Rotational-Temperature Measurements of Chemically Reacting CN Using Band-Tail Spectra

The vibrational state of chemically reacting CN radicals does not necessarily have a Boltzmann distribution because it may be influenced by the chemical reaction leading to the formation of the CN radicals. Here, we develop a new method to measure the rotational temperature of chemically reacting nonequilibrium cyanide radicals using the band tails of their emission spectra. Because of the very short relaxation time scales, both the translational and rotational states reach a thermal equilibrium even when the vibrational state does not have a Boltzmann distribution. The method proposed in this study has two advantages. First, it is not sensitively affected by self-absorption. Second, it does not require as high a wavelength resolution as other methods because it uses the overall shape of the tail part of the CN emission bands. Thus, our method is more suitable for high-speed temperature measurements, where a high-wavelength-resolution measurement is difficult to obtain. To investigate the validity of our method, we carried out laser-ablation experiments within an N-2 - H2O - CO2 - Ar gas mixture using graphite targets and measured the rotational temperature of laser-induced CN and C-2 radicals using the proposed method. The rotational temperatures exhibit reasonable trends as functions of time and beam cross sections, strongly suggesting that our method is useful for transiational-rotational-temperature estimation of chemically reacting nonequilibrium CN radicals.