The Influence of Small Hydrogen Admixtures up to 5?% to a Low Pressure Nonuniform Microwave Discharge in Nitrogen

We present the results of spectroscopic and 2D modeling study of the influence of hydrogen addition to non-uniform nitrogen plasma of electrode microwave discharge. The axial intensity distributions of the H_?? line and N₂ (2⁺: C³??_u????B³??_g, 1⁺: B³??_g????A³?? _u ⁺ ) and N₂ ⁺ (1^?: B²?? _u ⁺ ???X²?? _g ⁺ ) molecular bands are recorded for different incident microwave power input and hydrogen content. The corona model is used to determine electric field strength in nitrogen discharge from the intensity ratio of 2⁺ and 1^? system of nitrogen bands. By means of 2D modeling spatial distributions of nitrogen molecules in C³??_u state, microwave field strength, electron density, concentrations of N₂ ⁺, N₄ ⁺ are determined in nitrogen and in nitrogen?Chydrogen mixtures. The concentration of N₂H⁺ ions in nitrogen?Chydrogen mixtures is determined also. The general conclusion of 2D modeling is in agreement with experimental results and shows that the influence of hydrogen addition to discharge is related to the fast conversion reactions of nitrogen ions (N₂ ⁺, N₄ ⁺) to N₂H⁺ ion. These ion conversions lead to the change of ion plasma components transport properties, to the modification of microwave field strength in plasma, and consequently, to the alternation of all plasma parameters.     

Four‐bond deuterium isotope effects on the chemical shifts of amide nitrogens in proteins

An approach towards precision NMR measurements of four‐bond deuterium isotope effects on the chemical shifts of backbone amide nitrogen nuclei in proteins is described. Three types of four‐bond ¹⁵ N deuterium isotope effects are distinguished depending on the site of proton‐to‐deuterium substitution: ⁴ΔN(N_i‐1D), ⁴ΔN(N_i+1D) and ⁴ΔN(C_β,i‐1D). All the three types of isotope shifts are quantified in the (partially) deuterated protein ubiquitin. The ⁴ΔN(N_i+1D) and ⁴ΔN(C_β,i‐1D) effects are by far the largest in magnitude and vary between 16 and 75 ppb and ?18 and 46 ppb, respectively. A semi‐quantitative correlation between experimental ⁴ΔN(N_i+1D) and ⁴ΔN(C_β,i‐1D) values and the distances between nitrogen nuclei and the sites of ¹H‐to‐D substitution is noted. The largest isotope shifts in both cases correspond to the shortest inter‐nuclear distances. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling of an Atmospheric Pressure Nitrogen Glow Discharge Operating in High-Gas Temperature Regimes

A model of an atmospheric pressure nitrogen glow discharge in high-gas temperature regimes is developed. The model considers a fairly complete set of chemical reactions, including several processes with the participation of electronically exited nitrogen atoms describing the energy balance and charged particles kinetic processes in the discharge. It is shown that the thermal dissociation of vibrationally excited molecules plays an essential role in the production of N(⁴ S) atoms. The dominant ion within the investigated current range (52–187 mA) is the molecular N₂ ⁺ with an increasing proportion of atomic N⁺ towards high-current values. The process of production of electrons within the almost whole current range is controlled predominantly by associative ionization in atomic collisions N(² P) + N(² P) → N₂ ⁺ + e; being the N(² P) atoms mainly produced via quenching of N₂(A ³∑ _u ⁺ ) electronically excited molecules by N(⁴ S) atoms. The results of calculations are compared with the available experimental data and a good agreement is found.     

Ammonia-filled discharge tubes for optical emission spectrometric determination of 15N : 14N isotope ratios

A procedure is described for filling a discharge tube, permanently attached to a vacuum line, with ammonia in the pressure range 1.5–5 Torr, without any carrier gas. The ¹⁵N : ¹⁴N isotope ratio is determined from the N₂ spectrum emitted when the tube is excited by a 2450-MHz microwave source. Cooling one end of the discharge tube to —60°C enables the N₂ (1,0) bandheads at 316 nm to be used for ¹⁵N contents down to about 0.04 atom-%. Unidentified interfering bands and emission from NH, OH, H₂ and H are discussed. Samples containing 1–30 mg of nitrogen can be analysed with an accuracy and precision suitable for most soil—plant studies employing ¹⁵N as a tracer.     

