Temperature evolution in a pulsed CO₂–N₂ glow discharge measured using quantum cascade laser absorption spectroscopy

This work uses in situ narrowband quantum cascade laser (QCL) absorption spectroscopy to study the effect of N₂ on the time evolution of gas temperature, rotational temperature and the vibrational temperatures of CO₂ and CO in a pulsed glow discharge. Three colinear QCLs are used to scan three regions of about 1 cm⁻¹ between 2179.20 and 2253.51 cm⁻¹, including $(v_1,v_2^{l_2},v_3)\to (v_1,v_2^{l_2},v_3+1){CO}_2$ transitions up to the asymmetric stretch level v ₃ = 6, as well as (v _CO) → (v _CO + 1) CO transitions up to v _CO = 1. A fitting routine is used to extract temperatures from the measured absorption spectra. The time resolved measurements are performed in CO₂, admixed with up to 90% N₂, with the plasma operated with a 5–10 ms on-off cycle, a discharge current of 50 mA and a pressure of around 6.7 mbar. The time evolution of the gas temperature has been measured and agrees well with the time evolution of the rotational temperature. The asymmetric stretch vibrational temperature T ₃ of CO₂ reaches a maximum of 1060 K at 0.7 ms for pure CO₂, while T ₃ goes up to 2250 K for a N₂ content of 90% and stays constant until the plasma is switched off. Both T ₃ and the vibrational temperature of CO T _CO show a clear non-equilibrium with respect to the rotational temperature T _rot. Both do not equilibrate with the rotational temperature T _rot between consecutive plasma cycles for a N₂ content above 70%, although T ₃ and T _CO always equilibrate with each other in the afterglow. The symmetric stretch and bending mode temperature T ₁₂ is elevated more with respect to the rotational temperature for increasing N₂ content, while the maximum of the rotational temperature decreases for increasing N₂ admixtures, which might be attributed to the energy stored in the vibrational modes of N₂, CO₂ and CO. Additionally, an indication of an increase in the total pressure during the plasma on-time due to heating and a subsequent decrease in the afterglow due to cooling was found for a pure CO₂ plasma. (paper)