| 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 |