Vibrational analysis of N2(B 3Πg and C 3Πu) and CO(X) excited in N2 discharge and post discharge

A spectroscopic characterization of a N₂ radiofrequency discharge and N₂CO post discharge has been performed. The relative vibrational distribution of the excited B ³Π_g and C ³Π_u states of nitrogen and their correlation with the ground state have been analyzed. The analysis confirms the importance of the metastable molecules. N₂(A ³Σ⁺_u), in affecting the vibrational distribution of nitrogen in its ground state in the discharge and post discharge. The vibrational analysis of the CO ground state, excited in the post discharge by vibrationally excited N₂ molecules, confirms the high degree of vibrational non-equilibrium in the ground state of nitrogen, in the presence of a low first-level vibrational temperature.     

One-Step Synthesis of Silicon Oxynitride Films Using a Steady-State and High-Flux Helicon-Wave Excited Nitrogen Plasma

A steady-state and high-flux helicon-wave excited N₂ plasma was used to oxynitride Si substrates for the synthesis of silicon oxynitride (SiON) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been extensively used to characterize surface quality of the SiON films, and it is found that a large amount of nitrogen (N) can be incorporated into the films. The result of XPS depth profiles shows that the N concentration is high near the surface and the oxide/Si interface. In the UPS spectra, absence of the reappearance of surface states suggests a resistance to clustering of the oxynitride layer. The N₂ flux and Ar mixture quantity can facilitate tuning of the dissociation characteristics in N₂ discharge. By modulating the N₂ fractions, the N⁺ density reaches maximum at a N₂/(N₂ + Ar) flow-rate ratio of 0.5, resulting in incorporation of more N atoms into the SiON films. Considering the easy control of N₂ plasma, our work opens up a new avenue for achieving high-yield SiON films at low temperature.     

Numerical Investigation of a Parallel-Plate Atmospheric-Pressure Nitrogen/Ammonia Dielectric Barrier Discharge

In this paper, a planar atmospheric-pressure dielectric barrier discharge (AP-DBD) of nitrogen mixed with ammonia (0?C2?%) is simulated using one-dimensional self-consistent fluid modeling with cell-centered finite-volume method. This AP-DBD is driven by a 30?kHz power source with distorted sinusoidal voltages. The simulated discharge current densities are found to be in good agreement with the experiment data in both phase and magnitude. The simulated results show that the discharges of N₂ mixed with NH₃ (0?C2?%) are all typical Townsend-like discharges because the ions always outnumber the electrons very much which leads to no quasi-neutral region in the gap throughout the cycle. N₂ ⁺ and N₄ ⁺ are found to be the most abundant charged species during and after the breakdown process, respectively, like a pure nitrogen DBD. NH₄ ⁺ increases rapidly initially with increasing addition of NH₃ and levels off eventually. In addition, N is the most dominant neutral species, except the background species, N₂ and NH₃, and NH₂ and H are the second dominant species, which increase with increasing added NH₃. The existence of abundant NH₂ plays an important role in those applications which require functional group incorporation.     

Effect of excitation states of nitrogen on the surface nitridation of iron and steel in d.c. glow discharge plasma

