Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O₂ radiofrequency plasma

The influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O₂ radiofrequency plasma has been investigated experimentally. Spatially resolved mass spectrometry and laser induced fluorescence on OH radicals were used to estimate the intrusion of air in between the plasma torch and the substrate as a function of the torch-to-substrate separation distance. No air is detected, within the limits of measurement uncertainties, for separation distances smaller than 5 mm. For larger distances, the effect of ambient air can no longer be neglected, and radial gradients in the concentrations of species appear. The Ar 4p population, determined through absolute optical emission spectroscopy, is seen to decrease with separation distance, whereas a rise in emission from the N₂(C-B) system is measured. The observed decay in Ar 4p and N₂(C) populations for separation distances greater than 9 mm is partly assigned to the increasing collisional quenching rate by N₂ and O₂ molecules from the entrained air. Absorption measurements also point to the formation of ozone at concentrations from 10¹⁴ to 10 15 cm - 3, depending both on the injected O₂ flow rate and the torch-to-substrate separation distance.