Active species downstream of an Ar–O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring

Abstract

Self-consistent theoretical models have been developed in order to investigate the early and remote flowing afterglows of a surface-wave Ar–O2 microwave discharge generated at 2.45 GHz in a 0.5 cm diameter tube at pressures between 1 and 12 mbar. The early afterglow that occurs downstream of the discharge fills up the tube that connects the discharge region with the large-volume processing reactor, where the late afterglow develops. The models provide the time-dependent density profiles of different species along the afterglow and their 3D spatial distribution in the processing reactor. Systematic calculations are performed for all mixture compositions from pure Ar to pure O2 at different pressures.It is shown that the Ar+, and can survive up to 1–10 ms in the early afterglow depending on the mixture composition and pressure. In low O2 content mixtures the ion densities can increase in the early afterglow, depending on the operating conditions, as a result of Penning ionization involving the Ar(4s) states and forming Ar+, followed by charge transfer. In pure Ar the UV emitting resonant state atoms remain up to 0.1 ms in the afterglow, but with O2 addition their lifetime becomes considerably shorter. The oxygen species important for many applications, such as O(3P) atoms and O2(a) metastable molecules, survive up to 100 ms, thus are the main components of the late afterglow. It is shown that the O2 molecules are strongly dissociated in the discharge, dissociation being more efficient in high Ar content mixtures. However, the dissociation degree decreases to a few per cent in the early afterglow in about 10 ms. In the case of O2(a) molecules, yields above the threshold yield for the iodine laser operation are obtained at 12 mbar for afterglow times of up to 10 ms. In the large-volume reactor it has been found that at low pressure the density of O(3P) atoms decreases by about one order of magnitude towards the walls, while that of O2(a) changes about 20%, although with pressure the density decreases become more pronounced. Very similar density distributions are found at different mixture compositions for O(3P) atoms, while the quasi-homogeneous O2(a) distribution found in high Ar content mixtures progressively turns into a more inhomogeneous one with O2 addition.