High-resolution investigation of degenerate four-wave mixing in the γ(0, 0) band of nitric oxide

We report investigations of degenerate four-wave mixing (DFWM) in the NO A²Σ⁺←X²Π(ν′=0←ν″=0) band for combustion-diagnostic applications. We have performed high-resolution measurements of DFWM line shapes and intensities with varying foreign-gas pressures and compared the results to simple models. In addition, we have demonstrated the ability to detect flame-generated NO using DFWM, and have tested predictive capabilities of the models by comparing flame spectra to model spectra. Measurements were obtained using a single-mode laser (0.004 cm^-1 bandwidth) and a widely tunable multi-mode laser operating near 226 nm. We found that a two-level, moving-absorber model of Abrams et al. [Optical Phase Conjugation, (R. A. Fisher, Ed.), p. 234, Academic Press, 1983] gave an excellent description of DFWM line shapes of NO broadened by He, over a range of pressures spanning the Doppler and collisional broadening regimes. We also investigated the dependence of peak DFWM intensities on foreign-gas pressure, confirming a theoretically predicted rapid decrease in DFWM signal intensities with increasing foreign-gas pressure. However, observed intensities decreased more slowly than predicted. In addition, we observed a greatly reduced pressure dependence when the DFWM signals were strongly saturated. DFWM spectra from thermally generated NO in an H₂/O₂/N₂ diffusion flame were obtained with high signal-to-noise ratio and with relatively low saturation. A comparison of a flame spectrum with a model simulation exhibited excellent agreement. The model used the moving-absorber line-shape model and NO spectral data from the literature, and had no adjusted parameters.