DEVELOPMENT OF A CW LASER-ABSORPTION DIAGNOSTIC FOR MEASUREMENT OF CNIN SHOCK-TUBE EXPERIMENTS

Time-histories of the CN radical were monitored using a tunable ring d ye laser to measure absorption at the P-branch bandhead of the B2SIGMA + <-- X2SIGMA+ (0, 0) band of CN in a shock tube. Measurements of CN r esulting from the pyrolysis of highly dilute mixtures of C2N2 in Ar we re compared with a spectroscopic model, and the location of maximum ab sorption for the bandhead was determined to be 25743.72 cm-1 (lambda = 388.444 nm). A second set of experiments at the maximum absorption fr equency was conducted for temperatures between 2700 and 4900 K and pre ssures between 0.34 and 1.0 atm. These measurements were used to deter mine an expression for the effective collisional broadening coefficien t. Correlation of the data with the spectroscopic model led to a tempe rature- and pressure-dependent expression for the absorption coefficie nt. For P = 1 atm, this expression is of the form k(v)(T, 1 atm) [cm-1 atm-1] = a/T3 + b/T2 + c/T, where a = -9.95984 x 10(13), b = 1.13891 x 10(11), and c = -1.16132 x 10(7); it agrees with the spectroscopic m odel within 1% for the temperature range 2000 < T < 5000 K. The corres ponding uncertainty of the absorption coefficient for this expression is +/- 15% for T < 3000 K and +/- 5% for T > 3300 K. The detection lim it of the CN diagnostic is 0.1% absorption, which corresponds to a CN detection sensitivity of 0.02 ppm at T = 3600 K, P = 1 atm, and a path length of 14.3 cm.