Detection of methyl radicals in a flat flame by degenerate four-wave mixing.

We report the spatially resolved detection of methyl radicals in a methane-air flat flame, using degenerate four-wave mixing (DFWM). A frequency-tripled dye laser pumped with a frequency-doubled Nd:YAG laser was used to access the Herzberg beta(1) band of methyl near 216 nm. Using a nearly phase-conjugate geometry, we detected methyl with high spatial resolution [0.2 mm (0.3 mm) vertical (horizontal) and ~6 mm longitudinal] and with good signal-to-noise ratio in a rich (ø = 1.55) flame. Compared with laser absorption spectra, DFWM spectra were much less influenced by a broad featureless background. From the absorption data, we measured the peak methyl concentration to be 650 parts in 10(6), resulting in an estimated DFWM detection limit of 65 parts in 10(6).