Dissociation Of Molecular Nitrogen Research Articles

Coarse-grain model for internal energy excitation and dissociation of molecular nitrogen

A rovibrational collisional coarse-grain model has been developed to reduce a detailed mechanism for the internal energy excitation and dissociation processes behind a strong shockwave in a nitrogen flow. The rovibrational energy levels of the electronic ground state of the nitrogen molecule were lumped into a smaller number of bins. The reaction rate coefficients of an ab initio database developed at NASA Ames Research Center were averaged for each bin based on a uniform distribution of the energy levels within the bin. The results were obtained by coupling the Master equation for the reduced mechanism with a one-dimensional flow solver for conditions expected for reentry into Earth’s atmosphere at 10km/s. The coarse-grain collisional model developed allow us to describe accurately the internal energy relaxation and dissociation processes based on a smaller number of equations, as opposed to existing reduced models assuming thermal equilibrium between the rotational and translational energy modes.

Modeling and experimental study of molecular nitrogen dissociation in an Ar–N2 ICP discharge

The dissociation of nitrogen molecules in an Ar–N2 inductively coupled plasma (ICP) discharge is studied both experimentally and theoretically. To measure the absolute N atom density and emission intensity of Ar and N2 excited levels, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy and optical emission spectroscopy are used. We observe an increase in N atom density with increasing pressure whereas the N atom density decreases for pressures higher than 100 mTorr in a pure nitrogen discharge. On adding argon to the mixture, we observe that the dissociation rate is enhanced when going from a pure nitrogen discharge to an argon mixed discharge. To calculate the plasma parameters, a global (volume-averaged) model is developed. The variation of the electron temperature and the particle densities are calculated by solving the particle and energy balance equations. The model calculations are compared with the measurement results and the production and loss rates of each species are described under each discharge condition. From the model calculation, the dissociation of N2 molecules in the Ar–N2 mixed discharge occurs mainly by electron impact dissociation at low pressures, while at high pressures the dissociative recombination is enhanced by charge transfer between Ar+ and N2(X) as well as metastable–metastable pooling dissociation due to the high density. In addition, the surface sticking coefficient of nitrogen atoms in a planar ICP discharge (including glass and stainless steel walls) is deduced from TALIF measurements and is estimated to be 0.02 under our set-up conditions.

Numerical simulation of atomic nitrogen formation in plasma of glow discharge in nitrogen-argon mixture

We consider the problem of determining the content of atomic nitrogen as an active component responsible for the efficiency of metal surface modification in plasma of stationary low-pressure glow discharge in nitrogen-argon mixture (widely used in this technology). The influence of the gas mixture composition on the rate constant of molecular nitrogen dissociation, which determines the atomic nitrogen production, has been calculated, The parameters of plasma have been experimentally determined using the method of double probes. The electron energy distribution function is found by numerically integrating the Boltzmann equation in a two-term approximation for the molecular nitrogen-argon mixture.

Post-Discharge Nitriding: Mathematical Modeling and Numerical Simulation of the Layer Growth

Nitriding by microwave post-discharge process involves molecular nitrogen dissociation. It has been observed that nitrogen flux from surface to solid during the early stage does not follow a parabolic regime and that the growth rate of concomitant nitride layers is sensitive to atomic nitrogen concentration on the surface. In this work a mathematical model has been developed in order to describe the kinetics of the compound layer formation during a post-discharge nitriding process. The model is related to a moving boundary value problem and considers different stages: diffusion process, formation of the layers, layer growth and quasi-stabilization of the layer growth. Natural conditions on the nitrogen concentration consistent with the mass transfer mechanism are assumed. An analytical approximate solution of Goodman’s type is sought and numerical simulation is conducted to study the nitride layer growth.

Mathematical simulation of the layer growth kinetics during post-discharge nitriding: From early stage to quasi-steady stage

Nitriding by microwave post-discharge process involves molecular nitrogen dissociation. It has been observed that nitrogen flux from surface to solid during the early stage does not follow a parabolic regime and that the growth rate of concomitant nitride layers is sensitive to atomic nitrogen concentration on the surface. In this work a mathematical model has been developed in order to describe the kinetics of the compound layer formation during a post-discharge nitriding process. The model is related to a moving boundary value problem and considers different stages: diffusion process, formation of the layers, layer growth and quasi-stabilization of the layer growth. An analytical approximate solution of Goodman's type is sought and representations of the motion of the interfaces and the nitrogen concentration profiles are analyzed.

