Singlet oxygen generation in O2 flow excited by RF discharge: I. Homogeneous discharge mode: α-mode

Abstract

The production and transport dynamics of O2(a 1Δg) and molecules as well as O(3P) atoms has been studied in an O2 flow excited by a 13.56 MHz RF discharge in a quartz tube at pressures of 1–20 Torr. It has been shown that the densities of O2(a 1Δg) and O(3P) are saturated with increasing energy input into the discharge. The maximum yield of singlet oxygen (SO) and the O2 dissociation degree drops with pressure. It is demonstrated that depending on the energy input the RF discharge can exist in three modes: I—in the spatially homogeneous mode or α-mode; III—in the substantially inhomogeneous mode, when plasma jets are present outside the discharge; and II—in the transient mode between modes I and III. In this paper only the homogeneous mode of RF discharge in the O2 flow is considered in detail. A self-consistent model of the α-mode is developed, that allows us to analyse elementary processes responsible for the production and loss of O2(a 1Δg) and molecules as well as O(3P) atoms in detail. To verify both the kinetic scheme of the model and the conclusions, some experiments have been carried out at lower flow velocities and higher pressures (⩾10 Torr), when the stationary densities of O2(a 1Δg), and O(3P) in the discharge area were established not by the escape of particles but by the losses due to the volumetric and surface reactions. The density under these conditions is determined by the balance of production by both direct electron impact and electronic excitation transfer from metastable O(1D) atoms and deactivation by oxygen atoms and tube walls, including quenching by ozone in the afterglow. The O(3P) density is determined by the balance between the production through O2 dissociation by electron impact and heterogeneous loss at the wall recombination. The stationary density of O2(a 1Δg) is provided by the processes of O2(a 1Δg) production by direct electron impact and loss owing to quenching by the tube walls at a low pressure below 4 Torr, as well as by three-body recombination with oxygen atoms with increasing pressure above 7 Torr. The analysis of O2(a 1Δg) three-body quenching by oxygen atoms showed that this process could actually have a high rate constant and be able to provide a fast SO deactivation at high pressures. The approximate value of the rate constant—(1–3) × 10−32 cm3 s−1 has been obtained from the best agreement between the simulated and experimental data on transport dynamics of O2(a 1Δg) molecules and O(3P) atoms. It is shown that the RF discharge α-mode corresponds to a discharge with an effective reduced electrical field in a quasi-neutral plasma of about ∼ 30 Td, which makes possible a rather high efficiency of SO production of ∼3–5%.