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

Abstract

This work employs in situ rotational Raman spectroscopy to study the effect of N 2 and O 2 addition to CO 2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO 2 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 CO 2 , 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 CO 2 conversion factor α 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 CO 2 and its beneficial effect on CO 2 dissociation. Furthermore, the influence of admixtures on CO 2 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 CO 2 . On the other hand, the addition of O 2 leads to a decrease of α, which might be attributed to quenched vibrations of CO 2 , and/or to the influence of the back reaction in the presence of O 2 .

Absolute OH density measurements in a CO2–H2O glow discharge by laser-induced fluorescence spectroscopy

“…As expected, the gas cools down after the plasma pulse until 400 K when the next pulse begins. The plasma always stays above room temperature (even including error bars) as was observed in other studies with a comparable glow discharge as well [49]. When the plasma is on, a strong increase of the temperature until 800 K is observed.…”

Advances in non-equilibrium $$\hbox {CO}_2$$ plasma kinetics: a theoretical and experimental review

“…Another important step forward in the development of simulation codes for the CO 2 plasma description was made jointly by the group of Guerra et al in collaboration with the experimental groups of Guaitella et al and Engeln et al.They described in a very accurate way the behavior of a DC CO 2 glow discharges at low dissociation conditions, with the inclusion in the model of the first few (approximately 70) CO 2 mixed vibrational levels. A systematic mutual step-by-step validation procedure of their results allowed to separately investigate the role of V-V and V-T transfers [60,61], e-V excitation [62] and gas heating [63,64]. Two particularly revealing results of the benefit of a joint model / experiment approach are the identification of the best CO 2 dissociation cross section by electron impact for E/N values below 100 Td [89], and the evidence of the key role of CO(a 3 Π) in the 'back reaction mechanisms' with O 2 at low gas temperature giving back CO 2 [131,148].…”

“…Nevertheless T 3 , T 12 and the vibrational temperature of CO (T CO ) have been measured by infrared absorption spectroscopy using either FTIR or tuneable laser diodes in glow discharges [60,[107][108][109][110][111][112][113], and by Raman scattering in MW discharges [98,105]. Rotational Raman scattering has also been used in glow discharges [64,114] but only the degree of non-equilibrium characterized by the even and odd vibrationally averaged nuclear degeneracies could be obtained in addition to T rot . The difference T vib -T rot for T rot <1100 K in Fig.…”

“…It therefore appeared necessary to initiate a step-by-step model validation by comparing the results of calculations with measurements made in simplified plasma configurations. By selecting particular operating regimes of lowpressure glow discharges, it is possible to test one by one the accuracy of the EEDF calculations, the VV and VT coefficients, and the main chemical reactions under conditions of weak CO 2 excitation [60][61][62][63][64].…”

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

“…Different optical diagnostic methods have been used to investigate the CO2 excitation/relaxation process in various types of discharges. Compared with the Raman scattering [11][12][13] and IR emission [14,15] , the former gives us limited information about the Raman active symmetric bending and stretching modes and the latter presents only qualitative information due to the strong self-absorption, IR absorption spectroscopy is a powerful and preferential method for this topic. Particularly, the broadband IR light source in combination with the Fourier transform infrared spectroscopy (FTIR) provides a relatively large scan range, and thus ensures a simultaneous detection of multiple vibrational modes and even multiple species [16][17][18] .…”