The plasma nitriding phenomena that occur on the surfaces of iron and steel were investigated. In particular, the correlation between the kinds of nitrogen radicals and the surface nitriding reaction was investigated using a glow‐discharge apparatus. To control the excitation of nitrogen radicals, noble gas mixtures were used for the plasma gas. The highly populated metastables of noble gases selectively produce excited nitrogen molecules (N₂*) or nitrogen molecule ions (N₂⁺). The optical emission spectra suggested that the formation of N₂*‐rich or N₂⁺‐rich plasma was successfully controlled by introducing different kinds of noble gases. Auger electron spectroscopy and XPS were used to characterize the depth profile of the elements and chemical species on the nitrided surface. The nitride layer formed by a N₂⁺‐rich plasma had a much higher nitrogen concentration than that by a N₂*‐rich plasma, likely due to the larger chemical activity of the N₂⁺ species as well as the N₂⁺ sputtering bombardment to the cathode surface. The strong reactivity of the N₂⁺ species was also confirmed from the chemical shift of N 1s spectra for iron nitrides. An iron nitride formed by the N₂⁺‐rich plasma has higher stoichiometric quantity of nitrogen than that formed by the N₂*‐rich plasma. Besides the effect of nitrogen radicals on surface nitridation, the contribution of the chromium in steel to the nitriding reaction was also examined. This chromium can promote a nitriding reaction at the surface, which results in an increase in the nitrogen concentration and the formation of nitride with high nitrogen coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

Dissolution of hydrogen and nitrogen in silicon

Deuterium and¹⁵N were used as activable tracers for the study of the dissolution of hydrogen and nitrogen in silicon. Silicon was heated or zone-melted in D₂-Ar, or heated in¹⁵N₂-Ar after being covered with Si₃ ²⁵N₄. Depth profiles of D or¹⁵N in the resultant silicon samples were measured by the D(³He, p)⁴He or¹⁵N(, n)¹⁸F reaction combined with repeated HF–HNO₃ etching. These two measurements have proved to be highly reliable and sensitive and to offer useful techniques in the study of trace concentrations of hydrogen and nitrogen in solid matrices.     

Influence of the Excited States of Atomic Nitrogen N(<Superscript>2</Superscript>D), N(<Superscript>2</Superscript>P) and N(R) on the Transport Properties of Nitrogen. Part II: Nitrogen Plasma Properties

In this paper, the calculated values of the viscosity and thermal conductivity of nitrogen plasma are presented taking into account five (e, N, N⁺, N₂ and N₂⁺) or eight (e, N(⁴S), N(²P), N(²D), N(R), N⁺, N₂ and N₂⁺) species. The calculations are based on the supposition that the temperature dependent probability of occupation of the states is given by the Boltzmann factor. The domain for which the calculations are performed, is for p = 1 and 10 atm in the temperature range from 5,000 K to 15,000 K. Classical collision integrals are used in calculating the transport coefficients and we have introduced new averaged collision integrals where the weight associated at each interacting species pair is the probable collision frequency. The influence of the collision integral values and energy transfer between two different species is studied. These results are compared which those of published theoretical studies.     

Optical Emission Spectroscopy of Abnormal Glow Region in Nitrogen Plasma

Optical emission spectroscopy of the active species in N₂ plasma is carried out to investigate their concentration as a function of discharge parameters such as filling pressure (2.0–7.0 mbar), source power (100–200 W) and gas flow rate (50–300 mg/min). The primary motivation of this work is to obtain reliable information about the concentration of the active species of N₂ plasma, which play an important role in plasma surface nitriding processes. Emission intensity from the selected electronic excited states of molecular and atomic species is evaluated as a function of discharge parameters to investigate their concentration. The emission intensity ratio I(N₂⁺)/I(N₂) and I(N⁺)/I(N) of the electronic transitions is also evaluated as a function of discharge parameters to investigate the relative dependence of their concentrations. It is observed that the concentration of the active species of N₂ plasma is strongly affected by the filling pressure and source power whereas flow rate has no significant effect. An increased occurrence of N₂⁺ molecular ions in comparison with N₂ molecules, and N⁺ ions in comparison with N atoms is observed with source power whereas decreased occurrence of N₂⁺ molecular ions in comparison with N₂ molecules, and N⁺ ions in comparison with N atoms is observed with the rise in filling pressure.     

Characterization of a nitrogen atom beam by magnetic selection analysis

A nitrogen atom beam produced by a He-N₂ microwave discharge has been characterized using magnetic selection and mass spectrometry. The relative composition N/N₂ in the beam is ≈ 9%. The nitrogen atoms are nearly all in the two metastable states ²D_J and ²P_J, with relative population ≈ 2.5 : 1.     

