The nonlinear spectral lineshape of gas absorption near the solid surface in the field of one travelling monochromatic wave is analyzed theoretically. A 'far' zone of distances to the surface, where coherent particle polarization is formed already after their collisions with surface but interparticle collisions in the gas are not yet significant, is considered. It is demonstrated that the narrow saturation sub-Doppler resonances may be appeared on the Doppler lineshape in this zone. These resonances having no analogues in the gas laser spectroscopy. A formation mechanism of such resonances is discussed.
Effects of an intense saturating radiation on a probe lineshape under Doppler broadening are studied in a three-level system when the frequency of the probe light is higher than that of the saturating light. The problem is analysed in the approximation of weak probe field using an ensemble averaged density matrix. An analytical form of the lineshape is obtained for the resonant saturating field of high intensity at which the Rabi nutation frequency is larger than the Doppler width of the probe transition. The linewidth of probe absorption depends on the Doppler width and the frequency ratio of the probe light to the saturating light. The computer calculation is carried out to find the lineshape in detail at a light intensity where the Rabi nutation frequency is comparable to the Doppler width.