Vibrational band strengths and temperatures of nitric oxide by time-resolved infrared emission spectroscopy in a shock tube

Abstract

Infrared emission spectra from the fundamental vibration—rotation band of NO are observed as functions of time in a shock tube using a specially constructed infrared array spectrometer. Emission is observed from shock-heated, dilute NO/Ar mixtures behind reflected shocks at temperatures between 800 and 2500 K and pressures of ≈1 atm. The spectrally resolved measurements cover the range 4.5–6.6 μm at a spectral resolution of 0.2 μm/pixel. The spectral intensity distributions on the long wavelength side of the band sample higher vibrational levels and thus provide a sensitive measure of vibrational temperature. Absolute spectral intensities in optically thin portions of the band determine the NO number density once the temperature is known. The integrated band intensities determine the temperature-dependent photon yields for vibrationally excited NO( v), which are in turn reduced by a rigorous anharmonic oscillator analysis to determine the thermally averaged Einstein coefficient for the optical transition from v = 1 to v = 0. The resulting value is A( v = 1) = 13.2(± 10%) sec -1, which is in excellent agreement with several determinations at temperatures near and below 300 K. This result establishes temperature independence for the NO Einstein coefficients up to 2500 K.