Singlet oxygen generation in O2 flow excited by RF discharge: II. Inhomogeneous discharge mode: plasma jet

Abstract

The transport dynamics of the metastable oxygen molecules O2(a 1Δg) and as well as O(3P) atoms in an oxygen flow excited by an RF discharge in a jet-mode has been investigated. The production and loss processes of these active species have been analysed by comparing experimental data with simulation results from a self-consistent model of the RF discharge jet-mode. It is shown that both atomic and singlet oxygen (SO) production occur mainly in the plasma jet areas outside the electrode zone. The interelectrode space provides the necessary boundary conditions for the plasma jet existence. The energy efficiency of O2(a 1Δg) production with RF discharge excitation of the oxygen flow was analysed in detail. It is demonstrated that the homogeneous discharge α -mode, where the O2(a 1Δg) excitation efficiency reaches ∼3–5%, is the optimal one for singlet oxygen pumping. The O2(a 1Δg) excitation efficiency drops below 1% at the transition from the α -mode to a jet-mode, though the maximum O2(a 1Δg) concentration is reached just in the jet-mode. At oxygen pressures less than 4 Torr and in the case of an RF discharge jet-mode with extremely fast gas cooling, it is possible to provide an SO yield over the threshold necessary for obtaining generation in an oxygen–iodine laser. However, it only results from the strong oxygen dissociation in the discharge. The O2(a 1Δg) excitation efficiency slightly increases with pressure owing to the decreasing mean electron energy in the discharge volume. With increasing pressure, O2(a 1Δg) quenching with the three-body recombination with atomic oxygen becomes more essential. The removal of atomic oxygen from the gas flow, for example by binding oxygen atoms with any molecular additives, is necessary for scaling the electro-discharged SO generator on pressure. The products of such ‘binding’ processes should not deactivate O2(a 1Δg). It is experimentally shown that the application of RF generators with a higher frequency (40 MHz instead of 13.56 MHz) allows us to increase the O2(a 1Δg) excitation efficiency by 30–40%.