Electron Impact Dissociation Research Articles

Numerical Simulation of Nonequilibrium Plasma-Assisted Ignition for Gasoline Engine

Lean combustion in gasoline engine can reduce nitrogen oxide emission and fuel consumption greatly. However, as the fuel–air ratio approaches the lean-burn limit, the energy required to initiate the combustion increases dramatically, and the flame propagation speed decreases substantially. To enhance the lean combustion and expand the lean-burn limit of a spark-ignition engine, a high-voltage nanosecond pulsed plasma is introduced to initiate the combustion of the engine. Discharge process and chemical reactions are calculated separately due to an essential difference in their timescales. The effect of discharge induced active particles on the ignition of the gasoline engine is studied, which shows that the ignition is primarily associated with the faster development of chain reaction due to O and O3 produced by the electron-impact dissociation in the discharge phase; these active particles can greatly shorten ignition delay time. The combustion enhancement for lean mixture is more pronounced than the stoichiometric gases, which indicates that the nonequilibrium plasma has a great potential to extend the lean-burn limit.

Downstream etching of silicon nitride using continuous-wave and pulsed remote plasma sources sustained in Ar/NF3/O2 mixtures

Remote plasma sources (RPSs) are being investigated to produce fluxes of radicals for low damage material processing. In this computational investigation, the properties of a RPS etching system are discussed where an Ar/NF3/O2 gas mixture is flowed through an inductively coupled plasma source into a downstream chamber containing a silicon nitride coated wafer. The plasma is largely confined in the RPS due to the highly attaching NFx (x = 1–3) and an isolating showerhead although a weak ion-ion plasma maintained by [NO+] ≈ [F−] leaks into the downstream chamber. The etching of silicon nitride proceeds through iterative removal of Si and N subsites by isotropic thermal neutrals. When the fluxes to the wafer are rich in fluorine radicals, the etch rate is limited by the availability of NO molecules and N atoms which remove N subsites. As power deposition increases with continuous-wave excitation, the etch rate increases almost linearly with the increasing fluxes of NO and N atoms, as production of NO through endothermic reactions is aided by increasing gas temperature. Production of N atoms through electron impact dissociation of NO and NFx is aided by the increasing electron density. Similar trends occur when increasing the duty cycle during pulsed excitation. Addition of a plenum between the RPS and the downstream chamber aids in lateral diffusion of radicals before passing through the final showerhead and improves the uniformity of etching.

Ion energy distributions in highly transient EUV induced plasma in hydrogen

This work reports on the measurements of ion flux composition and ion energy distribution functions (IEDFs) at surfaces in contact with hydrogen plasmas induced by extreme ultraviolet (EUV) radiation. This special type of plasma is gaining interest from industries because of its appearance in extreme ultraviolet lithography tools, where it affects exposed surfaces. The studied plasma is induced in 5 Pa hydrogen gas by irradiating the gas with short (30 ns) pulses of EUV radiation (λ= 10–20 nm). Due to the low duty cycle (10–4), the plasma is highly transient. The composition and IEDF are measured using an energy resolved ion mass spectrometer. The total ion flux consists of H+, H2+, and H3+. H3+ is the dominant ion as a result of the efficient conversion of H2+ to H3+ upon collision with background hydrogen molecules. The IEDFs of H2+ and H3+ appear similar, showing a broad distribution with a cut-off energy at approximately 8 eV. In contrast, the IEDF of H+ shows an energetic tail up to 18 eV. Most probably, the ions in this tail gain their energy during their creation process by photoionization and dissociative electron impact ionization.

Electron-impact dissociation of molecular hydrogen into neutral fragments

We present convergent close-coupling calculations of electron-impact dissociation of the ground state of molecular hydrogen into neutral fragments over the range of impact energies from 6 to 300 eV. The calculations account for dissociative excitation, excitation radiative decay dissociation, and predissociation through all bound electronic triplet states, and singlet states up to the state. An estimate is given for the contribution from the remaining bound electronic singlet states. Our results are in agreement with the recommended data of Yoon et al. [J. Phys. Chem. Ref. Data 37 , 913 (2008)] in the low (6–12 eV) and high (60–70 eV) energy regions, but somewhat lower at the intermediate energies.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity.

