Ground State Nitrogen Research Articles

The role of highly vibrationally excited H2 initiating the nitrogen chemistry

The formation of hydrides by gas-phase reactions between H2 and a heavy element atom is a very selective process. Reactions with ground-state neutral carbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have high energy barriers because the H2 molecule has to be fragmented before a hydride bond is formed. In cold interstellar clouds, these barriers exclude the formation of CH, OH, NH, and SH radicals through hydrogen abstraction reactions. Here we study a very energetically unfavorable process, the reaction of N(4S) atoms with H2 molecules. We calculated the reaction rate coefficient for H2 in different vibrational levels, using quantum methods for v = 0−7 and quasi-classical methods up to v =12; for comparison purposes, we also calculated the rate coefficients of the analogous reaction S (3P)+ H2(v) → SH + H. Owing to the high energy barrier, these rate coefficients increase with v and also with the gas temperature. We implemented the new rates in the Meudon photodissociation region (PDR) code and studied their effect on models with different ultraviolet (UV) illumination conditions. In strongly UV-irradiated dense gas (Orion Bar conditions), the presence of H2 in highly vibrationally excited levels (v ≥ 7) enhances the NH abundance by two orders of magnitude (at the PDR surface) compared to models that use the thermal rate coefficient for reaction N(4S) + H2 → NH + H. The increase in NH column density, N(NH), across the PDR is a factor of ~25. We investigate the excitation and detectability of submillimeter NH rotational emission lines. Being a hydride, NH excitation is very subthermal (Trot ≪ Tk) even in warm and dense gas. We explore existing Herschel/HIFI observations of the Orion Bar and Horsehead PDRs. We report a 3σ emission feature at the ~974 GHz frequency of the NH NJ = 12 − 01 line toward the Bar. The emission level implies N(NH) ≃ 1013 cm−2, which is consistent with PDR models using the new rate coefficients for reactions between N and UV-pumped H2. This formation route dominates over hydrogenation reactions involving the less abundant N+ ion. JWST observations will quantify the amount and reactivity of UV-pumped H2 in many interstellar and circumstellar environments.

N–H collision integrals with study of repulsive interactions

Abstract Collision integrals are calculated and analyzed for the ground state nitrogen N ( S 4 ) and hydrogen H ( S 2 ) atoms involving the 3Σ and 5Σ electronic states in the temperature range of 1000–30 000 K. Influence of the quality of different known potential energy functions on the collision integrals was also studied and compared with the available data. Based on our analysis, the repulsive potential and its description beyond simple exponential also at small interatomic distances is necessary to obtain reliable collision integrals. Besides the ground state atoms, for the first time the collision integrals for excited nitrogen atoms (N ( D 2 ) and N ( P 2 ) ) with ground state hydrogen H ( S 2 ) were also calculated. The corresponding potential energy curves are also computed by using multireference configuration interaction (MRCI) method of electronic structure theory with aug-cc-pV6Z basis set. The resulting collision integrals were fitted to simple functional forms.

Influence of nitrogen impurities on the characteristics of helium discharge at high pressure

In this paper, a helium model with nitrogen impurities was established by COMSOL Multiphysics. In order to validate the model, the results of this model were compared with those of the pure helium model and the experimental data. A concept named “reaction chain” was applied to investigate the effects of the nitrogen impurities and elucidate the underlying mechanisms. In terms of discharge mechanisms, the Penning ionization of the He* and N2 can consume He* constantly and directly, which can shorten the reaction chain for the production of dominant ions. The direct ionization of ground state nitrogen molecules can produce more electrons to strengthen the effects of electron avalanche, which will further intensify the reactions. Therefore, the presence of nitrogen impurities can simplify the reaction pathways and intensify the reactions. As a result, the presence of nitrogen impurities can decrease the breakdown voltage remarkably.

Thermalization dynamics in a pulsed microwave plasma-enhanced laminar flame

Energy transfer in a pulsed-microwave enhanced flame is investigated using hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) to monitor both vibrational and rotational temperatures of nitrogen in an atmospheric pressure laminar premixed natural gas/air stagnation flame. Temperatures were measured throughout the laminar flame structure following a 30-kW peak power, 2 μs duration, 3 GHz microwave pulse in a resonant waveguide cavity. CARS measurements show a delayed increase in vibrational temperature, indicating energy loading via electron impact and subsequent energy cascade. Vibrational energy thermalization was observed over timescales faster than transport through the flame zone, but slower than predicted by known vibrational-translational rates, suggesting a long-lived pathway for increased vibrational temperature. Peak vibrational temperature increases of 100 K were observed and thermalize over 100′s of microseconds, resulting in a measurable increase in the rotational temperature over the same time interval. The magnitude of vibrational excitation and rate of thermalization in such plasma-assisted combustion environments is critical for applications including combustion ignition and control, and hybrid fs/ps CARS measurements provide the necessary detail on vibrational-translational relaxation processes of ground state nitrogen.

