Time evolution of CO₂ ro-vibrational excitation in a nanosecond discharge measured with laser absorption spectroscopy

CO₂ dissociation stimulated by vibrational excitation in non-equilibrium discharges has drawn lots of attention. Nanosecond (ns) discharges are known for their highly non-equilibrium conditions. It is therefore of interest to investigate the CO₂ excitation in such discharges. In this paper, we demonstrate the ability for monitoring the time evolution of CO₂ ro-vibrational excitation with a well-selected wavelength window around 2289.0 cm⁻¹ and a single continuous-wave quantum cascade laser with both high accuracy and temporal resolution. The rotational and vibrational temperatures for both the symmetric and the asymmetric modes of CO₂ in the afterglow of CO₂ + He ns-discharge were measured with a temporal resolution of 1.5 μs. The non-thermal feature and the preferential excitation of the asymmetric stretch mode of CO₂ were experimentally observed, with a peak temperature of T_{v 3, max} = 966 ± 1.5 K, T_{v 1,2, max} = 438.4 ± 1.2 K and T _rot = 334.6 ± 0.6 K reached at 3 μs after the nanosecond pulse. In the following relaxation process, an exponential decay with a time constant of 69 μs was observed for the asymmetric stretch (001) state, consistent with the dominant deexcitation mechanism due to VT transfer with He and deexcitation on the wall. Furthermore, a synchronous oscillation of the gas temperature and the total pressure was also observed and can be explained by a two-line thermometry and an adiabatic process. The period of the oscillation and its dependence on the gas components is consistent with a standing acoustic wave excited by the ns-discharge. (paper)