Time Evolution of the Dissociation Fraction in rf CO2 Plasmas: Impact and Nature of Back-Reaction Mechanisms

The time evolution of the dissociation fraction in a pulsed radio frequency (rf) CO2 discharge is studied by infrared absorption. A large parametric study performed in a closed reactor brings valuable information about both dissociation and recombination processes. The CO2 conversion shows a time evolution initially controlled by electron impact dissociation. For longer plasma-on times, the dissociation fraction reaches a steady-state that corresponds to a balance between dissociation processes and back-reaction mechanisms. The characteristic times of vibrational and rotational excitation of CO and CO2 are measured during a single plasma pulse. The dependence of the CO2 conversion as a function of pulse duration and frequency is then analyzed, showing the influence of the plasma excitation conditions on the back-reaction mechanisms. The study of CO2-O-2 and CO-O-2 gas mixtures gives further insight into the impact of the oxygen content in these mechanisms. The global back-reaction rate observed in these experiments is compared with calculated values from rate coefficients available in the literature. The experimental results and preliminary calculations reveal a key role of molecular oxygen and of the metastable electronically excited state CO(a(3)Pi(r)) in the back-reaction. Competing processes involving vibrational excited CO are not dominant in our discharge conditions but may become relevant at slightly higher vibrational temperatures.