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%.

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Capacitively coupled rf discharge in pure CF 4 was studied using a one-dimensional self-consistent particle-in-cell Monte Carlo model. Two different discharge modes are observed depending on the discharge conditions: the regime of electronegative plasma with high-electron temperatures in the bulk, and the regime of electropositive plasma with abnormally low electron temperatures in the bulk. The characteristic features of the two discharge modes are considered. A sharp transition from the former to the latter mode is observed with an increase in applied voltage. The dependence of the transition voltage on gas pressure is analyzed. In the studied range of gas pressures, the existence of a high-temperature mode in an electronegative gas like CF 4 is suggested by the balance between the ionization rate and attachment rate in the bulk region. As a result, the transition voltage increases with gas pressure because of the increased relative role of electron attachment. It is shown that the differences in the used electron cross-section sets may noticeably affect the simulation results and the discharge properties. Three different electron cross-section sets for CF 4 are considered. In particular, the transition voltage between the two discharge modes differs essentially for different cross-sections used. In order to analyze the fundamental causes of this difference, a detailed comparison of three cross-section sets was done on the basis of the Monte Carlo calculation of swarm parameters in constant electric fields.

A low-frequency capacitively coupled radio-frequency (rf) discharge in Ar excited at 1.76 MHz is studied both experimentally and theoretically. Experimental measurements of electron concentration, discharge voltage and current are presented for a wide range of rf input powers. The rf current shape is nonsinusoidal, close to the triangle one. The evolution of Ar(2p1) emission excitation function in the interelectrode gap during an rf cycle is measured using the phase-resolved optical emission spectroscopy technique. Theoretical study is based on the particle-in-cell Monte Carlo collision numerical simulation. Specific dynamic features of the low-frequency discharge are discussed. The important role of secondary electrons in discharge maintenance and power balance is shown. This study is crucial for understanding dual-frequency discharges with a corresponding value of low frequency.

This work is devoted to the study of the possibility of obtaining the highest O2(a 1Δg) yield in ED SOG at the high absolute O2(a 1Δg) concentration needed for developing a powerful oxygen–iodine laser pumped by electric discharge. A singlet oxygen was produced in a transversal rf discharge in the pressure range 10–30 Torr of pure oxygen in the small-diameter (7 mm) quartz tube with HgO coating of the inner walls for removing atomic oxygen to eliminate fast O2(a 1Δg) quenching. It is shown that pd scaling (p—pressure, d—tube diameter) of the rf discharge actually allows an increase of the absolute O2(a 1Δg) density. The increase in the rf frequency from 13.56 to 81 MHz results in the essential increase of the O2(a 1Δg) yield (beyond 15% at such a high oxygen pressure as 15 Torr), but the subsequent transfer to the higher rf frequency of 160 MHz only slightly influences the maximally obtained O2(a 1Δg) yield. The effect of the NO admixture on the O2(a 1Δg) production has been also studied. The rate constant of O2(a 1Δg) quenching by was directly measured. The NO admixture (up to 20%) resulted in the noticeable increase in the O2(a 1Δg) yield mainly at low energy inputs. But this gain in the O2(a 1Δg) concentration drops with increasing energy input. Nevertheless it is shown that by combining the O2 + NO mixture with the HgO coating of the discharge tube walls one can provide the O2(a 1Δg) yield on the level of ∼21% at 10 Torr, ∼17% at 20 Torr and ∼13% at 30 Torr of O2 with the efficiency of ∼4–6%. The analysis of the NO admixture influence on the discharge structure and O2(a 1Δg) production has been carried out by using the 2D model. It was found that at the low energy input the NO admixture acts as an easily ionized species that enlarges the region occupied by plasma. Thus, in the O2 + NO discharge the normal current density is lower than in the pure oxygen discharge. As a result a higher energetic efficiency of O2(a 1Δg) production is also observed in the case of the O2 + NO mixture and the low energy input. In order to provide the optimal conditions for O2(a 1Δg) production (with regard to the yield and efficiency) in the continuous wave transversal VHF discharge at such high oxygen pressures as of 10–30 Torr it is necessary to find out the range of energy inputs where the VHF discharge operates in the regime of normal current density on the boundary with the abnormal regime and to remove atomic oxygen produced in the discharge by some volume or surface processes.