Adsorption and dissolution of nitrogen in lithium—QM DFT investigation

Ab initio quantum mechanic density functional theory (QM DFT) calculations were used to study the interaction of molecular nitrogen (N 2) with metallic Li surface. The calculation has been made using Dmol commercial package provided by Accelrys Inc. A clusters of metal atoms, of various sizes were used to study the interaction of nitrogen with Li surface. It was demonstrated that the size is large enough to reach the asymptotic regime. The cluster had one, two, three and four atomic layers. It was also shown that due to low-cohesion energy of Li, the cluster had to be supported by external forces and/or limitation on atomic displacement of the atoms in the cluster. The interaction of single N atom with the Li surface was investigated. As it is expected, a single N atom is strongly attracted by the surface. A N 2 molecule interaction with Li surface was also investigated. In contrast to Al, Ga or In surfaces, in case of Li surface, the N 2 molecule is adsorbed in the molecular state, i.e. no dissociation of molecular nitrogen in the adsorption stage occurred. The N 2 adsorption process is barrierless, with the energy level for the physisorbed state about 0.5 eV below the far distance level. From the surface state, the N 2 molecule can penetrate into the lithium interior. The barrier for this transition is about 0.2 eV. It was also found that the energy of the molecular nitrogen dissolved in the Li is about 1.3 eV below the far distance level. The interatomic N–N distance on the N 2 molecule located in the Li interior is only slightly increased to 1.25 Å, which is 0.15 Å larger than the standard 1.1 Å distance for the free N 2 molecule. This confirms that the adsorption and dissolution of nitrogen in Li does not lead to the decomposition of the molecular N 2 structure.

Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production

An electromagnetic surface wave launched by a waveguide (type surfaguide [M. Moisan, Z. Zakrzewski, J. Phys., D., Appl. Phys. 24 (1991) 1025]) powered by a microwave generator (2.45 GHz) generates a plasma which produces atomic nitrogen by dissociation of molecular nitrogen. This plasma takes place in a quartz tube where an argon/nitrogen mixture is flowing. The dissociation rate depends directly on the power density injected into the discharge. The microwave power is therefore pulsated in order to be able to work with high instantaneous power while preserving a low mean power. The frequency of pulsation has a great influence on the effectiveness of the dissociation process: an optimal frequency has been be determined which depends on various parameters of the discharge such as power and tube diameter. The optimal conditions of pulsation seems to be due to a thermal effect and more precisely can be correlated with the radial profile of the gas temperature in the discharge. The dissociation fraction of the molecular nitrogen in atomic nitrogen is determined by following the decrease of the molecular nitrogen signal when the plasma is ignited. The latter is measured by mass spectrometry and by optical emission in the post-discharge. In order to understand the thermal effect inducing an optimum pulse frequency, the discharge is also characterized by optical spectroscopy: the change in concentration of species like argon atoms, neutral and ionized molecular nitrogen has been followed as a function of the pulse frequency and the gas discharge temperature measured by means of the rotational bands of N2+.

Enhancement of the molecular nitrogen dissociation and ionization levels by argon mixture in flue nitrogen plasma

In this work, the nitrogen molecular dissociation and ionization levels in Ar/N2 flue plasma are evaluated as functions of plasma parameters such as Ar mixture quantity and N2 flux in order to obtain the best condition for various applications such as thin film deposition and material surface modification. This plasma is operated at 10 kV and the nitrogen dissociation rate is determined by analyzing the optical emission of the nitrogen band. For different operating conditions, the dissociation rate [N] of N2 molecules was enhanced, as the mixture quantity of Ar increased from 0.06 m3/h to 0.9 m3/h and the max of enhancement factor is 4.3. This factor becomes bigger when the N2 flux becomes bigger. Moreover, the molecular nitrogen ionization density is calculated from the current intensity of the plasma. The ionization density was also enhanced, as the mixture quantity of Ar increased from 0.1 m3/h to 1.5 m3/h, under three different voltages. The max of enhancement factor of 1.96 is much smaller than the factor of the dissociation rate. These results are discussed in terms of the kinetics of the electrons, nitrogen ions, atoms and molecules.

