Theory of resonances of saturated absorption in low-pressure gases

A theory of saturated absorption resonances is developed for the transit range of pressures of an absorbing gas with an arbitrary degree of saturation. The rate equation approximation is used to obtain expressions for the absorption coefficient in the cases of one traveling wave, weak and strong opposite waves, and a standing wave with a rectangular radial distribution of the intensity. An analytic study is made of the behavior of resonance widths when the saturating field intensities are varied and it is shown that, in particular, at relatively low densities the widths are proportional to the fourth root of the intensity.