D.V. Lopaev

On the possibility of O2(a 1Δg) production by a non-self-sustained discharge for oxygen–iodine laser pumping

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.

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

The production and transport dynamics of O2(a?1?g) 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?1?g) 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 ?-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 ?-mode is developed, that allows us to analyse elementary processes responsible for the production and loss of O2(a?1?g) 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?1?g), 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?1?g) is provided by the processes of O2(a?1?g) 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?1?g) 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?1?g) molecules and O(3P) atoms. It is shown that the RF discharge ?-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%.

Negative ion destruction by O(3P) atoms and O2(a 1Δg) molecules in an oxygen plasma

Laser photodetachment was used to investigate the dynamics of negative ion density in the positive column of a pure oxygen dc glow discharge over a range of pressure (0.1–5 Torr) and current density (2–40 mA cm −2 ). Upon discharge current modulation, the negative ion O − concentration decayed with the characteristic loss times of oxygen O( 3 P) atom and metastable oxygen O2(a 1 � g) molecule concentrations over a wide range of discharge parameters. To determine the rate constants of negative ion loss by reaction with these species, the dynamics of O( 3 P) atoms and O2(a 1 � g) molecules was investigated using time-resolved actinometry and IR spectroscopy at 1.27 µm, respectively. At pressures greater than ∼0.5 Torr the attachment–detachment dominated regime of the oxygen discharge was realized and the main negative ion was O − . Under these conditions, electron attachment to O2 molecules to produce O − was compensated by detachment of O − with O( 3 P) and O2(a 1 � g). The rate constants of O − detachment with O( 3 P) atoms (O − +O ( 3 P) → O2 +e :k O d = (2.3 ± 0.5) × 10 −10 cm 3 s −1 ) and singlet O2(a 1 � g) molecules (O − +O 2(a 1 � g) → products + e: k �

Experimental and theoretical investigation of oxygen glow discharge structure at low pressures

An experimental and theoretical investigation of the oxygen glow discharge structure at low pressures has been performed. Radial dependencies of the electron energy distribution function, the ambipolar plasma potential, and the negative ion concentration, as well as the axial electric field and the concentrations of atomic and singlet oxygen were measured. A new approach to the application of laser photodetachment method has been used to measure the negative ion concentration. It allows one to obtain information about fast processes after the photodetachment at low frequencies (/spl sim/100-200 Hz) by using the simplest modulation technique. A self-consistent model involving the electrodynamics and kinetics of the discharge was developed. The observed variations of the negative ion densities with current density and oxygen pressure were explained in the model frame by a dependence of the detachment rate constant of the O/sup -/+O/spl rarr/e+O/sub 2/ process on the effective ion temperature (k=1.9/spl middot/10/sup -10/-/spl radic/1100/T/sub i//sup eff/). It was shown that the feature of oxygen dc discharge at low pressures is a possibility to change the basic type of negative ions from the O/sup -/ to the O/sub 2//sup -/. This effect become more pronounced with decreasing current density.

Experimental and theoretical study of RF plasma at low and high frequency

A capacitive coupled radio-frequency plasma at argon pressure of 100 mtorr, 13.56 and 81 MHz frequency and high specific input powers has been studied both experimentally and theoretically. The different numerical models were developed and used for simulation. It was shown that the main ionization source in low frequency (LF) discharge at all studied powers is due to secondary electrons emitted from the electrode by ion impact. The high-frequency (HF) discharge at the same powers is operated in alpha-mode. Applicability of fluid and kinetic models for simulation of LF and HF discharges are studied on the base of our experimental data

Kinetics of in oxygen RF discharges

Studies of the kinetics in the afterglow of three RF discharges having different configurations are presented. Experiments with a slow flow velocity transverse capacitive RF discharge were performed and detailed measurements of the spatial evolution of , O2(a1?g), O(3P) and flow temperature were taken. Simulations of three different experimental configurations were in good agreement with the data for the spatial decay of . The simulations also provided reasonable agreement with experimental data for atomic oxygen, O2(a1?g) and temperature, where the data were available. Updated electron impact cross-sections for oxygen dissociation were included in the simulations; this proved critical for proper modelling of the production of atomic oxygen and the subsequent decay of . At low oxygen atom densities the decay of was principally from heterogeneous quenching. For the slow flow velocity experiment, O atom recombination and deactivation by the walls play an important role; in the far downstream afterglow region a quasi-stationary concentration was found owing to a near-equilibrium between the pooling reaction and losses to the walls for this slow velocity case.

Nonlocal Effects in Stationary and Non-Stationary Discharges

In this report effects of EDF non-locality in stationary and non-stationary discharges are studied using kinetic approaches. Applicability of two-term approach in RF discharges, in positive column of a DC discharge and in plasma display panel (PDP) cell discharge is analyzed. Effective algorithm of self-consistent modeling of stationary and non-stationary discharges is proposed. The necessity of adequate description of negative ion kinetics and transport is demonstrated in the course of the experimental and theoretical investigations of discharges in electronegative gases.

New mechanism of O 2 (a 1 Δ g ) quenching in oxygen-contained plasmas

Singlet oxygen excitation in RF discharge at 13.56 MHz was studied experimentally and theoretically in the pressure range of 2 - 20 Torr. It was shown, that the saturation of O2(a1Δg) concentrations with input power at high oxygen pressures is due to quenching of O2(a1Δg) by atomic oxygen in three-body process. Removing of atomic oxygen using HgO coatings on the tube wall allows to remove saturation and to increase in three times the O2(a1Δg) concentrations at 15 Torr. The record values of singlet oxygen (SO) yield of about 10% are obtained for the first time in gas discharges at high oxygen pressures of 10 - 20 Torr.

Spatial Distribution and Kinetics of Negative Ions in Glow DC Discharge in Pure O2 and H2

In order to understand and insight into physics of electronegative gas plasma we started a comprehensive experimental and theoretical study of the discharge in the gases with substantially different extent of electronegativity: H2, O2 and CF4. For this aim we have used a classical object of discharge physics glow DC discharge in cylindrical geometry. We have carried out the experiments directed to investigate non-locality and anisotropy effects on EEDF, as well as to study negative ion kinetics, metastable particles and atom and radical kinetics. Those investigations gave us the basis for developing the self-consistent approaches to electronegative gas discharge treatment and allowed one to study some elementary processes in details. Here there are presented one of the investigations — a study of spatial distribution and kinetics of negative ions in DC discharge in pure H2 and O2.

CF2 Production by CF4 Electron Impact Dissociation in Gas Discharge

At present neither of available sets of CF4 electron scattering cross sections does enable one to interpret satisfactory the CF4 dissociation in discharge plasma. We have undertook the investigation directed to determine the self-consistent set of CF4 neutral dissociation cross-sections which would allows one to describe correctly the F and CFx(x=1–3) production rates in the discharge. We suggest to carry out a series of experiments on a study of CF and CF2 radical as well as F atom kinetics in positive column of CF4 DC discharge. It is the first work from this series the aim of which was to determine a rate constant of CF4 electron impact dissociation into CF2 radical production channel (e + CF4 ⇒ CF2 + products). The rate constant was determined monitoring the CF2 absolute concentration dynamics in modulated DC discharge by differential UV adsorption technique and laser induced fluorescence method. Following the experimental results and analysis of available data on electron scattering cross section sets on CF4 molecules we have attempted to specify the cross section for this CF4 dissociative channel.