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A numerical two-dimensional spatial model is used to describe experimental results from the literature on the concentrations of O2(a1Δg) and O2(b1 ) in a fast-flowing gas system free of plasma–chemical processes with the participation of electrons and ions. The concentration profiles of O2(a1Δg) and O2(b1 ) are found to depend on gas pressure, the fraction of oxygen atoms in O/N2 mixtures, and additions of O2 to the gas mixture. The model emphasizes the need to consider detailed vibrational kinetics of ozone and its formation on the surfaces of tube walls. A new interpretation is proposed for the three-body recombination of oxygen atoms on M = N2, O2, allowing for the reverse dissociation of the produced highly excited molecules. The resulting functional dependence of recombination rate coefficient krec(T) is obtained and agrees well with the available measured temperature dependences krec(T). Pathways for the subsequent relaxation of excited oxygen molecules and atoms are identified. [ABSTRACT FROM AUTHOR]

Oxygen molecules in the lowest metastable state, O2(a1Δg), play an important role in oxygen plasmas due to their high reactivity and significant concentrations. The accumulation of high densities of O2(a1Δg) occurs due to its low quenching rate. This paper demonstrates the existence, at high gas temperatures (700–1700 K), of fast quenching of O2(a1Δg) by O(3P) atoms, a process that has not been considered in previous models. Experiments were carried out at oxygen pressures of 10–100 Torr in an 81 MHz CCP discharge in a quartz tube with external electrodes. This setup provides high absorbed power density, leading to both high gas temperatures and significant O(3P) densities. We observe that the O2(a1Δg) density is significantly limited at high gas temperatures by rapid quenching by atomic oxygen. The results were interpreted using a self-consistent 1D discharge model. The observations can only be explained by the inclusion of a rapid quenching reaction: O2(a1Δg) + O(3P) → O2(X3Σg−) + O(3P), with an activation energy in the range of 0.54–0.69 eV. The rate constant was determined over a wide range of discharge conditions (P = 20–100 Torr and Tg = 800–1700 K), giving values between 3 × 10−11 exp(−8000/T) cm3 s−1 to 1.5 × 10−11 exp(−6300/T) cm3 s−1. A possible mechanism for this process is discussed. Measurements of the density of metastable O2(b1Σg+) molecules also indicated the existence of quenching by atomic oxygen, with a somewhat lower activation energy of ∼0.32 eV. The variations of the measured [O2(b1Σg+)]/N mole-fraction could be fitted by the model using a rate constant 2 × 10−11 × exp(−3700/T) cm3 s−1 for this process. These quenching processes of metastable O2(a1Δg) and O2(b1Σg+) molecules by oxygen atoms are important for oxygen plasmas and could have a significant impact on the kinetics of oxygen-containing mixtures at higher gas temperatures, for example in plasma-assisted combustion or in high-pressure plasma processing reactors. [ABSTRACT FROM AUTHOR]

Available data on the kinetic processes in H-O-O( aΔ) mixtures are analyzed theoretically, and the ranges in which the rate constants of these processes can vary are determined. The processes of energy transformation in an O( aΔ)-H-H-HO system in the approximations of the fast and slow (in comparison with the vibrational relaxation time of the HO radical) quenching of the electronically excited state are considered. The experiments on the quenching of singlet delta oxygen (SDO) molecules O( aΔ) excited in a microwave discharge at a temperature of 300 K and pressure of 6 Torr in a lean hydrogen-oxygen mixture are simulated (by a lean fuel mixture is meant a mixture in which the ratio of the fuel to the oxidizer mass fraction is less than the stoichiometric ratio, which is 2: 1 for a hydrogen-oxygen mixture). It is shown that, over the experimental observation times, the SDO quenching frequency depends on the concentration of molecular hydrogen, the residual odd oxygen fraction in the gas flow, and the ratio between the rate constants of kinetic processes involving HO and HO* radicals. Simulations of the microwave discharge and the transport of excited oxygen along the drift tube make it possible to determine the residual odd oxygen concentration in the gas flow. Recommendations on the choice of the rate constants for the O( aΔ) + HO) A″, v″ = 0) ↔ O + HO*( A′, v′ = 1), HO*( A′v′ ≤ 1) + O( aΔ) → HO*( A′,v′ ≥ 6) + O, and HO*( A′,v′ ≤ 1) + O( aΔ) → H + O + O processes are given. It is shown that, in the case of slow quenching in a H-O-O( aΔ) mixture at a low temperature, the intensity of hydrogen oxidation is enhanced due to the reaction + HO*( A′,v′ ≤ 1) + O(Δ) → H + O + O. [ABSTRACT FROM AUTHOR]

