Role of volume and surface processes in the atomic oxygen loss frequency in oxygen glow discharges in Pyrex

Abstract

This work aims at understanding and finding the most accurate way to estimate an effective O surface recombination probability ( γ O ) from the temporal evolution of O(³P) density measured in modulated current conditions in the positive column of oxygen glow discharges in Pyrex, which is relevant for experimental characterization and for model inputs. The procedure for deducing γ O from the O loss frequency ( ν O loss ) is not straightforward, since the processes determining the net losses of O(³P) are not known a priori. A global model describing plasma chemical kinetics is used in steady-state and current-modulated modes to assess the physical meaning of ν O loss , for a total of 66 experimental conditions in the pressure range 0.4-7.5 Torr with 7.8 sccm flow rate and 20-40 mA currents. An optimal γ O is derived from ν O loss and the accuracy of different hypotheses on the relevant processes for the net losses of O(³P) is addressed. The hypothesis that is found to be the most accurate indicates that the net loss of O(³P) is due not only to surface recombination, but also to recombination in volume and to flow losses. Volume processes account on average for 24% of O(³P) net losses, and that fraction tendentiously grows with pressure, reaching up to 71%. The verified hypothesis can be adopted for the estimation of γ O from the measured ν O loss in both discharge (partial modulation) and post-discharge (full modulation) conditions. Finally, a discussion is had on what steps can be taken for a complete validation of the oxygen glow discharge model.