Excited O2 Molecules Research Articles

Initiation of combustion of a CH4-O2 mixture in a supersonic flow with excitation of O2 molecules by an electric discharge

The possibility of intensification of ignition of a methane-oxygen mixture in a supersonic flow behind the front of an oblique shock wave by means of excitation of O2 molecules to the states a1Δg and b1Σg+ in an electric discharge is discussed. Through numerical simulations, activation of O2 molecules by an electric discharge is demonstrated to speed up chain reactions in the CH4-O2 mixture and to reduce the induction-zone length. Even a small amount of energy input to O2 molecules in the discharge (≈3·0−2 J/cm3) can reduce the ignition-delay length by a factor of hundreds and initiate combustion at distances of ≈1 m from the discharge zone at comparatively low temperatures of the gas behind the front (≈1000 K) and moderate pressures (≈105 Pa). Excitation of O2 molecules by an electric discharge is much more efficient than simple heating of the mixture.

On combustion enhancement mechanisms in the case of electrical-discharge-excited oxygen molecules

Combustion intensification mechanisms in a supersonic flow of a hydrogen-oxygen mixture behind the oblique shock wave front are investigated for the case when vibrations and the a1Δg and b1Σg+ electron states of a O2 molecule are excited by an electrical discharge. The presence of vibrationally excited and electronically excited O2 molecules in the oxygen plasma allows intensification of the chain mechanism in the H2-O2 mixture even if the energy put into O2 molecules in the discharge is low. Excitation of O2 molecules is several tens of times more efficient for acceleration of oxygen-hydrogen mixture combustion than mere heating of the gas by an electrical discharge. In addition, low-temperature inflammation of the mixture with electrical-discharge-excited O2 molecules makes it possible to raise the efficiency of conversion of the reactant chemical energy to heat compared with the conventional way of combustion initiation by heating.

Reaction of oxygen plasma active species with polyethylene

The composition of the polyethylene surface upon treatment in an oxygen plasma and its afterglow was studied by attenuated total reflectance IR spectroscopy and X-ray photoelectron spectroscopy. The oxidation of the surface at the lowest destruction rates was attained upon simultaneous action of excited O2(a1Δg) and ground-state oxygen molecules. However, O(3 P) atoms are involved in both the formation of oxygen-containing groups and their destruction accompanied by polymer degradation.

Pressure broadening and fine-structure-dependent predissociation in oxygen BΣu−3, v=

Both laser-induced fluorescence and cavity ring-down spectral observations were made in the Schumann-Runge band system of oxygen, using a novel-type ultranarrow deep-UV pulsed laser source. From measurements on the very weak (0,0) band pressure broadening, pressure shift, and predissociation line-broadening parameters were determined for the B 3sigma(u)-, v = 0,F(i) fine-structure components for various rotational levels in O2. The information content from these studies was combined with that of entirely independent measurements probing the much stronger (0,10), (0,19), and (0,20) Schumann-Runge bands involving preparation of vibrationally excited O2 molecules via photolysis of ozone. The investigations result in a consistent set of predissociation widths for the B 3sigma(u)-, v = 0 state of oxygen.

Kinetic mechanisms of initiating hydrogen-oxygen mixture combustion through the excitation of electronic degrees of freedom of molecular oxygen by laser radiation

Kinetic mechanisms resulting in the enhancement of combustion of H2+O2 mixtures when O2 molecules are excited to the a1Δg and b1Σg+ states with laser radiation (λ=1.268 and 0.762 µm) are analyzed. It is shown that the excitation of O2 molecules by the laser radiation leads to the appearance of new O, H, and OH formation channels; promotes the ignition of the starting mixture; and reduces the self-ignition temperature. With initial pressures in the range P0=103–104 Pa, the self-ignition temperature can be reduced to 300 K even at relatively low energies of the laser radiation with λ=0.762 µm.

Thermal Decomposition of Xenon Tetroxide at 490–780 K

The decomposition of XeO4 was studied over the temperature range 490–780 K. A reaction mechanism was proposed. A conclusion on the high vibrational excitation of O2 molecules formed in the elementary reaction O + XeO4 → O2 + XeO3 and the branched-chain character of the decomposition with branching in the reaction O + XeO3 → 2O + O2 + Xe was drawn.

Sterilization of medical productsin low-pressure glow discharges

Results are presented from experimental and theoretical studies of the sterilization of medical products by the plasmas of dc glow discharges in different gas media. The sterilization efficiency is obtained as a function of discharge parameters. The plasma composition in discharges in N2 and O2 is investigated under the operating conditions of a plasma sterilizer. It is shown that free surfaces of medical products are sterilized primarily by UV radiation from the discharge plasma, while an important role in sterilization of products with complicated shapes is played by such chemically active particles as oxygen atoms and electronically excited O2 molecules.