On the mechanisms of diode plasma nitriding in N 2–H 2 mixtures under DC-pulsed substrate biasing

The results obtained by the author proved that in the conditions of investigated processes the nitriding intensity was influenced by three components of the gas–plasma atmosphere: nitrogen and hydrogen (supplied as the working gas mixture) as well as argon (used as a component of the atmosphere at the stage of initial heating of the substrates before nitriding). The revealed correlation between composition of supplied N 2–H 2 mixture and: nitrided layer structures, plasma resistance, degree of molecular nitrogen dissociation and concentration of nitrogen molecular ions N 2 + shows that both ionized and neutral nitrogen particles participate directly in the nitriding process. The content of N 2, H 2 and Ar in the nitriding atmosphere determine characteristics of the discharge and relative concentration of nitrogen active species, thus nitriding ability of the individual nitriding processes. On the basis of obtained results, the author proposed the model of mechanisms of diode plasma nitriding taking into consideration intensity of different mechanisms at different nitriding temperatures.

Enhancement of the molecular nitrogen dissociation levels by argon dilution in surface-wave-sustained plasmas

In this work, the nitrogen molecular dissociation level in Ar/N2 surface-wave plasma is evaluated as a function of plasma parameters such as Ar percentage in the gas mixture, power absorbed in the plasma, and total pressure in order to design an efficient N-atom source that can be used for various applications such as thin-film deposition and materials surface modification. This plasma is operated at 40.68 MHz and the nitrogen dissociation rate is determined, in the remote plasma, by analyzing the optical emission of the first positive molecular nitrogen band. For all operating conditions, the dissociation rate ([N]/[N2]) of N2 molecules was enhanced, as the percentage of Ar in the mixture increased from 0 to ∼95%, and dissociation rates higher than 2.5% were measured. This gain in the dissociation rate became more pronounced when the plasma power and total pressure increased from 40 to 120 W and from 4 to 7.5 Torr, respectively. These results are discussed in terms of the kinetics of the electrons, nitrogen atoms, and molecules and confirm theoretical kinetic models presented in the literature.

Optical evidence for a nonmolecular phase of nitrogen above 150 GPa

Optical spectroscopy techniques, including visible and near infrared (IR) Raman and synchrotron IR methods have been applied to study solid nitrogen at megabar pressures. We find that nitrogen becomes totally opaque above 150 GPa, accompanied by the disappearance of Raman and IR vibrational excitations, while new broad IR and Raman bands become visible. Optical absorption measurements reveal that the semiconducting absorption edge responsible for the change of color is characterized by the presence of a wide Urbach-like tail and a high-energy (Tauc) region. These data are consistent with the dissociation of molecular nitrogen into a nonmolecular (possibly amorphous) phase.

Study of matrix isolation of nitrogen atoms in solid N2

The process of matrix isolation of nitrogen atoms in solid N2 is studied by the method of condensation from the gaseous phase. In order to vary the flow rates of nitrogen atoms and molecules at the sample surface over a wide range, N2–He mixtures of various compositions have been passed through the gas discharge zone. It is found that the EPR line width for N atoms in N2 increases upon a decrease in the nitrogen flow rate from the discharge zone to a cold substrate. It is shown that this increase is due to an increase in the concentration of atomic nitrogen in the matrix leading to an enhancement of the dipole–dipole interaction between atoms. It is established that a decrease in the intensity of recombination of atoms in the course of diffusion at the sample surface due to a decrease in the surface density of these atoms plays a decisive role in the growth of the concentration of matrix-isolated atoms in the experiments. It is found that for a constant flow rate of atoms from the discharge, their concentration in the matrix increases and attains saturation upon an increase in the discharge intensity. The dependence of concentration on the discharge intensity is attributed to a change in the degree of dissociation of molecular nitrogen in the discharge.

HCN formation under electron impact: Experimental studies and application to Neptune's atmosphere

Laboratory experiments simulating organic synthesis in Neptune's atmosphere have been performed. We have submitted to a spark discharge gaseous mixtures containing 9 mbar of molecular nitrogen and 3 mbar of methane (the p(N 2) p(CH 4) ratio is compatible with upper limits in Neptune's stratosphere) with varying quantities of molecular hydrogen. The spark discharge is used to model the energetic electrons produced by the impact of cosmic rays on the high atmosphere of Neptune. HCN is synthesized in the described experimental conditions, even with a low mixing ratio of molecular nitrogen. Studying the variation of HCN production with the initial composition of the gas mixture and extrapolating to high mixing ratio of molecular hydrogen allows to estimate HCN production in Neptune's atmosphere. The computed HCN production flux is 7×10 7 m −2 s −1, which is two orders of magnitude lower than the value predicted by chemical models for an internal source of N atoms. The major uncertainty in our extrapolation is the energetic distribution of electrons, implicitly assumed comparable in the discharge and in Neptune's atmosphere. We note that this distribution is also a source of uncertainty in chemical models. The chemical mechanism responsible for the local formation of HCN in the stratosphere probably occurs in the reactor too. We propose a simple characterization of the spark discharge. We thus link the molecular nitrogen dissociation cross section by electron impact to the measured parameters of the experiments (current, voltage, initial partial pressures) and to the resulting HCN partial pressures. However, other laboratory experiments with larger hydrogen pressures, requiring a more powerful electric source, have to be performed to yield a value of the cross section.

