Electron Impact Dissociation Of O2 Research Articles

A surface mechanism for O3 production with N2 addition in dielectric barrier discharges

Ozone, O3, is a strong oxidizing agent often used for water purification. O3 is typically produced in dielectric barrier discharges (DBDs) by electron-impact dissociation of O2, followed by three-body association reactions between O and O2. Previous studies on O3 formation in low-temperature plasma DBDs have shown that O3 concentrations can drop to nearly zero after continued operation, termed the ozone-zero phenomenon (OZP). Including small (<4%) admixtures of N2 can suppress this phenomenon and increase the O3 production relative to using pure O2 in spite of power deposition being diverted from O2 to N2 and the production of nitrogen oxides, N x O y . The OZP is hypothesized to occur because O3 is destroyed on the surfaces in contact with the plasma. Including N2 in the gas mixture enables N atoms to occupy surface sites that would otherwise participate in O3 destruction. The effect of N2 in ozone-producing DBDs was computationally investigated using a global plasma chemistry model. A general surface reaction mechanism is proposed to explain the increase in O3 production with N2 admixtures. The mechanism includes O3 formation and destruction on the surfaces, adsorption and recombination of O and N, desorption of O2 and N2, and NO x reactions. Without these reactions on the surface, the density of O3 monotonically decreases with increasing N2 admixture due to power absorption by N2 leading to the formation of nitrogen oxides. With N-based surface chemistry, the concentrations of O3 are maximum with a few tenths of percent of N2 depending on the O3 destruction probability on the surface. The consequences of the surface chemistry on ozone production are less than the effect of gas temperature without surface processes. An increase in the O3 density with N-based surface chemistry occurs when the surface destruction probability of O3 or the surface roughness was decreased.

Kinetic study of plasma assisted oxidation of H2 for an undiluted lean mixture

For the past couple of decades, electrical discharges (or plasmas) have been widely investigated in pursuit of the advancement in combustion and fuel reforming. Particularly, nonthermal plasma has attracted researchers’ attention to improve ignition characteristics, promote flame stability, and reform hydrocarbons. Nevertheless, due to the nonthermal plasma's complex physicochemical nature, most of the experimental findings have not been fully explained yet. Recently, plasma-chemical kinetic studies have been initiated to address the important roles of plasma chemistry in hydrocarbon chemistry including combustion phenomena. However, we still have a long way to go to fully understand the underlying mechanisms and predict experimental outcomes. Here, we present a kinetic study of plasma-assisted low-temperature oxidation of H2 for an undiluted H2/O2 mixture. The aim of this study is to establish a foundation for low-temperature plasma assisted combustion as well as high-temperature plasma assisted reforming processes. We employed a plasma-chemical reaction mechanism and plasma-chemical kinetic modeling platform (KAUSTKin) and a temperature controlled dielectric barrier discharge reactor to study the plasma assisted oxidation of H2. As a result of systematically varying the gas temperature and discharge power, we found a non-linear oxidation behavior highlighting a Negative Temperature Coefficient (NTC)-like trend in a temperature range of 600–750 K. We investigated the effects of both the reduced electric field and the temperature on the plasma assisted oxidation chemistry. We found that (i) the oxidation is initiated by the electron impact dissociation of O2, which is governed by the reduced electric field and controls the oxidation degree, (ii) HO2 is the key intermediate for the full oxidation to H2O, and (iii) O3 and H2O2 production negatively affect the oxidation for temperatures below 400 K and over 600 K, respectively. We believe that these findings will further contribute to a better description and deeper understanding of the plasma chemistry with hydrocarbons as well as other H2 mixtures.

Kinetics of Low-Temperature Plasma-Assisted Ignition of Ethanol-Gasoline Surrogate under Gasoline Engine like Conditions

ABSTRACT The plasma-assisted reaction kinetic mechanism for high-voltage nanosecond discharge at high pressure was developed for lean and stoichiometric ethanol-gasoline surrogate/O2/N2/AR mixture. The numerical results show that plasma-assisted ignition of ethanol-gasoline surrogate is mainly associated with the electron impact dissociation of O2, nC7H16, iC8H18 and C2H5OH molecules and excited O2* produced from electron impact during discharge phase. The generation of H and O atoms, radicals and O2* facilitates plasma-assisted ignition (PAI). The formation of C2H5OH+, C8H18 + and O− increases ignition delay time. PAI improves the ignition stability of lean mixture mainly by increasing the reaction rate in reaction iC8H18+ HO2⇋C8H17+ H2O2. The reaction CH3+ HO2⇋CH4+ O2 promotes auto-ignition. But it delays the plasma-assisted ignition of stoichiometric mixture while it facilitates plasma-assisted ignition in lean mixture. The ignition delay time in lean-burn conditions can be shortened only by a proper timing of low-temperature plasma discharge at low temperature.

On the quenching of excited electronic states of molecular nitrogen in nanosecond pulsed discharges in atmospheric pressure air

Emission measurements are carried out to study the quenching of excited electronic states of nitrogen, N2∗, in nanosecond pulsed discharges in atmospheric pressure air and nitrogen. The results reveal that ground state N2 quenches N2(C) and N2(B) at rates less than dissociative quenching by ground state O2 by a factor of 4 and 2.5, respectively. Kinetic simulations with the inferred quench rates indicate that the dissociative quenching of N2∗ by O2 is responsible for 82% of atomic oxygen production while electron-impact dissociation of O2 is for 5% under these discharge conditions.