Kinetics of processes in the middle atmosphere during the laser excitation of O2 molecules

Kinetic processes taking place in the atomic-molecular system O-O2-O3 in the middle atmosphere with the participation of oxygen molecules in the excited electronic states O2(a1Δg) and O2(b1Σg+) are analyzed in detail. The possibility of increased ozone production under the influence of solar radiation during the laser excitation of O2 molecules in the a1Δg state is demonstrated on the basis of numerical modeling. Upper and lower bounds are determined for the densities of O2(a1Δg) molecules at which the ozone concentration increases in the irradiated zone.

Spectroscopy and Photophysics of O2 in the Neighborhood of the Dissociation Limit in Rare Gas Solids

Abstract The energy levels, potentials and relaxation mechanism of O2 doped in rare gas (Ar, Kr) and N2 solids in the neighborhood of the dissociation limit have been studied by means of absorption and emission spectroscopy. The environmental effects of the solids on the energy level shift are not large, but the cage effect is important in considering the relaxation mechanism following the laser irradiation. The efficient cross-relaxation among nested Herzberg states and specific energy transfer dependent on the energy gap between relevant levels, in particular from A′ state to c state, have been found. Due to the very slow vibrational relaxation, vibrationally excited O2 molecules have been accumulated to a high concentration in the solid. Based on the B–X transition energy values from the vibrationally excited O2 molecules, the B state potential curve has been determined. This shows that the cage effect retards the atomic dissociation in the B state.

Bimolecular surface photochemistry: Mechanisms of CO oxidation on Pt(111) at 85 K

Results from a photoinduced bimolecular surface reaction are presented. The reaction, occurring from CO coadsorbed with O2 on Pt(111) at 85 K, is O2+CO+hν→O+CO2. Surface analysis techniques employed include electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS), photon-induced desorption spectroscopy (PID), and low energy electron diffraction (LEED). The incident power, photon energy, and polarization dependences of the photochemical processes, O2 photodesorption and CO2 photoproduction, were characterized, with the cross section for both processes being 3×10−19 cm2 at 240 nm. Electronic EELS studies were performed to acquire information on the electronic structure of O2 on Pt(111). The experimental results are compared to predictions of models describing direct dipole excitation of the O2–Pt system and substrate mediated hot carrier mechanisms. Reaction mechanisms involving photogenerated hot O atoms or excited O2 molecules on the surface are considered. The implications of this work on surface reaction dynamics are discussed.

Formation of O(3P j) photofragments from the Hartley band photodissociation of ozone at 226 nm

The photodissociation of ozone at 226 nm is studied for O3→O2(X3 Σ−g)+O(2p3Pj) by probing O(3Pj) atomic photofragments with a resonance-enhanced multiphoton ionization method. Angular and kinetic energy distributions are determined by measuring time-of-flight spectra as a function of the angle between the polarization vector of the dissociation laser and the detector axis. The Doppler width of O(3Pj) photofragments is also measured. The translational energy distribution is well represented by assuming the formation of vibrationally excited O2 molecules with an average vibrational quantum number of 12. The anisotropy parameter β for the angular distribution is found to be 0.7±0.1. The predissociation dynamics from the photoprepared O3(1B2) state to the repulsive potential surface is discussed.

Formation mechanism of vibrationally excited O2 molecules in the multiphoton absorption of NO2

A pump–probe two-color experiment has been performed to elucidate the formation mechanism of O2 X 3Σ−g in the multiphoton absorption of NO2 over the region 460–540 nm. A probe dye laser was employed to excite O2 X 3Σ−g into the B 3Σ−u state and the UV emission intensity of Schumann–Runge bands was measured under the various experimental conditions. The maximum vibrational level of O2 X 3Σ−g formed is v″max =24 which corresponds to Evib=30 968 cm−1. The rotational distribution of O2 X 3Σ−g (v″=24) was almost of Boltzmann with Trot=1300 K at low pressures. The isotopic 1:1 mixture of N16O2 and N18O2 has been photolyzed to test whether the O2 molecules are formed by unimolecular dissociation or through chemical reactions. From the product branching ratio of 16O2:16O18O:18O2 and the maximum vibrational levels observed, the vibrationally excited O2 molecules are concluded to be mainly generated by the chemical reaction of O(1D)+NO2→O2+NO, ΔH=32 000 cm−1. The O(1D) atoms are formed by a sequential three-photon absorption of NO2, where the initial two-photon absorption occurs through the 1 2B2←X̃ 2A1 transition in a cyclic manner and a certain collision-induced process takes place in a dense system of the predissociative states locating in 11 900–19 400 cm−1 above the dissociation threshold of NO(2Π)+O(3P).

