A CW LASER-ABSORPTION DIAGNOSTIC FOR METHYL RADICALS

Absorption of narrow-line laser radiation by methyl radicals produced at high temperatures was studied in the Herzberg beta1 band near 216 n m. cw radiation for these measurements was generated in a ring dye las er system using intracavity BBO frequency doubling. Methyl radicals we re produced by the shock wave heating of five selected source compound s: azomethane, methyl iodide, tetramethyl tin, tetramethyl silane and ethane. The variation with wavelength of the CH3 absorption coefficien t between 210 and 225 nm was measured at 1625 K and 1.25 atm using eth ane/Ar and methyl iodide/Ar gas mixtures. The variation of the absorpt ion coefficient with temperature from 1350 to 2450 K was measured at 2 16.615 nm using four source compounds. Using this wavelength for CH3 m easurements resulted in a typical detectivity limit of 2 ppm at 1650 K , 1 atm, L = 10 cm, with a SNR of unity and a detection bandwidth of 5 00 kHz.