Numerical simulations of the distribution of atomic oxygen and nitric oxide in the thermosphere and upper mesosphere

A suite of one, two and three-dimensional models of the thermosphere and upper mesosphere have been used to make a theoretical and numerical study of the distribution of atomic oxygen and nitric oxide (NO), and their responses to varying solar and geomagnetic activity at equinox and solstice. The primary source of nitric oxide is from the reaction of excited atomic nitrogen, N( 2D), with molecular oxygen. The atomic nitrogen is created by a number of ion-neutral reactions and by direct dissociation of molecular nitrogen by photons, photo-electrons and auroral electrons. At mid and low latitudes, peak nitric oxide density occurs at 115 km as a direct consequence of ionisation and dissociation of molecular nitrogen by photoelectrons produced by the solar flux at wavelengths below 30.0 nm (XUV). Peak Nitric oxide densities at low latitudes near 115 km altitude are shown to vary by a factor of seven over the solar cycle, due to the estimated change in the solar XUV flux. This is in good agreement with the Solar Mesosphere Explorer (SME) satellite observations. The diurnal variation at the peak height is relatively small, since the production by solar XUV and loss by solar dissociation both have similar zenith angle dependencies. At higher altitudes, above 150 km, NO density varies by more than an order of magnitude during the day. The modelled latitudinal distributions of NO at equinox predicts an equatorial peak, which is contrary to the observations by SME. The geomagnetic source dominates at high latitudes, due to the ionisation and dissociation by auroral particles. For all but extremely quiet geomagnetic conditions, peak NO concentrations occur at high latitudes, where the density of NO often exceeds the low latitude values by more than an order of magnitude. At solstice, peak NO concentrations occur within both the summer and winter polar regions. At solstice, within the summer polar region, solar and auroral production adds constructively, while in the winter polar region, the peak in NO density is due to auroral production, in the absence of solar photo-dissociation of NO. Atomic oxygen is created by the dissociation of molecular oxygen, by the solar flux from a broad UV and EUV spectral range, and also by several neutral-neutral and ion-neutral chemical reactions. A strong seasonal/latitude variation in atomic oxygen density is created by transport of the species by the global circulation due to its long life-time in the thermosphere. Upwelling throughout the summer hemisphere depletes atomic oxygen densities, while the compensating downwelling in the winter hemisphere, particularly at middle and high latitudes, enhances the atomic oxygen concentration. Geomagnetic energy sources further modify the distribution of atomic oxygen by causing additional upwelling of the atmosphere in the polar regions, with associated horizontal advection of the species by the enhanced global circulation. Concentrations of atomic oxygen vary by more than a factor of five in the upper and lower thermosphere as viewed on a constant pressure surface. Significant seasonal and latitudinal changes occur as low as 90 km altitude.

Electron-impact dissociation of molecular nitrogen in atmospheric-pressure nonthermal plasma reactors

This letter presents measurements of the specific energy consumption (eV per molecule) for electron-impact dissociation of N2 (e+N2→e+N+N) in a pulsed corona and an electron beam reactor. Measurements were done using 100 pm of NO in N2. In this mixture the removal of NO is dominated by the reduction reaction N+NO→N2+O. By measuring the specific energy consumption for reduction of NO, these experiments provide a good measure of the specific energy consumption for electron-impact dissociation of N2. The specific energy consumption using pulsed corona processing is 480 eV per dissociated N2 molecule. For electron beam processing, the specific energy consumption is 80 eV per dissociated N2 molecule.

Carbon Nitride Films Using a Filtered Cathodic ARC

AbstractCarbon nitride films were deposited on sapphire, silicon, molybdenum and quartz substrates using a filtered cathodic arc with a radio frequency (rf) inductively coupled plasma, Previous researchers have used the cathodic arc but obtained low concentrations of nitrogen in their films. the addition of the rf coils was used to enhance dissociation of molecular nitrogen in the vicinity of the substrate. the resultant films were analyzed for chemical bonding and composition by several methods including Electron Energy Loss Spectroscopy (EELS), augei Electron Spectroscopy (AES), and infrared spectroscopy (IR). EELS and IR provided bonding information and showed that the films were largely composed of sp2 bonding and contained carbon-nitrogen double bonds, respectively. also, chemical composition of the films was determined using aES, IR and EELS.

