Quantitative High Temperature Absorption

A spectral survey of NCO absorption near the P1 + 'Qli head of the [A 2Z +(00°0)«-A'J/71<00I0)] band was obtained at 1450°K, 0.6 atm using a remotely located cw ring dye laser source and a shock tube. Mixtures of hydrogen cyanide, oxygen and nitrous oxide diluted in argon were shock heated to provide a reproducible steady-state concentration of NCO, and narrow-line absorption was measured in repeated experiments with the laser set at different wavelengths. The peak absorption was found at 440.479 nm (vac). The experimental spectrum was compared with a theoretical model to yield an average Voigt parameter o=:0.1. Additional experiments, in mixtures of cyanogen, oxygen and nitrous oxide diluted in argon, provided a known plateau level of NCO, which was used to infer an absolute absorption coefficient /?(1450°K, 0.60 atm) = 110(—50, +130) cm-'atm"1 at 440.479 nm. This value of /? corresponds to an oscillator strength of 0.0026 for the (0000*-00'0) band. Similar experiments were conducted to monitor the absorption around the R, head of the [B2n,(l0'<i)-X2n/i0QlQ)] band of NCO, using a frequency doubled cw ring dye laser. The observed spectrum displayed strong broadening, indicating predissociation of the upper state. At the peak absorption wavelength (304.681 nm, vac), we inferred /?(1470°K, 0.63atm) = 40(-19,+48) cm-'atm"' and oa9. This value of fi corresponds to an oscillator strength of 0.0031 for the (10'0«-00'0) band. INTRODUCTION The species NCO is of interest in combustion; for example, it serves as a critical intermediate in the formation of NO during combustion of nitrogen-containing fuels.1 The spectral characteristics of NCO have been studied extensively, but few attempts have been made to perform quantitative measurements of NCO in a combustion environment. Dixon studied the [A2Z + *-X2n& spectrum of NCO from 360 to 450nm2 and the [fi2fl, — X2n,] system from 265 to 320 nm.3 Bolman et al. also investigated the [A2Z + *-X2n] spectrum.4 Measurements of the radiative lifetimes of NCO [A2Z+] that lead to total oscillator strengths have been reported by Reisler et al.,5 Charlton et al.6 and Sullivan et al.1 In addition, Sullivan et al* examined radiative lifetimes in the [S2/7J level using laser-induced fluorescence. Measurements of NCO concentrations are limited. Anderson et al.9 observed the NCO spectrum in a flame by intracavity laser excitation and reported relative NCO concen tration profiles. Bullock and Cooper10 monitored relative NCO absorptions at 438.48 nm in a kinetic study of the gas-phase reactions of CN at low temperature. In this study, two novel laser absorption diagnostics of NCO were demonstrated behind incident shock waves in a shock tube. Mixtures of QNj, N2O and O2 diluted in argon and mixtures of HCN, N2O and O2 diluted in argon were shock heated to generate reproducible levels of NCO under specified conditions of pressure (0.6 atm) and tem perature (1450°K). Narrow-linewidth absorption spectra around 440.5 and 304.7 nm were mapped out by conducting a series of nearly identical experiments, each at a different laser wavelength. Using the known levels of NCO from the C2N2 mixtures, absolute peak absorption coefficients near 440.5 and 304.7 nm were inferred and compared with theoretical models to extract band oscillator strengths and Franck-Condon factors at both wavelengths. In this paper, the experimental facility and optical techniques will first be described, then the spectroscopic models and results for the two techniques will be presented.