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 β1 band near 216nm. cw radiation for these measurements was generated in a ring dye laser system using intracavity BBO frequency doubling. Methyl radicals were produced by the shock wave heating of five selected source compounds: azomethane, methyl iodide, tetramethyl tin, tetramethyl silane and ethane. The variation with wavelength of the CH3 absorption coefficient between 210 and 225 nm was measured at 1625 K and 1.25 atm using ethane/Ar and methyl iodide/Ar gas mixtures. The variation of the absorption coefficient with temperature from 1350 to 2450 K was measured at 216.615 nm using four source compounds. Using this wavelength for CH3 measurements resulted in a typical detectivity limit of 2ppm at 1650 K, 1 atm, L = 10 cm, with a SNR of unity and a detection bandwidth of 500 kHz.