Temperature measurements of the background gas and metastable argon atoms in N2/Ar nanosecond repetitively pulsed discharges

Characterization of plasma properties that underpin kinetic processes in nanosecond repetitively pulsed discharges (NRPDs) is necessary to understand and manipulate the behavior of these discharges for a wide variety of applications. Here, the neutral gas temperature in N2 and 50% N2/50% Ar NRPDs during the discharge is determined by characterizing the rotational temperature of rovibrational spectra from the N2 2nd positive system. At the conditions investigated, it is shown that the timescale for rotational–translational relaxation is shorter than the effective lifetime of the N2(C) state, thereby, rendering the rotational temperature measurements a reasonable representation of the background gas temperature. The measurements show that the translational temperature of ground state nitrogen molecules does not increase significantly above ambient temperature during the discharge generated at a constant pressure of 20 Torr, 10 kHz pulse repetition frequency, and pulse energy of 50 μJ. An absorption based detection technique with a 2 ns time-resolution used to measure the translational temperature history of the metastable argon atoms (Ar(1s3)) in the N2/Ar NRPDs shows that the Ar(1s3) atoms and the neutral background gas are in thermal non-equilibrium during the discharge. Furthermore, the addition of nitrogen gas is shown to significantly reduce the translational energy enhancement of the metastable argon atoms produced in the N2/Ar discharges compared to that in pure argon discharges.

The dynamic evolution and interaction with dielectric material of the discharge in packed bed reactor

In this paper, dielectric columns with different dielectric constants are employed as dielectric materials in the packed bed reactor to investigate the dynamic behaviors of plasma interaction processes. The effects of the dielectric constants (zirconia: ε = 25 and PTFE: ε = 2.5) on the production of reactive species are studied for plasma catalysis applications. Comparison studies of discharge images, electrical characteristics, discharge dynamic evolution and spatial-temporal resolved optical emission spectroscopy are carried on when zirconia and PTFE columns are employed. The results show that there are four discharge processes existing in the packed bed reactor: surface streamer on the dielectric column, local discharge at the contact point, surface discharge on the grounded dielectric plate, and the volume discharge. The production of reactive species such as N2(C3Пu), N2+(B2Σu+) and O(3p5P) depend on the discharge processes to a great extent. The production of the N2+(B2Σu+) always accompanies the formation of the streamer by electrons direct impact process to excite the ground state nitrogen molecules to N2+(B2Σu+). The O(3p5P) is generated in two different ways, which plays a major role during the voltage pulse raising and falling time, respectively. The first way is the direct and fast one-step ionization and excitation by high energy electrons with O2. The second way is the energy transfer from the nitrogen metastable N2(A3Σu+) and energetic electrons, in which the O is first ionized from O2 and then excited to O(3p5P). Furthermore, compared with a zirconia column, a PTFE column is more conductive to the generation of reactive species.

A Filamentary Plasma Jet Generated by Argon Dielectric-Barrier Discharge in Ambient Air

A filamentary plasma jet was produced by a cylindrical dielectric-barrier discharge (DBD) equipped with a thin quartz tube. This plasma jet consists of many filaments, and the filaments bifurcate into thinner branches close to the ground electrode. Only a single filament is produced in each current pulse, and this filament is distorted in the long gas gap. The maximum electron density in the filament is on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Optical emission examination shows a decrease of optical emissions from OH and Ar but an increase of optical emissions from N2 along the plasma jet. The vapor in the working gas argon rather than in the ambient air dominates the optical emissions from the de-excitation of OH along the plasma jet. Near the ground electrode, the nitrogen emission spectra mainly resulted from the direct electron effect on the ground state or metastable nitrogen and the collisions between nitrogen metastables. The rotational temperature is lower than the vibrational temperature but much higher than the room temperature. This feature makes the filamentary plasma jet only suitable for processing samples bearing high temperature.