Electron impact dissociative excitation of O2: 1. Kinetic energy distributions of fast oxygen atoms

The kinetic energy distribution of oxygen atoms resulting from electron impact dissociation of O2 has been measured. A high‐resolution vacuum ultraviolet spectrometer was employed for the measurement of the OI(115.2 nm, 130.4 nm, 135.6 nm) fine‐structure emission line profiles at 35 eV and 100 eV electron impact energies. The deconvolved line profiles of atomic oxygen reveal the existence of a broad line profile (∼30–40 mÅ full width at half maximum) produced from a combination of dissociative excitation and dissociative ionization excitation processes. The atomic oxygen kinetic energy distributions at 100 eV electron impact energy spans the energy range from 1 to 10 eV, with a peak value near 1 eV. The excitation functions of OI(115.2 nm, 130.4 nm, 135.6 nm) were measured in the vicinity of threshold, from 14 to 50 eV. The PACS classification is 34.80.Gs (electron scattering‐molecular dissociation) and 33.50Dq (molecular spectra‐fluorescence).

Selective detection of O(1S) following electron impact dissociation of O2 and N2O using a XeO* conversion technique

We have observed the metastable species O(1S0) following the dissociation of molecules by pulsed electron impact. A crossed beam apparatus was used to obtain time-of-flight spectra of the fragments. We have employed a novel detector which consists of a layer of freshly deposited Xe. O(1S0) atoms impinging on the surface quickly form XeO* excimers which rapidly decay (&amp;lt;1 μs) producing easily detected photons. Our present results indicate that this method is sensitive (with high quantum efficiency) solely to the 1S state of oxygen. Other metastables (such as 5S and 1D) and ground state oxygen atoms have not been detected. Low resolution optical spectra of the XeO emission reveal bands at 375, 550, and 725 nm. Time-of-flight and fragment kinetic energy spectra for both target gases at various electron impact energies are presented together with excitation functions from threshold to 1000 eV. These have been made absolute using a Bethe–Born calibration technique for N2O. O2 data are calibrated relative to N2O. Maximum cross sections for O(1S) production are 2.25×10−17 cm2 at 45 eV and 2.1×10−18 cm2 at 80 eV for N2O and O2 targets, respectively.

Electron-impact dissociation of oxygen

The electron-impact dissociation of O2 to form two oxygen atoms is observed in a crossed beam experiment at electron energies between 13.5 and 198.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the O2 dissociation are consistent with production of O(1D)+O(3P) fragments following electron impact excitation to the B 3Σu−, B′ 3Σu−, and 2 3Πu states, and production of O(3P)+O(3P) fragments from excitation to the (unresolved) c 1Σu−, A′ 3Δu, and A 3Σu+ states. Absolute cross sections for the electron impact dissociation of O2 are measured.

Kinetics of O(1S) and O(1D) metastable atoms in a DC oxygen glow discharge

The concentrations of O(1D) and O(1S) metastable atoms are measured in the positive column of a DC oxygen glow discharge by vacuum ultraviolet absorption spectroscopy at 115 and 122 nm respectively. In the investigated range of pressures p=0.2-2 torr and discharge currents Id=10-80 mA, the concentrations of O(1D) and O(1S) atoms range from 1011-1012 and from 1010-1011 cm-3 respectively. In these conditions the main creation and destruction processes are discussed: the O(1D) atoms are created by electron impact dissociation of O2(X3 Sigma ) and O2(a1 Delta ), and excitation of O(3P), and lost through quenching by oxygen molecules. It is shown that it is necessary to add to the electron impact excitation another creation term of O(1S) atoms to balance the destruction through quenching by O2(X3 Sigma ), O2(a1 Delta ) and O(3P).

Electron impact dissociation of O2

The production of metastable O(5S) atoms by electron impact dissociative excitation of O2 has been observed using the technique of translational spectroscopy. The kinetic energy spectra show two distinct structures with maxima near 0.13 and 2.5 eV. These distributions indicate that two distinct excitation processes are involved in the production of the O(5S) species, one from the dissociation of O2* bound states in their repulsive region, one coming from the dissociation of completely repulsive O2* potentials. Excitation functions for both these excitation processes have been measured and compared with earlier data.

The auroral 6300 Å emission: Observations and modeling

Measurements of the auroral atomic oxygen (³P‐¹D) emission line at 6300 Å made by the Atmosphere Explorer visible airglow experiment are analyzed using a tomographic inversion. Emission altitude profiles are compared to the results from an electron transport and chemical reaction model. The model incorporates measurements of the energetic electron flux, neutral composition, ion composition, and electron density. Reasonable correspondence is obtained using primarily the “classical” sources of O(¹D) excitation: electron impact on atomic oxygen and dissociative recombination of O2+. The reaction of N(²D) with O2 is considered to be a minor source. Small contributions are also calculated for cascade from O(¹S), electron impact dissociation of O2, reaction of N+ with O2, and energy transfer from O+(²D) and thermal electrons to O(³P). A possible minor source from the quenching of N(²D) or N(²P) by O(³P) is also discussed.

Dissociation of O2 in N2/O2 mixtures

An explanation is given of the unexpectedly high ozone yield of discharges in N2/O2 mixtures. In addition to the direct electron impact dissociation of O2 a substantial number of additional oxygen atoms are provided by reactions involving excited nitrogen molecules and nitrogen atoms. The calculations based on recent electron impact cross sections are found to be in good agreement with measurements of ozone yield and the time constant of ozone formation.

Energy Spectra of Metastable Oxygen Atoms Produced by Electron Impact Dissociation ofO2

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