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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.

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.