Oxygen atom densities were measured in situ in a CO 2 glow discharge, at pressures between 0.2 and 5 Torr . Two measurement techniques were compared, namely optical emission actinometry (using Ar as the actinometer) and High-Resolution Two-photon Absorption Laser Induced Fluorescence (HR-TALIF) normalised to Xe, and were found to give consistent results. The variation of the atomic oxygen density with gas pressure shows two different regimes with a transition around 1 Torr. Measurements of the O atom loss frequency under plasma exposure showed that this behaviour is caused by a change in the O atom loss mechanisms, which are dominated by surface processes in our experimental conditions. The corresponding recombination probabilities on Pyrex γ O are found to vary with the gas temperature near the wall for a constant surface temperature, similarly to what has recently been obtained in pure O 2 . However, the measured values are more than two times lower than γ O obtained in a O 2 plasma in similar conditions. The O atom densities are also compared to the dissociation fraction of CO 2 determined by infra-red absorption. The obtained CO and O densities show different behaviour as a function of the energy input. The simultaneous measurement of gas temperature, electric field, O, CO and CO 2 densities and O atoms loss frequency in the same conditions provides an ideal set of constraints for validating CO 2 plasma kinetic models.

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This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v1 = 2 and v2 = v3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge, operating at pressures in the range 0.4 - 5 Torr (53.33 - 666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes; On the other hand, two main mechanisms contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture: the back reaction, CO(a3Πr) + O2 → CO2 + O and the recombinative detachment O- + CO → e + CO2.

This work proposes a complete and consistent set of cross sections for electron collisions with water molecules to be published in the IST-Lisbon database on LXCat. The set is validated from the comparison between experimental and computed electron swarm parameters. The former are collected from literature while the latter are calculated using a space-homogeneous two-term Boltzmann solver, assuming isotropic scattering in inelastic collisions. Rotational cross sections, based on the Born approximation, are optimised by means of the electron swarm analysis technique. Superelastic rotational and vibrational collisions are accounted for in the calculations and found to be particularly important for low-energy electrons interacting with water molecules. The set can be used with codes assuming space-homogeneous conditions, in particular common two-term Boltzmann solvers, ensuring a good agreement with experiments. Therefore, it constitutes an important tool for fast calculations and modelling of complex plasma chemistries.

This work presents a reaction mechanism for oxygen plasmas, i.e. a set of reactions and corresponding rate coefficients that are validated against benchmark experiments. The kinetic scheme is validated in a DC glow discharge for gas pressures of 0.2–10 Torr and currents of 10–40 mA, using the 0D LisbOn KInetics (LoKI) simulation tool and available experimental data. The comparison comprises not only the densities of the main species in the discharge - O2(X3Σ− g), O2(a1Δg), O2(b1Σ+ g ) and O(3P) - but also the self-consistent calculation of the reduced electric field and the gas temperature. The main processes involved in the creation and destruction of these species are identified. Moreover, the results show that the oxygen atoms play a dominant role in gas heating, via recombination at the wall and quenching of O2(X3Σ− g, v) vibrations and O2 electronically-excited states. It is argued that the development and validation of kinetic schemes for plasma chemistry should adopt a paradigm based on the comparison against standard validation tests, as it is done in electron swarm validation of cross sections.