Time evolution of CO <sub>2</sub> ro-vibrational excitation in a nanosecond discharge measured with laser absorption spectroscopy
Yanjun Du, Tsanko Tsankov, D Luggenhölscher, Uwe Czarnetzki
Abstract
Abstract CO 2 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 CO 2 excitation in such discharges. In this paper, we demonstrate the ability for monitoring the time evolution of CO 2 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 CO 2 in the afterglow of CO 2 + He ns-discharge were measured with a temporal resolution of 1.5 μ s. The non-thermal feature and the preferential excitation of the asymmetric stretch mode of CO 2 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 μ s after the nanosecond pulse. In the following relaxation process, an exponential decay with a time constant of 69 μ s 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.