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Fluid modeling of radical species generation mechanism in dense methane-air mixture streamer discharge

Atmospheric dielectric barrier discharge (DBD) was found to be promising in the context of plasma chemistry, plasma medicine, and plasma-assisted combustion. In this paper, we present a detailed fluid modeling study of abundant radical species produced by a positive streamer in atmospheric dense methane-air DBD. A two-dimensional axisymmetric fluid model is constructed, in which 82 plasma chemical reactions and 30 different species are considered. Spatial and temporal density distributions of dominant radicals and ions are presented. We lay our emphasis on the effect of varying relative permittivity (εr = 2, 4.5, and 9) on the streamer dynamics in the plasma column, such as electric field behavior, production, and destruction pathways of dominant radical species. We find that higher relative permittivity promotes propagation of electric field and formation of conduction channel in the plasma column. The streamer discharge is sustained by the direct electron-impact ionization of methane molecule. Furthermore, the electron-impact dissociation of methane (e + CH4 = >e + H+CH3) is found to be the dominant reaction pathway to produce CH3 and H radicals. Similarly, the electron-impact dissociations of oxygen (e + O2 = >e + O+O(1D), e + O2 = >e + O+O) are the major routes for O production.

FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov–Gerasimenko

Abstract The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov–Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H2O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H2O, CO2, and O2. CO emissions were found to be due to both electron and photoexcitation processes. Thus, we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and CO2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.

Non-equilibrium plasma kinetics of reacting CO: an improved state to state approach

Non-equilibrium plasma kinetics of reacting CO for conditions typically met in microwave discharges have been developed based on the coupling of excited state kinetics and the Boltzmann equation for the electron energy distribution function (EEDF). Particular attention is given to the insertion in the vibrational kinetics of a complete set of electron molecule resonant processes linking the whole vibrational ladder of the CO molecule, as well as to the role of Boudouard reaction, i.e. the process of forming CO2 by two vibrationally excited CO molecules, in shaping the vibrational distribution of CO and promoting reaction channels assisted by vibrational excitation (pure vibrational mechanisms, PVM). PVM mechanisms can become competitive with electron impact dissociation processes (DEM) in the activation of CO. A case study reproducing the conditions of a microwave discharge has been considered following the coupled kinetics also in the post discharge conditions. Results include the evolution of EEDF in discharge and post discharge conditions highlighting the role of superelastic vibrational and electronic collisions in shaping the EEDF. Moreover, PVM rate coefficients and DEM ones are studied as a function of gas temperature, showing a non-Arrhenius behavior, i.e. the rate coefficients increase with decreasing gas temperature as a result of a vibrational–vibrational (V–V) pumping up mechanism able to form plateaux in the vibrational distribution function. The accuracy of the results is discussed in particular in connection to the present knowledge of the activation energy of the Boudouard process.

Characterization of Ar/N2/H2 middle-pressure RF discharge and application of the afterglow region for nitridation of GaAs

This work characterizes the production and destruction of nitrogen and hydrogen atoms in RF capacitively coupled middle-pressure discharge in argon/nitrogen/hydrogen mixtures. Input power, electron concentration, electric field strength and mean electron energy were determined on the basis of electrical measurements. Gas temperature and concentration of Ar atoms in 1s states were determined from spectral measurements. On the basis of experimentally determined plasma characteristics, main production and loss mechanisms of H and N atoms were discussed. The plasma produced radicals were applied for the nitridation and oxide reduction of gallium arsenide in the afterglow region of discharge. After plasma treatment the GaAs samples were analyzed using x-ray photoelectron spectroscopy (XPS) technique. Successful nitridation of GaAs sample was obtained in the case of Ar/5% N2 discharge. In this gas mixture the N atoms were generated via dissociative recombination of created by charge transfer from Ar+. The treatment in Ar/5% N2/1% H2 mixture resulted in the reduction of oxide signals in the XPS spectra. Negligible formation of GaN in the latter mixture was connected with reduced concentration of N atoms, which was, in turn, due to less efficient mechanism of N atom production (electron impact dissociation of N2 molecules) and additional loss channel in reaction with H2.

Rosetta Mission: Electron Scattering Cross Sections—Data Needs and Coverage in BEAMDB Database

The emission of [O I] lines in the coma of Comet 67P/Churyumov-Gerasimenko during the Rosetta mission have been explained by electron impact dissociation of water rather than the process of photodissociation. This is the direct evidence for the role of electron induced processing has been seen on such a body. Analysis of other emission features is handicapped by a lack of detailed knowledge of electron impact cross sections which highlights the need for a broad range of electron scattering data from the molecular systems detected on the comet. In this paper, we present an overview of the needs for electron scattering data relevant for the understanding of observations in coma, the tenuous atmosphere and on the surface of 67P/Churyumov-Gerasimenko during the Rosetta mission. The relevant observations for elucidating the role of electrons come from optical spectra, particle analysis using the ion and electron sensors and mass spectrometry measurements. To model these processes electron impact data should be collated and reviewed in an electron scattering database and an example is given in the BEAMD, which is a part of a larger consortium of Virtual Atomic and Molecular Data Centre—VAMDC.

