Reduction of degenerate four-wave mixing spectra to relative populations II. Strong-field limit

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Abstract
The degenerate four‐wave‐mixing (DFWM) signal is said to be saturated when the population difference of the two levels involved in the resonant transition oscillates with a rate (Rabi frequency) greater than the relaxation and dephasing rates. The field intensity at which this occurs is referred to as the DFWM saturation intensity. We find that the DFWM saturation behavior predicted by nondegenerate two‐level models is in close agreement with the observed power dependence of (0,0) band transitions of the CH A 2Δ−X 2Π system. Furthermore, when the linear polarization states of the excitation fields are varied, the saturation intensity does not change significantly. In contrast, large differences in the DFWM signals are observed as a function of input field polarization and rotational branch. These differences are nearly independent of laser intensity. The DFWM signal differences are rationalized using the diagrammatic perturbation theory (DPT) expressions described in the preceding paper. We find that the DPT expressions are accurate to 10%–30% at saturating laser intensities. The important aspects of the reduction of saturated DFWM signals to relative internal‐state distributions are outlined in environments where population relaxation and dephasing events are dominated by collisions, and a rotational temperature analysis is presented of the CH radical in an atmospheric‐pressure oxyacetylene flame. Rotational temperatures determined using saturated DFWM are estimated to be accurate to 5%.