An overview is presented of experimental and theoretical research in the field of physics and engineering of singlet delta oxygen (SDO) production in low-temperature plasma of various electric discharges. Attention is paid mainly to the SDO production with SDO yield adequate for the development of an electric discharge oxygen–iodine laser (DOIL). The review comprises a historical sketch describing the main experimental results on SDO physics in low-temperature plasma obtained since the first detection of SDO in electric discharge in the 1950s and the first attempt to launch a DOIL in the 1970s up to the mid-1980s when several research groups started their activity aimed at DOIL development, stimulated by success in the development of a chemical oxygen–iodine laser (COIL). A detailed analysis of theoretical and experimental research on SDO production in electric discharge from the mid-1980s to the present, when the first DOIL has been launched, is given. Different kinetic models of oxygen low-temperature plasma are compared with the model developed by the authors. The latter comprises electron kinetics based on the accompanying solution of the electron Boltzmann equation, plasma chemistry including reactions of excited molecules and numerous ion–molecular reactions, thermal energy balance and electric circuit equation. The experimental part of the overview is focused on the experimental methods of SDO detection including experiments on the measurements of the Einstein coefficient for SDO transition and experimental procedures of SDO production in self-sustained and non-self-sustained discharges and analysis of different plasma-chemical processes occurring in oxygen low-temperature plasma which brings limitation to the maximum SDO yield and to the lifetime of the SDO in an electric discharge and its afterglow. Quite recently obtained results on gain and output characteristics of DOIL and some projects aimed at the development of high-power DOIL are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/40/2/R01/pdf

Physics and engineering of singlet delta oxygen production in low-temperature plasma

Rate coefficients for excitation of the b 1Σ+g state of O2 by low energy electrons have been measured using a drift tube technique. The time dependence of the absolute intensity of the 762 nm band emission was measured for O2 densities between 1016 and 2×1018 molecules/cm3. When corrected for electron attachment, ionization, and metastable diffusion, the number of b 1Σ+g molecules produced per centimeter of electron drift and per O2 molecule calculated from the 762 nm emission varied from 1.3×10−18 cm2 at E/N=5×10−17V cm2 to 2.1×10−16 cm2 at E/N=2×10−15V cm2. These values of electric field to oxygen density ratio E/N correspond to mean electron energies of 0.75 and 6 eV, respectively. Measured decay constants for the 762 nm radiation yield a value for the product of the diffusion coefficient and the O2 density of (5.0±0.3) ×108 cm−1 sec−1 and a quenching coefficient for the b 1Σ+g state of (3.9±0.2) ×10−17 cm3 sec−1. Comparison of measured excitation coefficients with values calculated using a recommended...

Excitation of the b 1Σ+g state of O2 by low energy electrons

The products of reaction and etch rates of Si and SiO2 in SF6‐O2 plasmas have been studied as a function of feed composition in an alumina tube reactor at 27 mHz, 45 W, and 1 Torr. There is a broad chemical analogy with CF4‐02 plasmas. As in CF4‐02 mixtures, the rate of Si etching and 703.7‐nm emission from electronically excited F atoms each exhibit distinct maxima as a function of feed gas composition; these data support a model in which fluorine atoms, the etching species, compete with oxygen atoms for chemisorption on the Si surface. Without oxygen in the feed or Si in the reactor, no stable products could be detected. With an SF6‐O2 mixture in the absence of silicon, the final reaction products are F2, SOF4, and SO2F2. The product distribution was unaffected by small SiO2 substrates. When Si is etched, SiF4 is the only stable silicon‐containing etch product and SOF2 is formed in oxygen‐poor mixtures. Rapid etch rates (≳104 A/min for Si) can be obtained with a high selectivity in favor of silicon (Si:...