Combined Application of Optical Emission Spectroscopy and Kinetic Numerical Modelling to Determine the Ions Densities in a Flowing N<Subscript>2</Subscript> Post-Discharge

The combined application of optical emission spectroscopy (OES) and kinetic numerical modelling was employed to determine the N₂⁺(X²\( \Sigma_{\text{g}}^{ + } \)), N₃⁺, and N₄⁺ densities in the post-discharge (pink afterglow; PA) of a nitrogen flowing DC discharge. We measured the relative densities of the N₂(C³Π_u) and N₂⁺(B²\( \Sigma_{\text{u}}^{ + } \)) states along the post-discharge region by OES. The density values were attained as functions of the post-discharge residence time. We fitted the experimental densities with densities calculated from a kinetic numerical model developed to calculate the temporal density of several nitrogen species in the nitrogen afterglow. Analysis of the rate balance equations of these ions indicated that these densities can be determined from data generated from both the model and experimental N₂⁺(B²\( \Sigma_{\text{u}}^{ + } \)) density. Thus, we determined the ions density profiles in the nitrogen post-discharge and observed that the N₃⁺ density is dominant in the PA. This is followed by that of the N₂⁺(X²\( \Sigma_{\text{g}}^{ + } \)) and N₄⁺ ions. Such behaviour has been previously reported in a study that employed mass spectrometry to analyse the ions in the PA generated by a nitrogen high-frequency discharge. In our study, the DC discharge was operated at a gas flow rate of 0.9 Slm^?1, a discharge current of 30 mA, and a gas pressure range of 400–700 Pa.     

15N/14N Isotopic Exchange in the Dissociative Adsorption of N2 on Tantalum Nitride Cluster Anions Ta3N3-

Adsorption and activation of dinitrogen (N₂) is an indispensable process in nitrogen fixation. Metal nitride species continue to attract attention as a promising catalyst for ammonia synthesis. However, the detailed mechanisms at a molecular level between reactive nitride species and N₂ remain unclear at elevated temperature, which is important to understand the temperature effect and narrow the gap between the gas phase system and condensed phase system. Herein, the ¹⁴N/¹⁵N isotopic exchange in the reaction between tantalum nitride cluster anions Ta₃¹⁴N₃^- and ¹⁵N₂ leading to the regeneration of ¹⁴N₂/¹⁴N¹⁵N was observed at elevated temperature (393-593 K) using mass spectrometry. With the aid of theoretical calculations, the exchange mechanism and the effect of temperature to promote the dissociation of N₂ on Ta3N3? were elucidated. A comparison experiment for Ta₃¹⁴N₄^-/¹⁵N₂ couple indicated that only desorption of ¹⁵N₂ from Ta₃¹⁴N₄¹⁵N₂^- took place at elevated temperature. The different exchange behavior can be well understood by the fact that nitrogen vacancy is a requisite for the dinitrogen activation over metal nitride species. This study may shed light on understanding the role of nitrogen vacancy in nitride species for ammonia synthesis and provide clues in designing effective catalysts for nitrogen fixation.     

Setting up of an Isotope Analyser for Quantitative Determination of Percentage Abundance of 15N in Nitrogen Gas

Abstract

An ¹⁵N isotope analyser has been assembled for the quantitative determination of the percentage abundance of ¹⁵N isotope in nitrogen gas employing molecular optical emission spectroscopic technique. The (2,0) band of the second positive system of N₂ was excited in a high frequency discharge. The band heads of the isotopic species ¹⁴N¹⁴N and ¹⁴N¹⁵N molecules were resolved using a 0.5 m monochromator and their intensity peaks were measured. The ratio of the peak heights enabled the quantitative determination of ¹⁵N in the N₂ sample. The performance of the isotope analyser assembled was evaluated and it was found to be quite good as inferred from the analysis of primary standards. The analytical error in the ¹⁵N concentration-range 0.36 to 24% is <6% and rsd is <4%. A salient feature of the method adopted is that it is direct and does not call for the use of comparision standards.     