De-Excitation of Excited States in a Nitrogen Switch After Discharge

The de-excitation of nitrogen excited states after spark gap discharge was investigated with time-resolved optical emission spectroscopy method. Recombination of ionized particles was experimentally proved less than several tens microseconds. After the recombination process, it was shown that atomic nitrogen in ground state [N(4S)] and molecular nitrogen in first excited state [N2( $\text{A}^{3} {\Sigma }_{u}^{+}$ )] was long-lived particles (metastable state) after gas discharge. Characteristics of decay for N(4S) and [N2( $\text{A}^{3} {\Sigma }_{u}^{+}$ )] were studied, and their lifetime and decay of density were estimated in this paper. Peculiar unexponential decay of N2( $\text{A}^{3} {\Sigma }_{u}^{+}$ ) showed there was reproduction process of N2( $\text{A}^{3} {\Sigma }_{u}^{+}$ ) after gas discharge. N(4S) could be the source for reproduction process under their mechanism of three-body recombination. Low ionization energy and high ionization collision cross section of N2( $\text{A}^{3} {\Sigma }_{u}^{+}$ ) make it a possible factor limiting the switch recovery. The de-excitation of metastable states opens novel perspectives for understanding the recovery of gas switch.

Mechanism of N2 dissociation and kinetics of N(4S) atoms in pure nitrogen plasma

This work deals with kinetics of the ground state nitrogen atoms N(4S) and N2 dissociation mechanism in pure nitrogen plasma. The experiment was carried out in positive column of DC glow discharge at a range of parameters p = 5 - 50 Torr, J = 20 - 100 mA. The use of axial homogeneous glow discharge allowed considering N(4S) balance for spatially uniform conditions controlled by only two terms: source (characterized by effective production rate kdisseff) and loss (characterized by effective loss time τNloss). Analysis of these parameters gains considerably better understanding of N2 dissociation mechanism in pure nitrogen plasma that was the main goal of the given work. So N/N2 dissociation rate as function of discharge parameters was obtained using two independent emission optical methods: actinometry on Ar atoms and N2 2+ band emission decay at discharge modulation. Measurements of N/N2 radial profiles allowed estimating N atom surface loss probability γNloss and correspondingly τNloss. It was revealed that γNloss depends on N(4S) concentration and thereby discharge conditions through the sorption balance of physisorbed N atoms. Simple phenomenological model taking into account basic surface processes provides γNloss data in good agreement with experiment. Finally, kdisseff was obtained as function of reduced electric field E/N and it was shown that even EEDF self-consistently calculated with accounting for N2 vibrational excitation is unable to provide observed values of kdisseff. Reasons of that fact are discussed in detail.

Isomerization and fragmentation of acetonitrile upon interaction with N(4S) atoms: the chemistry of nitrogen in dense molecular clouds

We experimentally show that the reaction between ground state nitrogen atoms N(4S) and acetonitrile CH3CN can lead to two distinct chemical pathways that are both thermally activated at very low temperatures. First is CH3CN isomerization which produces CH3NC and H2CCNH. Second is CH3CN decomposition which produces HNC and CH3CNH+CN− fragments, with the possible release of H2. Our results reveal that the mobility of N(4S)-atoms is stimulated in the 3–11 K temperature range, and that its subsequent encounter with one acetonitrile molecule is sufficient for the aforementioned reactions to occur without the need for additional energy to be supplied to the CH3CN + N(4S) system. These findings shed more light on the nitrogen chemistry that can possibly take place in dense molecular clouds, which until now was thought to only involve high-energy processes and therefore be unlikely to occur in such cold and dark interstellar regions. The reaction pathways we propose in this study have very important astrochemical implications, as it was shown recently that the atomic nitrogen might be more abundant, in many interstellar icy grain mantles, than previously thought. Also, these reaction pathways can now be considered within dense molecular clouds, and possibly affect the branching ratios for N-bearing molecules computed in astrochemical modelling.

Formation of simple nitrogen hydrides NH and NH2 at cryogenic temperatures through N + NH3→ NH + NH2 reaction: dark cloud chemistry of nitrogen.

Although NH3 molecules interacting with ground state nitrogen atoms N((4)S) seem not to be a very reactive system without providing additional energy to initiate the chemical process, we show through this study that, in the solid phase, at very low temperature, NH3 + N((4)S) reaction leads to the formation of the amidogen radical NH2. Such a dissociation reaction previously thought to occur exclusively through UV photon or energetic particle irradiation is in this work readily occurring just by stimulating the mobility of N((4)S)-atoms in the 3-10 K temperature range in the solid sample. The N((4)S)-N((4)S) recombination may be the source of metastable molecular nitrogen N2(A), a reactive species which might trigger the NH3 dissociation or react with ground state nitrogen atoms N((4)S) to form excited nitrogen atoms N((4)P/(2)D) through energy transfer processes. Based on our obtained results, it is possible to propose reaction pathways to explain the NH2 radical formation which is the first step in the activation of stable species such as NH3, a chemical induction process that, in addition to playing an important role in the origin of molecular complexity in interstellar space, is known to require external energy supplies to occur in the gas phase.