OH and O radicals production in atmospheric pressure air/Ar/H2O gliding arc discharge plasma jet

Atmospheric pressure air/Ar/H2O gliding arc discharge plasma is produced by a pulsed dc power supply. An optical emission spectroscopic (OES) diagnostic technique is used for the characterization of plasmas and for identifications of and radicals along with other species in the plasmas. The OES diagnostic technique reveals the excitation Tx ≈ 5550–9000 K, rotational Tr ≈ 1350–2700 K and gas Tg ≈ 850–1600 K temperatures, and electron density under different experimental conditions. The production and destruction of and radicals are investigated as functions of applied voltage and air flow rate. Relative intensities of and radicals indicate that their production rates are increased with increasing content in the gas mixture and applied voltage. reveals that the higher densities of and radicals are produced in the discharge due to more effective electron impact dissociation of and molecules caused by higher kinetic energies as gained by electrons from the enhanced electric field as well as by enhanced The productions of and are decreasing with increasing air flow rate due to removal of Joule heat from the discharge region but enhanced air flow rate significantly modifies discharge maintenance properties. Besides, significantly reduces with the enhanced air flow rate. This investigation reveals that plays a significant role in the production of and radicals.

Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH4 mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H2, C2 and C3 hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C2 and C3 formation dramatically decreased, and syngas (H2 and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH4 mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH3, CH2, CH, H, O and O (1D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.

R-matrix study of electron impact excitation and dissociation of CH+ ions

Electron impact excitation and electron impact dissociation of CH+ ions are studied in the framework of the R-matrix method using the diatomic version of the UK molecular R-matrix codes. A configuration interaction calculation is first performed to yield the potential energy curves of the lowest eight singlet and triplet states of CH+. Scattering calculations are then performed to yield vibrationally-resolved electronic excitations to the lowest three bound states, namely the a , A and the b . Electron impact dissociation cross sections are obtained from the assumption that all electronic excitations above the dissociation threshold result in dissociation. Bound states of CH and resonance positions and widths of Feshbach resonances in the e–CH+ system are also calculated at the CH+ equilibrium bond length 2.137 a0.

Kinetic-energy release of fragments from electron-impact dissociation of the molecular hydrogen ion and its isotopologues

We calculate the kinetic-energy release distributions of fragments produced for electron-impact dissociation of the vibrationally excited molecular hydrogen ion ${{\mathrm{H}}_{2}}^{+}$ and its isotopologues ${{\mathrm{D}}_{2}}^{+}$ and ${{\mathrm{T}}_{2}}^{+}$. Here we apply the adiabatic-nuclei convergent close-coupling method and compare results with several different methods, including the $\ensuremath{\delta}$ approximation. Results are presented for a number of dissociative excitation transitions and dissociative ionization as a function of the initial vibrational state of the molecule. We confirm that the square root approximation is a good approximation for the adiabatic-nuclei kinetic-energy release cross sections of ${{\mathrm{H}}_{2}}^{+}$. Agreement with experiment, where available, is good.

Ignition by Electrical Discharges

Abstract The ignition of combustible/air mixtures by electrical discharges includes several physical and chemical processes. In process safety often the total available electrical energy is taken as a risk measure. However, to get a more detailed insight in the ignition process, also energy losses to the surrounding have to be considered. Additionally, for specific discharges not only the dissipation leading to thermal heating is of importance. Especially in the case of non-equilibrium plasma, a specific fractional amount of the discharge energy is used for electron impact dissociation, excitation, and ionization reactions, producing active radicals and ions during the discharge phase. While the electrical energy can be measured easily, it is difficult to determine energy losses. In this paper three different electrical discharges are examined experimentally and numerically to yield a better understanding of the ignition by electrical discharges.

Electron-impact dissociation and ionization of CN+ ions

Absolute cross sections for various pathways of electron-impact ionization and dissociation of CN${}^{+}$ ions are measured, which provides strong support for more accurate modeling of relevant processes in astrophysical plasma environments.

