DETECTION OF METHYL RADICALS IN A FLAT FLAME BY DEGENERATE 4-WAVE-MIXING

We report the spatially resolved detection of methyl radicals in a met hane-air flat flame, using degenerate four-wave mixing (DFWM). A frequ ency-tripled dye laser pumped with a frequency-doubled Nd:YAG laser wa s used to access the Herzberg beta(1) band of methyl near 216 nm. Usin g a nearly phase-conjugate geometry, we detected methyl with high spat ial resolution [0.2 mm (0.3 mm) vertical (horizontal) and similar to 6 mm longitudinal] and with good signal-to-noise ratio in a rich (phi = 1.55) flame. Compared with laser absorption spectra, DFWM spectra wer e much less influenced by a broad featureless background. From the abs orption data, we measured the peak methyl concentration to be 650 part s in 10(6), resulting in an estimated DFWM detection limit of 65 parts in 10(6). (C) 1995 Optical Society of America