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In situ quantum cascade laser (QCL) absorption spectroscopy is used to investigate the effect of admixed water in a pulsed CO2 glow discharge on the vibrational excitation of CO2 and CO and the conversion of CO2. Time-resolved transmittance spectra of the non-equilibrium CO2 plasma are measured with a 100 μs time resolution. A custom fitting routine is used to extract the time evolution of the gas temperature, rotational temperature and vibrational temperatures of CO2 and CO, while the CO2 conversion is determined from measured CO2 and CO number densities. Rotational Raman scattering is additionally performed in the centre of the reactor to verify measured rotational and vibrational temperatures from line-of-sight absorption spectroscopy. The plasma is operated at 6.7 mbar, with up to 10% water admixed, and is pulsed with a 5–10 ms on-off cycle, with a current of 50 mA supplied during the plasma on-time. Vibrational temperatures and CO2 conversion are not significantly affected by water admixtures below 0.5%. However, the asymmetric stretch temperature of CO2 (T 3) shows considerable quenching upon admixing 10% water vapour, with the maximum elevation above the rotational temperature (T rot) decreasing from 580 ± 86 K to 230 ± 63 K. For the vibrational temperature of CO (T CO), a similar trend is measured. However, the slopes of T 3 and T CO within the first few hundred μs after the start of the plasma remain unchanged, even when admixing 10% water vapour, suggesting equal excitation of the vibrational modes through e–V and V–V interactions. The conversion decreases by almost a factor of 4 when admixing 10% water. We argue that vibrational quenching of CO2 by water can explain part of the decrease. Changes in electron density and temperature and reactions between CO and OH can also play a role.

We report first measurements of time-resolved absolute CO number densities and rotational temperatures in a non-thermal CO 2 plasma environment using TALIF with a nanosecond pulsed laser. Two-photon excitation spectra from the B 1Σ+(v′ = 0) ← X 1 Σ+ (v″ = 0) Q-branch are recorded and fitted to extract rotational temperatures. Absolute number densities are determined from the frequency-integrated excitation spectrum. The plasma under investigation is a pulsed glow discharge operated at a frequency of 60 Hz with an plasma-on time of 5 ms per plasma cycle, 50 mA plasma current and a pressure of 6.67 mbar. CO number densities range from (2.6 ± 0.6) × 10 22 m -3 to (1.2 ± 0.3) × 10 22 m -3, while rotational temperatures range from 370 ± 40 K to 700 ± 70 K at the beginning and end of the plasma-on phase, respectively. Our results show fair agreement with literature data.

International audience; This work employs in situ rotational Raman spectroscopy to study the effect of N2 and O2 addition to CO2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO2 and 25%, 50% and 75% of N2 or O2 admixture, in a 5–10 ms on-off cycle, 50 mA plasma current and 6.7 mbar total pressure. The rotational temperature profile is not affected by adding N2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O2 and N2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N2 containing mixtures show that CO2 conversion factor α increases from 0.15 to 0.33 when the content of N2 is increased from 0% to 75%, demonstrating the potential of N2 addition to enhance the vibrational pumping of CO2 and its beneficial effect on CO2 dissociation. Furthermore, the influence of admixtures on CO2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O2 leads to a decrease of α, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O2.

International audience; The excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed, being (1) time-resolved in situ Fourier transform infrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The decay rate ρT3-Trot follows the relation ρT3-Trot = 388 s-1 exp((Trot - 273 K)/(154 K)). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05 to 0.18 for 10 mA to 50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge.

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,v2-l 2,v 3)\to (v 1,v 2-l2,v3+1) C O 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.

The implementation of in situ rotational Raman spectroscopy is realized for a pulsed glow discharge in CO2 in the mbar range and is used to study the rotational temperature and molecular number densities of CO2, CO, and O2. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5-10 ms on-off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing in situ Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO2 conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO2 are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO2, acquired in the FTIR study.

Vibrational temperatures of CO2 are studied in a pulsed glow discharge by means of time-resolved in situ Fourier transform infrared spectroscopy, with a 10 μs temporal resolution. A method to analyze the infrared transmittance through vibrationally excited CO2 is presented and validated on a previously published CO2 spectrum, showing good agreement between fit and data. The discharge under study is pulsed with a typical duty cycle of 5-10 ms on-off, at 50 mA and 6.7 mbar. A rapid increase of the temperature of the asymmetric stretch vibration (T 3) is observed at the start of the pulse, reaching 1050 K, which is an elevation of 550 K above the rotational temperature () of 500 K. After the plasma pulse, the characteristic relaxation time of T 3 to strongly depends on the rotational temperature. By adjusting the duty cycle, the rotational temperature directly after the discharge is varied from 530 to 860 K, resulting in relaxation times between 0.4 and 0.1 ms. Equivalently, as the gas heats up during the plasma pulse, the elevation of T 3 above decreases strongly.