A rotational Raman study under non-thermal conditions in pulsed CO2-N2 and CO2-O2 glow discharges

This work employs in situ rotational Raman spectroscopy to study the effect of N-2 and O-2 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 N-2 or O-2 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 N-2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O-2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O-2 and N-2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N-2 containing mixtures show that CO2 conversion factor a increases from 0.15 to 0.33 when the content of N-2 is increased from 0% to 75%, demonstrating the potential of N-2 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 N-2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O-2 leads to a decrease of a, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O-2.