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%.

O2(a 1Δg) production in a non-self-sustained discharge (ND) in pure oxygen and oxygen mixtures with inert gases (Ar and He) has been studied. A self-consistent model of ND in pure oxygen is developed, allowing us to simulate all the obtained experimental data. Agreement between the experimental and simulated results for pure oxygen over a wide range of reduced electric fields was reached only after taking into account the ion component of the discharge current. It is shown that the correct estimation of the energetic efficiency of O2(a 1Δg) excitation by discharge using the EEDF calculation is possible only with the correct description of the energy deposit into the plasma on the basis of an adequate discharge model. The testing of an O2(a 1Δg) excitation cross-section by direct electron impact, as well as a kinetic scheme of processes involving singlet oxygen, has been carried out by the comparison of experimental and simulated data. The tested model was then used for simulating O2(a 1Δg) production in ND in oxygen mixtures with inert gases. The study of O2(a 1Δg) production in Ar : O2 mixtures with small oxygen content has shown that the ND in these mixtures is spatially non-uniform, which essentially decreases the energetic efficiency of singlet oxygen generation. While simulating the singlet oxygen density dynamics, the process of three-body deactivation of O2(a 1Δg) by O(3P) atoms was for the first time taken into account. The maximal achievable concentration of singlet oxygen in ND can be limited by this quenching. On the basis of the results obtained and the model developed, the influence of hydrogen additives on singlet oxygen kinetics in argon–oxygen–hydrogen mixtures has been analysed. The simulation has shown that fast quenching of O2(a 1Δg) by atomic hydrogen is possible due to significant gas heating in the discharge that can significantly limit the yield of singlet oxygen in hydrogen-containing mixtures.

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Quenching of O2(1g) molecules both in the gas phase and on a reactor surface has been investigated in binary mixtures of hydrogen and oxygen by using the fast-flow quartz reactor and infrared emission spectroscopy. Rate constants of the O2(1g) deactivation by H2 and O2 at room temperature have been determined to be (1.5±0.5) 10-18 cm3 s-1 and (1.6±0.2) 10-18 cm3 s-1, respectively. Heterogeneous quenching of O2(1g) on quartz walls has been studied both in pure oxygen and in H2:O2 mixtures. A model of O2(1g) heterogeneous quenching in binary mixtures has been developed and has allowed us to describe all observed features of singlet oxygen kinetics. Active surface complexes formed by `chemisorbed' atomic oxygen and `physadsorbed' molecules of O2 and H2 are assumed to be responsible for the O2(1g) heterogeneous deactivation. It has been shown that the higher rate of O2(1g) quenching in pure oxygen is connected with a quasi-resonant transfer of the O2(1g) electronic excitation to physadsorbed oxygen molecules. The observed effect of the wall passivation by hydrogen is conditioned both by the absence of the similar resonance in the hydrogen surface complex and by the higher bond energy of H2 in this complex. Bond energies of O2 (3750±450 K) and H2 (4050±450 K) in the surface complexes have been determined from the model parameters by fitting calculations to experimental results.

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