Measurements and analysis of rotational temperatures obtained with Raman and optical emission spectroscopy in a nonequilibrium nitrogen plasma

Abstract

Abstract Rotational temperature measurements of N2(B3Πg), N2(C3Πu) and N 2 + ( B 2 Σ u + ) obtained by optical emission spectroscopy (OES) are compared with previous gas temperature measurements obtained with Raman spectroscopy in a nonequilibrium recombining nitrogen plasma at atmospheric pressure. The plasma is produced in a state of thermal and chemical equilibrium at about 7000 K using an inductively coupled plasma torch. It is then forced out of chemical equilibrium by imposing rapid cooling through a water-cooled tube. The gas temperature measured by Raman spectroscopy is about 3200 K at the exit of the tube. The rotational temperatures of N2(B) and N2(C) measured with OES are found to be in good agreement with the gas temperature, but the rotational temperature of N2 +(B) is found to be significantly above the gas translational temperature. We propose that this higher rotational temperature is the result of N2 +(B) being formed by the charge exchange reaction between N2(A3Σu +) and N+. Kinetic and QSS simulations are performed to support this assumption and a rate constant for this reaction is proposed.