Oxygen atom kinetics in CO₂ plasmas ignited in a DC glow discharge

Oxygen atom densities were measured in situ in a CO₂ glow discharge, at pressures between 0.2 and 5 Torr (26.7-666.6 Pa). Two measurement techniques were compared, namely optical emission actinometry (using Ar as the actinometer) and high-resolution two-photon absorption laser induced fluorescence 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 O₂. However, the measured values are more than two times lower than γ _O obtained in a O₂ plasma in similar conditions. The O atom densities are also compared to the dissociation fraction of CO₂ 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 CO₂ densities and O atoms loss frequency in the same conditions provides an ideal set of constraints for validating CO₂ plasma kinetic models.