Results of experiments on the quenching of singlet delta oxygen (SDO) O( aΔ) in lean hydrogen-oxygen mixtures at temperatures of 500-1020 K and pressures of 26-90 Torr behind the shock front are analyzed theoretically. The processes affecting SDO quenching are simulated taking into account the temporal characteristics of the experiment and various mechanisms of energy transformation in an O( aΔ)-H-H-HO system. It is demonstrated that the approximations of both fast and slow (in comparison with the vibrational relaxation time of the HO radical) quenching of the electronically excited state of the radical are in good agreement with the experimental data on the effective rate constant of SDO deexcitation at temperatures of up to 700 K. It is shown that the available data on the kinetics of reactions involving SDO in H-O-O( aΔ) mixtures overestimate the SDO quenching rate in comparison with the experimental results obtained at temperatures above 850 K. The decrease in the rate constant of the reaction H + O( aΔ)→ products by one order of magnitude makes it possible to match the simulation results to the experimental data. The existence of the processes restoring SDO in the presence of atomic hydrogen that are not considered in the current models of the H-O-O( aΔ) kinetics is supposed. [ABSTRACT FROM AUTHOR]

Influence of electron excited singlet delta oxygen (SDO) molecules produced in direct current (DC) glow discharged plasma on the induction length decrease in the H2–O2 mixture has been studied via comprehensive two-dimensional (2D) numerical simulations. The sensitivity analysis of the induction length on SDO mole fractions and residual mole fractions of odd oxygen was carried out. The influence of the reaction H2 + O2(1Δg) → H + HO2 on the ignition length was found to be negligible due to the presence of residual odd oxygen in the oxidizer flow. Two stages specify the ignition time decrease for the studied conditions: (1) chain initiation in reactions with residual odd oxygen and (2) the following chain-branching enhancement due to reaction H + O2(1Δg) → O + OH. The last reaction is the key process for studied conditions when the concentration of SDO exceeds 4%. The estimated rate constant of this reaction was found to be about 2.5 · 10−13 cm3/s at 780 K. The quenching reaction H + O2(1Δg) → H + O2 does not affect the ignition length. [ABSTRACT FROM AUTHOR]

N2 dissociation in pure nitrogen plasma has a long history of research. It seems to be a complex process which comprises many reactions involving various electronic and vibrational nitrogen states whose contributions can vary depending on conditions. In this paper, we studied N2 dissociation in the stationary N2 discharge both experimentally and theoretically. We used a DC glow discharge in a quartz tube in pure N2 at moderate pressures (5–50 Torr). The degree of dissociation, atomic nitrogen loss rate and gas temperature were measured by applying optical emission spectroscopy (OES) and as a result an ‘effective’ rate constant for nitrogen dissociation was obtained across a wide range of the reduced field E/N. The analysis of N2 dissociation was carried out using a specially developed 1D radial self-consistent model which takes into account the spatial inhomogeneities of species concentrations, E/N, electron energy distribution function, Tgas etc, together with fairly complete plasma-chemical kinetics and all the cross-sections known to date for electron kinetics. The model was successfully validated through the experimental results obtained for electric field, gas temperature and N atom density. Comprehensive analysis of closely coupled processes in nitrogen plasmas—gas heating, VDF formation and N2 dissociation—was carried out. Simulations reproduced the experimental data on well and allowed us to evaluate the different contributions of the various dissociation channels considered. It was shown that the nitrogen dissociation mechanism in the stationary N2 discharge is provided by direct electron impact via the excitation of the pre-dissociative states from the vibrationally excited nitrogen molecules N2(X, υ). The upper limit for the rate constant of the processes N2(A)  +  N2(14  ⩽  υ  ⩽  19)  →  N  +  N  +  N2 was estimated to be 5 · 10−14 cm3 s−1. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive analysis of the reaction H  +  O2(a1Δg)  →  products and its channels, including available experimental data, theoretical estimations and kinetic studies, was carried out. A possible intrinsic mechanism of the reaction H  +  O2(a1Δg)  →  products, taking into account Renner–Teller coupling between the lowest doublet 2A′ and 2A″ states of a HO2 molecule was suggested. The proposed mechanism allows qualitative justification of the available data on the high probability for the quenching reaction: H  +  O2(a1Δg)  →  H  +  O2(3Σg). The effect of the reactions, including electronically excited molecules on the evaluation of temperature dependencies for the total and branching rate constants of the reaction H  +  O2(a1Δg)  →  products at low temperatures, was investigated. The value of the reaction H  +  O2(a1Δg)(+M)  →  HO2(2A′,2A″)(+M) rate constant was evaluated from shock tube experiments on O2(a1Δg) quenching in a lean H2–O2–O2(a1Δg) mixture. Based on the obtained results, temperature and pressure dependencies for the rate constants of the reaction H  +  O2(a1Δg)  →  product channels were recommended for the simulations, together with the proposed kinetic sub-mechanism for O2(a1Δg) chemistry in H2–O2 mixtures. [ABSTRACT FROM AUTHOR]