Enhanced dissociation of molecular nitrogen in a microwave plasma with an applied magnetic field

The dissociation of nitrogen in a 2.45 GHz microwave plasma has been significantly enhanced by the application of a magnetic field perpendicular to the oscillating electric field. The degree of dissociation measured in the emergent beam was found to be strongly dependent on the magnetic field strength, nitrogen pressure and microwave power. These results have been interpreted in terms of microwave absorption resonances. Dissociation fractions up to 0.67 have been measured.

Modeling the solar cycle change in nitric oxide in the thermosphere and upper mesosphere

Measurements from the Solar Mesosphere Explorer (SME) satellite have shown that low‐latitude nitric oxide densities at 110 km decrease by about a factor of 8 from January 1982 to April 1985. This time period corresponds to the descending phase of the last solar cycle where the monthly smoothed sunspot number decreased from more than 150 to less than 25. In addition, nitric oxide was observed to vary by a factor of 2 over a solar rotation, during high solar activity. A one‐dimensional, globally averaged model of the thermosphere and upper mesosphere has been used to study the height distribution of nitric oxide (NO) and its response to changes in the solar extreme ultraviolet radiation (EUV) through the solar cycle and over a solar rotation. The reference spectra for EUV have been extended, to wavelengths down to 0.1 nm, to include all radiation likely to affect the region above 70 km altitude, the lower boundary of the model. The primary source of nitric oxide is the reaction of excited atomic nitrogen, N(²D), with molecular oxygen. The atomic nitrogen is created by a number of ion‐neutral reactions and by direct dissociation of molecular nitrogen by photons and photoelectrons. The occurrence of the peak nitric oxide density at or below 115 km is a direct consequence of ionization and dissociation of molecular nitrogen by photoelectrons, which are produced by the solar flux below 30.0 nm (XUV). Nitric oxide is shown to vary over the solar cycle by a factor of 7 at low latitudes in the lower thermosphere E region, due to the estimated change in the solar EUV flux, in good agreement with the SME satellite observations. The NO density is shown to be strongly dependent on the temperature profile in the lower thermosphere and accounts for the difference between the current model and previous work. Wavelengths less than 1.8 nm have little impact on the NO profile. A factor of 3 change in solar flux below 5.0 nm at high solar activity produced a factor of 2 change in the peak NO density, consistent with SME observations over a solar rotation; this change also lowered the peak to 100 km, consistent with rocket data.

Nitric oxide in the lower thermosphere

Satellite measurements of nitric oxide in the lower thermosphere (100–120 km) show the density to be highly variable. The variation at low latitudes is correlated with solar activity and the variation at polar latitudes is connected with geomagnetic activity. A study of a one-dimensional, photochemical model shows that calculations of nitric oxide density in the lower thermosphere are sensitive to uncertainties in the branching ratios for the production of excited and ground state nitrogen by two reactions: the dissociative recombination of ionized nitric oxide and the electron impact dissociation of molecular nitrogen. To a lesser extent, the calculations are also sensitive to uncertainties in the following four reactions: the reaction of excited nitrogen atoms with molecular oxygen, the photodissociation of nitric oxide, the ion-molecule reaction between ionized molecular nitrogen and atomic oxygen, and the deactivation of excited atomic nitrogen by atomic oxygen. The large variability in polar nitric oxide is produced by the variation in the auroral electron flux. A calculation using the model demonstrates that the relationship between the electron flux and the nitric oxide density is non-linear. The Joule heating associated with auroral activity also produces variations in the nitric oxide density. The variability in low latitude nitric oxide is produced by variations in the solar soft x-ray flux. A model calculation shows the relationship between the x-ray flux and the nitric oxide density to be nearly linear.

Role of electronic excitations and atomic quenching on dissociation of nitrogen in dc discharges

It is shown that if the rate coefficients of quenching of the N 2(X 1Σ + g, ν) states by nitrogen atoms is taken about two orders of magnitude higher than that by the N 2(X 1Σ + g, ν=0) molecules, then collisional-radiative model calculations closely reproduce experimental observations on dissociation of molecular nitrogen in dc discharges.