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

Abstract

Frequency modulation detection of NH2 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, 15 cm shock tube diameter). This corresponds to a minimum detectable absorption of 0.01% and 0.005%, respectively, which represents an improvement of more than a factor of 20 when compared to conventional laser absorption detection. The feasibility of quantitative absolute concentration measurements is demonstrated using CH3NH2 as a precursor for the preparation of known NH2 concentrations. The uncertainty for absolute concentration measurements is estimated to be ±10% if a suitable precursor for direct shock tube calibration measurements can be used, and ±15% if an alternative calibration scheme based on the detection of the signal generated by a scanning etalon in reflective mode is used. FM detection has been applied to determine the rate coefficient of the thermal decomposition of CH3NH2: CH3NH2 + M → CH3+NH2+M over the temperature range 1530–1975 K and at pressures near 1.3 atm. The rate coefficient was found to be: k1=8.17×1016 exp(−30710/T) (±20%) [cm3 mol−1 s−1] This is in good agreement with a recent determination using conventional laser absorption detection of the NH2 radical. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 445–453, 1999