M. Votsmeier et al., Sensitive detection of NH2 in shock tube experiments using frequency modulation spectroscopy, INT J CH K, 31(6), 1999, pp. 445-453
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.