A Cornucopia of Iridium Nitrogen Compounds Produced from Laser-Ablated Iridium Atoms and Dinitrogen

The reaction of laser-ablated iridium atoms with dinitrogen molecules and nitrogen atoms yield several neutral and ionic iridium dinitrogen complexes such as Ir(N₂), Ir(N₂)⁺, Ir(N₂)₂, Ir(N₂)₂⁻, IrNNIr, as well as the nitrido complexes IrN, Ir(N)₂ and IrIrN. These reaction products were deposited in solid neon, argon and nitrogen matrices and characterized by their infrared spectra. Assignments of vibrational bands are supported by ab initio and first principle calculations as well as ^14/15N isotope substitution experiments. The structural and electronic properties of the new dinitrogen and nitrido iridium complexes are discussed. While the formation of the elusive dinitrido complex Ir(N)₂ was observed in a subsequent reaction of IrN with N atoms within the cryogenic solid matrices, the threefold coordinated iridium trinitride Ir(N)₃ could not be observed so far.     

Design of Advanced MnO/N‐Gr 3D Walls through Polymer Cross‐Linking for High‐Performance Supercapacitor

Three‐dimensional, vertically aligned MnO/nitrogen‐doped graphene (3D MnO/N‐Gr) walls were prepared through facile solution‐phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross‐links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as‐prepared 3D MnO/N‐Gr hybirdes provide a large surface area (91.516 m² g^?1) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g^?1 at 0.25 A g^?1 and an excellent charge/discharge stability (93.7 % capacity retention after 8000 cycles) in aqueous 1 m Na₂SO₄ solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N‐Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg^?1 and a power density of 437.5 W kg^?1.     

Metal‐Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N?N bond energy. Photochemical strategies that drive N₂ fixation are scarcely developed. Here, the synthesis of a dinuclear N₂‐bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H⁺/2 e^? transfer of the pincer ligand. A three‐step cycle is demonstrated for N₂ to nitrile fixation that relies on electrochemical reduction, photochemical N₂‐splitting and thermal nitrogen transfer.     

活性氮与碘乙烷反应化学发光研究

在流动余辉装置上, 利用N₂空心阴极放电制备活性氮, 研究了活性氮与碘乙烷(C₂H₅I)反应的化学发光. 在620～820 nm波长范围内观察到了较强的发射光谱, 拟合得到的光谱常数表明它来源于NI(b¹Σ^＋→X³Σ^－)跃迁, 并对35个谱峰进行了振动归属. 最后讨论了活性氮中主要成分与C₂H₅I反应的可能过程, 结合辅助性实验分析表明, 活性氮中的N(²P)与C₂H₅I直接反应很可能产生激发态NI(b¹Σ^＋)自由基. 这是利用化学反应直接产生激发态NI(b¹Σ^＋)的首次报道, 观察到的激发态最高振动能级为v'＝6.     

Dynamic 15N NMR studies of iron phosphine complexes containing coordinated dinitrogen

The ¹⁵N‐labelled iron dinitrogen complexes trans‐[FeH(N₂)(PP)₂]⁺[BPh₄]^? (PP = dppe, depe, dmpe) and cis‐[FeH(N₂)(PP₃)]⁺[BPh₄]^? were prepared in situ by exchange of unlabelled coordinated dinitrogen with ¹⁵N₂. ¹⁵N NMR chemical shifts and coupling constants are reported. The ¹⁵N spectra exhibit separate signals for the metal‐bound and terminal nitrogen atoms of the coordinated N₂. The ¹⁵N resonances display ¹⁵N, ¹⁵N coupling as well as ³¹P, ¹⁵N coupling and long‐range ¹⁵N, ¹H coupling when there is a metal‐bound hydrido ligand. Exchange between free and coordinated dinitrogen was monitored by magnetization transfer between ¹⁵N‐labelled sites using an inversion–transfer–recovery experiment. Exchange between the metal‐bound and terminal nitrogen atoms of coordinated N₂ was also monitored by magnetization transfer and this could proceed by N₂ dissociation or by an intramolecular process. Copyright © 2003 John Wiley & Sons, Ltd.