Sensitive detection of NH2 in shock tube experiments using frequency modulation spectroscopy

Frequency modulation detection of NH, in shock tube kinetic experiments is demonstrated with sensitivities of 0.5 ppm in a single pass and 0.25 ppm in a double pass configuration (1500 K, 1.3 atm, detection bandwidth 1 MHz, 1 5 cm shock tube diameter). This corresponds to a minimum detectable absorpt ion of 0.01% and 0.005%, respectively, which represents an improvement of m ore than a factor of 20 when compared to conventional laser absorption dete ction. The feasibility of quantitative absolute concentration measurements is demo nstrated using CH3NH2 as a precursor For the preparation of known NH2 conce ntrations. The uncertainty for absolute concentration measurements is estim ated to be +/- 10% if a suitable precursor for direct shock tube calibratio n measurements can be used, and +/- 15% if an alternative calibration schem e based on the detection of the signal generated by a scanning etalon in re flective mode is used. FM detection has been applied to determine the rate coefficient of the ther mal decomposition of CH3NH2: CH3NH2 + M --> CH3 + NH2 + M over the temperature range 1530-1975 K and at pressures near 1.3 atm. The r ate coefficient was found to be: k(1) = 8.17 x 10(16) exp(-30710/T) (+/- 20%) [cm(3) mol(-1) s(-1)] This is in good agreement with a recent determination using conventional la ser absorption detection of the NH2 radical. (C) 1999 John Wiley & Sons, In c.