Analysis of degenerate four-wave mixing spectra of NO in a CH4/N-2/O-2 flame

We report comparisons of degenerate four-wave mixing (DFWM) spectra of NO m easured in a CH4/N-2/O-2 flame to spectral simulations based on a two-level theory for stationary, saturable absorbers by Abrams et al. Temperatures d etermined from least-squares fits of simulations to experimental spectra in the A(2)Sigma(+) <-- X(2)Pi(+) (0, 0) band are compared to temperatures ob tained from OH absorption spectroscopy and a radiation-corrected thermocoup le. We find that DFWM rotational temperatures derived from e-branch spectra agree with thermocouple and are independent of pump laser intensity for lo w to moderate saturation (I approximate to I-sat). However, the temperature s are systematically low and depend on pump intensity if the analysis negle cts saturation effects. We demonstrate a method for obtaining an effective pump saturation intensity for use with the two-level model. This approach f or analyzing saturated DFWM line intensities differs from previous work in that the use of the theory of Abrams et al. rather than a transition-dipole -moment power law allows treatment of a much wider range of saturation. Bas ed on the observed signal-to-noise ratio an NO detection sensitivity of 25 ppm is projected, limited by a DFWM background interference specific to hyd rocarbon flames.