Detection of Methyl Radicals in a Flat Flame by Degenerate Four-Wave Mixing

Abstract

We report the spatially resolved detection of methyl radicals in a methane–air f lat 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 b1 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 sf _ 1.55d 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 106, resulting in an estimated DFWM detection limit of 65 parts in 106.