Influence of effective pumping speed on oxygen atom density in a plasma post-glow reactor

The density of neutral oxygen atoms in a post-glow reactor was measured with a fibre-optics catalytic probe. The source of O atoms was a microwave discharge generated in Ar-O2 gas with different flow rates up to 3000 sccm/min. The O density was measured at different power of the microwave generator between 40 and 160 W and different effective pumping speed between 7 and 28 m3/h. It was found to depend on the ratio between O2 and Ar flow rates. At a constant O2 flow of 200 sccm/min there was a broad maximum of O density between the Ar flow of 200 and 1000 sccm/min, independent of the effective pumping speed. At a constant O2 and Ar flow rates of 200 and 1000 sccm/min, respectively, the O density was found to increase both with increasing power and increasing effective pumping speed. The results were explained with collision phenomena in ionized gases and heterogeneous recombination of O atoms on surfaces.