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

Abstract

CO 2 dissociation stimulated by vibrational excitation in non-equilibrium discharges has drawn lots of attention. 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 CW quantum cascade laser (QCL) 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 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 v3, max = 966  1.5 K, Tv12, max = 438.4  1.2 K and Trot = 334.6  0.6 K reached at 3 s after the nanosecond pulse. In the following relaxation process, an exponential decay with a time constant of 69 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 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.

Nanosecond resolved ro-vibrational CO2 excitation measurement

“…Up to now various methods for obtaining the molecular excitation, e.g. Raman scattering [10][11][12], IR emission spectroscopy [13,14], Fourier transform infrared spectroscopy [15][16][17][18], and more recently, also tunable diode laser absorption spectroscopy using narrowband lasers (∼MHz) are intensively adopted to measure the CO 2 ro-vibrational excitation [19][20][21][22][23]. However, none of these methods has come close to a nanosecond resolution.…”

“…However, none of these methods has come close to a nanosecond resolution. For example, in a recent work of ours [23] a temporal resolution of 1.5 µs has been achieved, which was so far the common standard.…”

Fast O Atom Exchange Diagnosed by Isotopic Tracing as a Probe of Excited States in Nonequilibrium CO2–CO–O2 Plasmas

“…Among these diagnostics we can mention two photon absorption laser-induced fluorescence (TALIF), 30,31 Raman scattering, 32 Fourier transform infrared (FTIR) absorption spectroscopy, 33 tunable diode lasers, 34,35 or quantum cascade lasers. 36,37 Most of them focus on the measurement of the dissociation fraction, i.e., CO and CO 2 (or their ratio), and their vibrational kinetics. However, atomic and molecular oxygen also play a key role in the vibrational kinetics of CO 2 plasmas 38 and in the backreaction mechanisms.…”

“…In situ diagnostics are essential to provide information on the interactions between all these species. Among these diagnostics we can mention two photon absorption laser-induced fluorescence (TALIF), , Raman scattering, Fourier transform infrared (FTIR) absorption spectroscopy, tunable diode lasers, , or quantum cascade lasers. , Most of them focus on the measurement of the dissociation fraction, i.e., CO and CO 2 (or their ratio), and their vibrational kinetics. However, atomic and molecular oxygen also play a key role in the vibrational kinetics of CO 2 plasmas and in the back-reaction mechanisms. , Atomic oxygen could also contribute to increasing the overall CO 2 dissociation. , In spite of that, few studies focus on the kinetics of O and O 2 in CO 2 plasmas. − The chemical paths are still elusive and need further clarification in order to take advantage of nonequilibrium for an efficient CO 2 conversion.…”

Modeling of vibrational excitation dynamics in a nanosecond CO2 discharge

“…In recent experiments [10,11], the dynamics of CO 2 vibrationally excited states in a nanosecond discharge between plane parallel electrodes, separated by a gap of 0.1 cm, in the mixture CO 2 :He = 1:9 at a total pressure of 150 mbar has been studied with high temporal resolution. The obtained results are very helpful for verification of CO 2 kinetic schemes, as they allow the rate constants of the excitation of the asymmetric vibrational mode by electron impact to be estimated.…”