Diffusion and reactivity of ground-state nitrogen atoms N(4S) between 3 and 15 K: application to the hydrogen abstraction reaction from methane under non-energetic conditions

Diffusion and reactivity of ground-state nitrogen atoms N(4S) between 3 and 15 K: application to the hydrogen abstraction reaction from methane under non-energetic conditions

Kinetics of positive ions and electrically neutral active particles in afterglow in neon at low pressure

Kinetics of positive ions and electrically neutral active particles formed during breakdown and successive discharge in neon-filled tube at 6.6 millibars pressure had been analyzed. This analysis was performed on the basis of mean value of electrical breakdown time delay t¯d dependence on afterglow period τ (memory curve). It was shown that positive ions are present in the 1μs &amp;lt; τ &amp;lt; 30 ms interval, which is manifested through t¯d slow increase with the increase of τ. A rapid t¯d increase in the 30 ms &amp;lt; τ &amp;lt; 3 s interval is a consequence of significant decrease of positive ions concentration and dominant role in breakdown initiation have ground state nitrogen atoms, which further release secondary electrons from the cathode by catalytic recombination process. These atoms are formed during discharge by dissociation of ground state nitrogen molecules that are present as impurities in neon. For τ &amp;gt; 3 s, breakdown is initiated by cosmic rays and natural radioactivity. The increase of discharge current leads to decrease of t¯d due to the increase of positive ions concentration in inter electrode gap. The increase of applied voltage also decreases t¯d for τ &amp;gt; 30 ms due to the increase of the probability for initial electron to initiate breakdown. The presence of UV radiation leads to the decrease of t¯d due to the increased electron yield caused by photoelectrons. The influence of photoelectrons on breakdown initiation can be noticed for τ &amp;gt; 0.1 ms, while they dominantly determine t¯d for τ &amp;gt; 30 ms.

Processes in afterglow responsible for initiation of electrical breakdown in xenon at low pressure

AbstractThe processes responsible for initiation of electrical breakdown in xenon-filled tube with two spherical iron electrodes at 2.7-mbar pressure have been analyzed. The analysis is based on the experimental data of electrical breakdown time delay as a function of afterglow period. It is shown that positive ions remaining from previous discharge, as well as positive ions created in mutual collisions of metastable atoms in afterglow, have a dominant role in secondary emission of electrons from the cathode which lead to initiation of breakdown in early afterglow. In late afterglow, dominant role in initiation of breakdown is taken by N(4S) atoms formed during the discharge by dissociation of ground state nitrogen molecules that are present as impurities in xenon. When the concentration of N(4S) atoms decreases sufficiently, the initiation of breakdown is caused by cosmic radiation. Small doses of gamma-ray irradiation also contribute to the initiation of breakdown, but only for large values of the afterglow period.

Vibrational and rotational CARS measurements of nitrogen in afterglow of streamer discharge in atmospheric pressure fuel/air mixtures

The use of nonequilibrium plasma generated by nanosecond discharges to ignite fuel/air mixtures, known as transient plasma ignition (TPI), has been shown to effectively reduce ignition delay and improve engine performance relative to spark ignition for combustion engines. While this method is potentially useful for many engine applications, at present the underlying physics are poorly understood. This work uses coherent anti-Stokes Raman spectroscopy (CARS) to measure the rotational and vibrational excitation of nitrogen molecules in the discharge afterglow in a variety of fuel/air mixtures outside the limits of combustion in order to elucidate the thermal behaviour of TPI. The time evolution of relative populations of vibrationally excited states of nitrogen in the electronic ground state are reported for each gas mixture; it is shown that generation of these vibrationally excited states is inefficient during the discharge in air but that generation occurs at a high rate roughly 5 µs following the discharge; with the addition of fuels vibrationally excited states are observed during the discharge but an increase in population is still seen at 5 µs. Possible mechanisms for this behaviour are discussed. In addition, rotational temperature increases of at least 500 K are reported for all gas mixtures. The effect of this temperature increase on ignition, reaction rates, and thermal energy pathways are discussed.

Plasma diagnostics of barrier-torch discharge in argon flow in a capillary by cross-correlation spectroscopy

The radiation kinetics of the plasma of barrier-torch disrcharge in argon flow in a capillary has been studied by cross-correlation spectroscopy. It was established that the discharge emission spectrum consists of peaks of electronically excited states of argon, bands of hydroxyl radicals, and a second positive system of nitrogen. An analysis of the spatio-temporal distributions of emission intensity for the selected spectral indicators showed that the causes of the torch are ionization waves that extend through the capillary from the electrode system with a speed of 105 m/s and project up to 3–4 mm. It was established that the formation of electronically excited molecules of nitrogen N2(C3Πu) in the torch of discharge occurs mainly on the reaction between metastable electronically excited atoms of argon and molecules of nitrogen in the electronic ground state.

