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 gamma(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 gamma(O) obtained in a O-2 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.