Production of O atoms in Ar-O2 and N2-O2 microwave flowing post-discharges

Production of O atoms has been determined in microwave flowing post-discharges by NO titration for Ar-O2 and N2-O2 gas mixtures and by fibre optics catalytic probe (FOCP) for Ar-O2. It was found that the NO titration and FOCP diagnostics give similar results in Ar-O2 gas mixtures. With a gas flow rate of 1 Sl/min, a pressure of 200 Pa (1.5 Torr) in the post-discharge reactor and a post-discharge time of approximately 10 ms (distance from the discharge end divided by the gas velocity), it has been found that the O atom density increases with the O2 percentage into Ar to stabilise after 10%. Also the O atom density increases with microwave power to saturate after 100 watts. The dissociation rate of O2 reaches a maximum value of approximately 70% in a gas mixture with 5% O2 in Ar, for microwave power 100–160 watts. The NO titration diagnostic was also applied to N2-O2 gas mixtures. Here, it was necessary to separate the effect of NO produced by the N2-O2 plasma from the effect of NO introduced in the post-discharge for O atom titration. Reliable results of NO titration for less than 10% O2 into N2 were found. At constant gas flow rate of 1 Sl/min, a pressure of 200 Pa and a microwave power of 100 watts, the N atom density decreases with the O2 percent into N2 to disappear at 7% O2. At the inverse, the O atom density continuously increased by a factor 2 from 3 to 10% O2.