State-to-state modeling of a recombining nitrogen plasma experiment

An atmospheric pressure nitrogen/argon plasma flows at high velocity through a water-cooled test-section in which it is forced to recombine within 250μs. At the test-section inlet, the plasma is in Local Thermodynamic Equilibrium (LTE) at about 7200K. Because recombination rates are finite, the plasma reaches a state of chemical nonequilibrium at the exit of the test-section, at a temperature of about 4715K. At the test-section exit, the radiation emitted by the plasma, measured by emission spectroscopy, shows significant departures from equilibrium in the populations of the excited electronic states of nitrogen (N2 B3Πg, N2 C3Πu) and of the nitrogen ion N2+B2Σu+. This experiment is thus proposed as a test-case to validate collisional-radiative (CR) models. A vibrationally specific CR model is then used to predict the emission of these states. The rate coefficients of the model are calculated with the Weighted Rate Coefficient method based on elementary cross-section data. These rates depend explicitly on the kinetic temperatures of electrons (Te) and heavy species (Tg). The predictions of the CR model are in good agreement with the measured vibrational population distribution in the N2 B state. A method is then proposed to determine ground state nitrogen atom densities based on the measured vibrational population distribution of the N2 B state.

Absolute ground-state nitrogen atom density in a N2/CH4 late afterglow: TALIF experiments and modelling studies

Following a first study on a late afterglow in flowing pure nitrogen post discharge, we report new two-photon absorption laser-induced fluorescence (TALIF) measurements of the absolute ground-state atomic nitrogen density N(4S) and investigate the influence of methane introduced downstream from the discharge by varying the CH4 mixing ratio from 0% up to 50%. The N (4S) maximum density is about 2.2 × 1015 cm−3 in pure N2 for a residence time of 22 ms and does not change significantly for methane mixing ratio up to ∼15%, while above, a drastic decrease is observed. The influence of the residence time has been studied.A kinetic model has been developed to determine the elementary processes responsible for the evolution of the N (4S) density in N2/CH4 late afterglow. This model shows the same decrease as the experimental results even though absolute density values are always larger by about a factor of 3. In the late afterglow three-body recombination dominates the loss of N (4S) atoms whatever the CH4 mixing ratio. For high CH4 mixing ratio, the destruction process through collisions with CH3, H2CN and NH becomes important and is responsible for the observed decrease of the N (4S) density.

Detection of metastable excited molecules N2(A3Σu+) in an atmospheric pressure nitrogen discharge by Raman scattering

AbstractRaman scattering from metastable excited nitrogen molecules N2(A3Σu+) created by an impulse discharge in nitrogen at atmospheric pressure was detected. A pulsed Nd:YAG laser at a wavelength of 266 nm was used as the light source, and Raman scattering from N2(A3Σu+) at a wavelength of 277 nm was detected using an interference filter and photomultiplier tube. The filter had sufficient rejection of Rayleigh scattering of laser light at a wavelength of 266 nm and of Raman scattering from ground‐state nitrogen molecules N2(X1Σg+) at a wavelength of 284 nm. The temporal variation of the signal intensity of Raman scattering from N2(A3Σu+) was measured by transmitting the laser light at different time delays relative to sparkover. The results showed that the signal intensity decayed with a time constant of about 200µs. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(6): 34–40, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10205

Characterization of an N2 flowing microwave post-discharge by OES spectroscopy and determination of absolute ground-state nitrogen atom densities by TALIF

A flowing microwave post-discharge source sustained at 2.45 GHz in pure nitrogen has been investigated by optical emission spectroscopy (OES) and two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. Variations of the optical emission along the post-discharge (near, pink and late afterglow) have been studied and the gas temperature has been determined. TALIF spectroscopy has been used in the late afterglow to determine the absolute ground-state nitrogen atomic densities using krypton as a reference gas. Measurements show that the microwave flowing post-discharge is an efficient source of N (4S) atoms in late afterglow. In our experimental conditions, the maximum N (4S) density is about 2.2 × 1015 cm−3 for a pressure of 22 Torr, at 300 K. The decay of N (4S) density as a function of the time spent in the quartz tube has been modelled and a wall recombination probability γ of (2.1 ± 0.3) × 10−4 is obtained.