O2 dissociation in Ar–O2 surface-wave microwave discharges

A self-consistent model is developed in order to investigate the dissociation of O2 molecules in Ar–O2 surface-wave microwave discharges. The dissociation degree of O2 molecules ([O]/2[O2]0) is determined for surface-wave discharges generated in 0.5 cm and 2.8 cm diameter tubes with 2.45 GHz and 915 MHz microwave field frequencies, respectively, in the 0.5–25 mbar pressure range at different mixture compositions from pure O2 to 95%Ar–5%O2. The dissociation degree varies non-monotonically with pressure—it passes through a minimum—which is more pronounced in high Ar content mixtures. This behaviour is found to be a consequence of the electron collision processes, in particular the evolution of the non-Maxwellian shape of the electron energy distribution function with pressure. These changes make the electron impact dissociation coefficient follow the same non-monotonic trend with pressure, despite leading to the expected monotonic variation of the ionization rate coefficient. The minimum in dissociation is in correlation with the momentum transfer cross-section profiles of the mixture's components, and occurs approximately at the pressure that satisfies the condition νce = ω, which explains its dependence on ω.