Time resolved CO₂ ro-vibrational excitation in a nanosecond discharge measured with quantum cascade laser absorption spectroscopy

CO₂ conversion is of growing interest and non-thermal plasmas are promising to be efficient in this regard due to their unique parameters (electron, vibrational, rotational, and gas tem­peratures). On the short time scale of a nanosecond pulsed discharge, energetic electrons lead to vibrational excitation instead of gas heating. Ladder-climbing population of higher vi­brational levels due to the small energy difference (∼0.3 eV) is a more efficient dissociation pathway. Temporally resolved measurement of the ro-vibrational excitation is important to gain insight into the excitation and relaxation processes and to validate detailed kinetic models of CO₂ dissociation. Measurements are performed in a nanosecond pulsed discharge at 150 mbar CO₂ (10 %) in He (90 %). A quantum cas­cade laser and a fast detector are used for measuring the absorption with down to 8 ns temporal resolu­tion. The tuning range of the laser allows for simul­taneous measurement of rotational and vibrational temperature for the symmetric and asym­metric modes. In order to obtain the multiple temperatures (T_rot, T_v1, T_v2 and T_v3) and the CO₂ concentration, spectra as a function of the temperatures are fitted to the measured ones. The evolution of the rota­tional and vibrational temperatures of CO₂ within the nanosecond discharge pulse of 150 ns length is obtained and analyzed.