Plasma etching of Si and SiO2 in SF6–O2 mixtures

In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium?oxygen mixture plasmas (He?O2, helium with 5000?ppm admixture of oxygen) for wide air impurity levels of 0?500?ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.

https://iopscience.iop.org/article/10.1088/0963-0252/22/1/015003/pdf

Chemical kinetics and reactive species in atmospheric pressure helium?oxygen plasmas with humid-air impurities

The various analytic approaches to the solution of the electron Boltzmann equation in non-uniform glow discharge plasmas in atomic gases are presented. For slow electrons with kinetic energy w > epsilon 1, the approximation of continuous energy losses can be used. The simplest approximation corresponds to the neglect of scattering. In this case, simple analytic expressions for the distribution of the fast electrons, for the multiplication factor and so on can be derived, and the structure of normal and abnormal cathode regions for plane and hollow cathodes can be analysed.

https://iopscience.iop.org/article/10.1088/0963-0252/4/2/004/pdf

Electron kinetics in non-uniform glow discharge plasmas

The linearized one-dimensional Boltzmann equation was solved for the electron gas in the positive column of a low-current DC-discharge. A sinusoidal perturbation of the electric field was assumed in the column and the perturbed distribution function was found. The problem was solved both numerically and approximately. It is shown that in the absence of an electron-electron interaction strong deviations from the solution of the hydrodynamic quasihomogeneous equations appear for certain wavelengths, which were called “resonant”. These wavelengths almost exactly agree with the characteristic wavelengths of the ionization waves of the s- and p-variety. A physical interpretation of this resonant behaviour is given and its connection with the long correlation length in a plasma with non-interacting electons is pointed out.

On non-hydrodynamic properties of the electron gas in the plasma of a DC discharge

The solution of the Boltzmann equation found in the foregoing paper was utilized to calculate the dispersion of the fast as well as slow ionization waves (moving striations) in the positive column of a low-current neon discharge. Two balance equations of heavy particles were used: atomic ions guiding the fast waves and metastable atoms guiding the slow waves. The theory yields basically two kinds of waves: hydrodynamic r-variety and those waves that occur due to the spatial resonances of the electron gas: s-, s′- and p-variety, which are all experimentally known. Moreover, the theory predicts a new “p′”-variety as a fast wave with the same characteristic potential as the p-wave. Fixation of the characteristic potentials of the low-current ionization waves as well as high ratio of the group to phase velocity naturally follows from the theory as is shown by means of an approximative method. Numerical solutions are also presented giving full agreement for the optimum wavelength forE/p
0≲3 V/cm. torr, but for higherE/p
0 having by a few per cent lower values. The phase-shifts between various perturbed plasma parameters are also computed showing certain differences if compared with those expected from a hydrodynamic description but having in all cases the production term for the particles leading the wave in the right place demanded by the ionization mechanism of amplification and propagation of the wave.

A theory of the low current ionization waves (striations) in inert gases

The calculations of the distribution function of electrons and the collisional rates were carried out on the basis of a numerical solution of the Boltzmann equation for a weakly ionized discharge in O2 rangingE/N from 5 Td to 200 Td. The distribution function, Townsend ionization and dissociative attachment coefficients calculated were found to be in good agreement with experiments. A comparison of the results on the collisional rates with the values available in the literature was performed as well.

Electron collision rates in oxygen glow discharge

In this paper numerical calculations of the electron energy distribution and some swarm parameters are presented on the basis of a numerical solution of Boltzmann equation with the cross-sections assumed to be given. The method requires that the set of the cross-section data be well known. The accuracy of all the results at moderate values ofE/N is then dependent only on the cross-sections whilst the calculations for high values ofE/N can be, furthermore, distorted by an inadequate approximation (1) which is otherwise commonly used for these purposes. In fact, it can be said that all our swarm parameter curves lie within the limit of the spread of the experimental points. It implicates that there is no possibility to reduce possible imperfections of the present calculations till better experimental values of both the cross-sections and swarm parameters become available. The reasons remain probably the same even if other methods for analyzing the behaviour of the electron swarms were applied.

Electron gas in molecular oxygen discharge

On the basis of good knowledge of the energy distribution function of electrons corresponding to the conditions of low-current discharge the electrons as well as the excited helium atoms are balanced. A model of helium atom possessing four excitation levels (23S, 21S, 23P, 21P) is assumed. The calculated volt-ampere characteristic is compared with an experiment and the relative densities of excited atoms are given. For these quantities approximate formulae are derived. The laws of similarity, ionization mechanism and energy balance of the electron gas are discussed.

T. Růžička

Papers

On non-hydrodynamic properties of the electron gas in the plasma of a DC discharge

A theory of the low current ionization waves (striations) in inert gases

Electron collision rates in oxygen glow discharge

Electron gas in molecular oxygen discharge

Low-current helium discharge at low pressures