Experimental and Theoretical Study of Singlet Delta Oxygen Production in Microcathode Sustained Discharges.

Abstract

In rare gas/oxygen mixtures, the generation of O2(a∆g) states by microplasmas has been studied experimentally and theoretically. Microplasmas refer to electric discharges created in very small geometries able to operate in DC mode at high pressure without glow to arc transition. A microplasma configuration which has proven to be stable at atmospheric pressure and up to a power density of some 100 kW/cm is the Micro Hollow Cathode Discharge (MHCD) developed by Schoenbach and coworkers. MHCDs are created by applying a voltage between two closely spaced hollow electrodes separated by a dielectric layer. The thickness of the dielectric and the diameter of the hole are both on the order of some 100's microns. These MHCDs can be used as plasma cathodes for generating a diffuse discharge between the MHCD cathode and a third positively-biased electrode placed some distance away. This is the so-called Micro Cathode Sustained Discharge (MCSD) configuration. It is shown in the present paper that these MCSDs can be operated as nonself-sustained discharges with low values of the reduced electric field and of the gas temperature. As a result, these MCSDs can efficiently generate large amounts of singlet delta oxygen. In argon-oxygen mixtures, O2(a∆g) relative yields of 13 % were measured 23 cm downstream in the afterglow of the MCSD discharge, while in helium-oxygen mixtures, O2(a∆g) number densities above 10 cm were achieved at atmospheric pressure for flow rates in the range 5-30 ln/mn. Moreover, it has been demonstrated that MCSDs can be operated in parallel in this non-self-sustained discharge mode. Such arrays of MCSDs offer a new possibility to produce large amounts of O2(a∆g) states, which could give rise to a large spectrum of applications