPPLER-BROADENED SPECTRA IN THE PRESENCE OF WEAK COLLISIONS

Steady-state saturation of Doppler-broadened spectra of gases by stron g near-resonant field in the presence of weak velocity-changing collis ions (VCC), which causes the Gaussian-Markovian frequency fluctuations , is considered nonperturbatively. A stochastic rate equation is deriv ed, and its applicability to different systems is discussed. The field -matter interaction is approximated by a delta-function sink in the ve locity space. The necessary and sufficient conditions for the validity of this contact model (CM) are obtained: The CM is shown to hold for not too strong fields and/or sufficiently frequent VCC. The CM has all owed us to obtain-nonperturbative analytical results for the velocity distribution of the populations of the resonant transition,the integra l populations, and the absorption (induced emission) rate. We show tha t the CM, together with the quasistatic limit, describes the saturatio n for all values of the parameters, except for the region of the trans ition between the above models. For the exactly resonant case the CM, together with the previously obtained nonperturbative solution, provid es the full solution of the problem. The most striking feature of the saturation in the case of weak VCC is the intermediate saturation, i.e ., the existence of the interval of the field intensities, where the p opulations and absorption (emission) rate are almost independent of th e field. In this region the sink is very strong, whereas the coupling of the field with nonresonant atoms is still weak, so that the saturat ion is controlled by the diffusion of the active atoms in the velocity space towards the sink. The results of the present theory are shown t o differ significantly from the results of the strong-collision model of the VCC. The present theory is verified by independent numerical ca lculations. [S1050-2947(98)01310-9].