High‐resolution auroral observations of the OI(7774) and OI(8446) multiplets

High resolution observations of the OI(7774) and OI(8446) multiplet auroral emissions were made using a Fabry‐Perot spectrometer at Churchill, Canada during March, 1984. The intensity of each of these lines, relative to N2+ (4278) may depend on atmospheric composition. The OI(8446) emission linewidths are under 40 mÅ while the OI(7774) emission linewidths have two components; one narrow (∼ 40 mÅ) and one broad (∼ 275 mÅ). The broad to narrow intensity ratio increases with decreasing red to blue ratio. These results are consistent with electron impact excitation of O being the dominant source of both emissions at high altitudes. However, at lower thermospheric altitudes, dissociative excitation of O2 molecules may be an important source of OI(7774) emission.

Quenching of the 1Σ+g state in liquid oxygen isotopes

A nonexponential decay of the 1Σ+g state in liquid oxygen is observed for high number densities (≫1017 cm−3) of excited O2 molecules. At low excitation intensities the decay of O2(1Σ+g) is exponential with rate constants 1.2×105 and 6.7×103 s−1 for liquid natural O2 and 18O2, respectively (at T=77 K). There is direct experimental evidence that O2(1Σ+g) is quenched to the 1Δg state. Calculations of the ratio of the transition probabilities for collisional deactivation of 16O2(1Σ+g) and 18O2(1Σ+g) are in satisfactory agreement with the experimental results.

Generation of O2(c 1Σu−, C 3Δu, A 3Σu+) from oxygen atom recombination

The spectrum produced in the afterglow of an O2–He discharge has been studied between 4000 and 8000 Å. Until recently, only the O2(A 3Σu+→X 3Σg−) emission bands had been observed in this region. The work of Lawrence et al. established that, under the appropriate conditions, the O2(c 1Σu− →X 3Σg−) system could also be observed. We were able to duplicate their results, and in addition we have discovered three O2 band systems not previously seen in gas phase laboratory spectra. These systems are C 3Δu→a 1Δg, C 3Δu →X 3Σg−, and c 1Σu−→a 1Δg and were positively identified by isotopic substitution experiments. The 0–v″ progression in the C–a system is the first gas phase spectrum involving a large range of the higher vibrational levels in the a 1Δg state. Analysis of the system establishes a new set of a 1Δg vibrational constants : ωe=1510.23 ±0.34 cm−1, and ωexe=13.368±0.12 cm−1. The C–a system involves only the Ω=1 spin component of the C 3Δu state. The C 3Δu→X 3Σg− system, on the other hand, radiates mainly through the Ω=2 component. By combining the present data on the C 3Δu(v=0) level, the absorption measurements of Herzberg on the v=5 and v=6 levels, the known dissociation energy, and the high-pressure bands measured by Herman and by Finkelnburg and Steiner, we have established vibrational constants for the C 3Δu state. These are ωe=803.5±1.0 cm−1, ωexe=8.18±0.13 cm−1, and ωeye =−0.872±0.006 cm−1. From data existing in the literature, combined where necessary with the present measurements, we have estimated radiative lifetimes for the three O2 metastable states that we observed. From the band strength data of Hasson et al., one may calculate τ=250–160 msec for A 3Σu+(v=0–6), and deduce an estimate of τ=25–50 sec for c 1Σu−(v= 0–10). For C 3Δu(Ω=1, v=0–6), the estimated lifetime is 5–50 sec, and for C 3Δu(Ω=2, v=6), the estimated lifetime is 10–100 sec. The O2 spectra have been produced not only in O2-He afterglows, but also in NO-titrated N2–He afterglows (i.e., in an O2-free environment), proving that the source of excited O2 molecules is oxygen atom recombination. The results are pertinent to O2 emissions in the terrestrial and Venusian air glows, and to combustion processes.

Energy transfer effects of excited molecule production by surface-catalyzed atom recombination

Surface-catalyzed atom recombination often yields internally excited molecules. If these molecules are not quenched, by or in the vicinity of the surface, there can be an appreciable reduction in the rate of energy transfer to that surface. To assess the importance of homogeneous and heterogeneous quenching on the wall energy transfer under continuum diffusion conditions, we examine a simple physicochemical model leading to closed-form predictions of the quenching and energy transfer effects and relevant dimensionless parameters. This model, together with available experimental data on the selective production and quenching of electronically excited O2 molecules during heterogeneous O-atom recombination, are used to estimate: (i) the importance of gas phase and wall quenching in experimental determinations of atom recombination coefficients or the apparent heat of atom recombination at catalyst surfaces, and (ii) the likelihood of appreciable reductions in the aerodynamic heating of hypersonic glide vehicles.

Vibrationally Excited O2 Molecules from the Reactions of O Atoms with NO2 and with O3

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