Calculated cross sections for electron collisions with NF3, NF2 and NF with applications to remote plasma sources

Electron impact cross sections sets are constructed for the nitrogen trifluoride, nitrogen difluoride and nitrogen monofluoride molecules. These cross sections are based on ab initio R-matrix calculations augmented by other procedures. Cross sections are presented for elastic collisions, momentum transfer, dissociative electron attachment, electron impact dissociation, ionisation and dissociative ionisation. For NF process occurring via the metastable a and b states are also considered. A semi-empirical method of estimating the products of electron impact ionisation is proposed and tested for ammonia. The cross sections are extended to high energy where appropriate. The cross section set constructed is tested in a global model simulation of a low pressure, inductively coupled plasma based on a Ar/NF3/O2 initial gas mixture.

Simulation of spatio-temporal variation of OH radical density in atmospheric-pressure streamer discharge

The spatio-temporal variation of OH radical density in an atmospheric-pressure plasma discharge is investigated via two-dimensional numerical simulation. The behaviours of OH are characterized in four regions in the inter-electrode gap, namely the ‘Hot anode’, ‘Secondary streamer’, ‘Primary streamer’, and ‘Near-cathode’ regions. In these regions, temporal variations of averaged reduced electric field, electron density, and gas temperature differ and they affect the temporal variation of OH density. In all regions, a relatively large amount of OH is produced by the dissociation reactions of H2O with electronically excited nitrogen molecules and oxygen atom rather than the direct electron-impact dissociation reactions. In the Hot anode and Secondary streamer regions, the instantaneous density of OH just after the discharge is high. However, the subsequent OH decay rates differ in these regions owing to the difference in gas temperatures. The reaction mechanism is discussed by dividing the effect of the diffusion-convection term and chemical reaction term in the continuous equation for OH radical density.

Experimental scaling of plane-Born cross sections and ab initio assignments for electron-impact excitation and dissociation of XF4 (X = C, Si, and Ge) molecules.

Electron energy loss spectra of carbon tetrafluoride, silicon tetrafluoride, and germanium tetrafluoride molecules (CF4, SiF4, and GeF4) have been measured for incident electron energies of 50-360 eV at 1.5°-15.5° and for 30 eV and 30° scattering angle, while sweeping the energy loss over the range 9.0-20.0 eV. Low-lying valence excited triplet and singlet states are investigated by quantum chemical ab initio calculations. The Rydberg series converging to the (lowest) ionisation energy limits of XF4 (X = C, Si, Ge) are also identified and classified using the systematic behaviour according to the magnitude of the quantum defects. A generalized oscillator strength analysis is employed to derive oscillator strength f0 value and the apparent Born integral cross sections from the corresponding differential cross sections by using the Vriens formula for the optically allowed transitions. The f0 value is compared with the optical oscillator strength of the photoabsorption, pseudo-photon measurements, and theoretical values. The binary-encounter and f-scaled Born cross sections of the most intense optically allowed transitions have been also derived from the excitation threshold to the high energy region where the Born approximation is valid. Potential energy curves were obtained along the XF3 + F coordinate with two different basis sets to lend support on electron impact dissociation processes yielding radical formation. We found that in CF4, the lowest-lying dissociative character is due to intramolecular conversion from Rydberg 3s to valence character (σ*(C-F)), whereas in SiF4 and GeF4, an antibonding behaviour prevails.

Hydrogen atom kinetics in capacitively coupled plasmas

Hydrogen (H) atom kinetics has been investigated in capacitively coupled very high frequency (VHF) discharges at powers of 16–780 mW cm–2 and H2 gas pressures of 0.1–2 Torr. The H atom density has been measured using vacuum ultra violet absorption spectroscopy (VUVAS) with a micro-discharge hollow cathode lamp as a VUV light source. The measurements have been performed in two different electrode configurations of discharges: conventional parallel-plate diode and triode with an intermediate mesh electrode. We find that in the triode configuration, the H atom density is strongly reduced across the mesh electrode. The H atom density varies from ∼1012 cm−3 to ∼1010 cm−3 by crossing the mesh with 0.2 mm in thickness and 36% in aperture ratio. The fluid model simulations for VHF discharge plasmas have been performed to study the H atom generation, diffusion and recombination kinetics. The simulations suggest that H atoms are generated in the bulk plasma, by the electron impact dissociation (e + H2 e + 2H) and the ion–molecule reaction (H2+ + H2 + H). The diffusion of H atoms is strongly limited by a mesh electrode, and thus the mesh geometry influences the spatial distribution of the H atoms. The loss of H atoms is dominated by the surface recombination.