Hybrid Monte Carlo — Fluid model for studying the effects of nitrogen addition to argon glow discharges

A computer model is developed for describing argon/nitrogen glow discharges. The species taken into account in the model include electrons, Ar atoms in the ground state and in the 4s metastable levels, N₂ molecules in the ground state and in six different electronically excited levels, N atoms, Ar⁺ ions, N⁺, N₂⁺, N₃⁺ and N₄⁺ ions. The fast electrons are simulated with a Monte Carlo model, whereas all other species are treated in a fluid model. 74 different chemical reactions are considered in the model. The calculation results include the densities of all the different plasma species, as well as information on their production and loss processes. The effect of different N₂ additions, in the range between 0.1 and 10%, is investigated.     

Dissociation patterns in N2O following electron impact

A number of dissociation channels in N₂0 have been observed by time-of-flight spectroscopy following electron impact excitation. The metastable atoms and molecules produced were directly detected. Excited N₂ molecules were produced in the A³Σ_u⁺, B³Π_g and B′³Σ_u^? states in conjunction with ground state oxygen atoms. A number of additional dissociation channels were monitored by observing the production of oxygen and nitrogen atoms in Rydberg states. The results indicate the existence of potential minima in some of the repulsive surfaces involved.     

Room-Temperature Near-Infrared Metastable Species Measured by Phase-Sensitive Lock-in and Fourier Transform Techniques

An ultrasensitive detecting system coupled with either a lock-in or Fourier transform technique has been used to detect near-infrared (1000-2500 nm) metastable species in room temperature solutions. These species include O₂(¹Δ_g), O₂(¹σ⁺_g), the solvent induced satellite peak of molecular singlet oxygen (¹Δ_g) emission, and the triplet state of bis(triisobutylsiloxy)silicon-2,-3-naphthalocyanine (SilNC). Using the O₂(¹Δ_g) emission in benzene as a standard, the quantum yield and radiative decay rate of SiINC triplet state have been determined. Depending on types of spectral acquisition, special techniques such as phase-distinguishing and step scan capabilities were utilized. Their advantages and disadvantages are discussed.     

O n + clusters produced by the sputtering of solid oxygen

A brief survey is provided of the formation of cluster ions in the sputtering of frozen gases with keV-range energy rare gas atoms or ions, and of radiation damage in the residual solid. The sputtering of solid oxygen produces cluster ions, primarily the series O _3n+2 ⁺ . The constitution of these clusters has been investigated by observing collision-induced dissociation, laser-induced photo-dissociation, and metastable decay in a triple quadrupole mass spectrometer. It is proposed that the dominant clusters contain O ₅ ⁺ as the central ion, surrounded by molecules of ozone. Upon electronic excitation of one of the ozone molecules, its departure from the cluster competes with more extensive disintegration, presumably preceded by excitation energy transfer from the excited ozone to O ₅ ⁺ .     

Singlet oxygen in the environmental sciences

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, ³P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a¹Δ_g is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O₂(a¹Δ_g) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O₂(a¹Δ_g) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b¹Σ⁺ _g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(¹D) to O₂. At night, recombination of O(³P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a¹Δ_g and b¹Σ⁺ _g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O₂ and, especially, N₂, can generate O₂(b¹Σ⁺ _g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O₂(b¹Σ⁺ _g) by the atmospheric gases yields O₂(a¹Δ_g). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the ¹S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.     

Vacuum UV emission from reactions of metastable inert gas atoms: Chemiluminescence of ArO and ArCl

Vacuum UV chemiluminescence from bound upper states of ArO, KrO and ArCl is observed from the quenching of excited inert gas atoms by N₂O, O₃, Cl₂ and CCl₄. In these systems, the metastable ³P₂ states of Ar and Kr strongly resemble ground state alkali atoms in their chemical properties.     

Electronically excited and ionized states of the chlorine molecule

Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl₂ electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl⁺+Cl^? or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl₂ which dissociate into the ground state atoms only the ³Π_u state is not repulsive. The calculated D₀ value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl₂ laser line is confirmed to result from a ³Π_g → ³Π_u emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve ¹Σ_u⁺ and ¹Π_u states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O₂ it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 ¹Σ_u⁺), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.     

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^? process, while oxygen can be fully reduced to water by a 4 e^?/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^?. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.     

First principles studies on the interaction of O2 with X@Al12 (X = Al−, P+, C,Si) clusters

The interaction of O₂ with the doped icosahedral X@Al₁₂ (X = Al^?, P⁺, C, Si) clusters with 40 valence electrons were investigated using density functional theory methods. A different behavior exhibited between Al₁₃^? and X@Al₁₂ (X = P⁺, C, Si) when they interact with O₂. The dissociation of O₂ on Al₁₃^? is strongly dependent on spin state of oxygen molecule. But X@Al₁₂ (X = P⁺, C, and Si) is not the case. The transform of spin moment from O₂ to Al₁₃^? is much faster. Small molecularly binding energy and relatively high energy barrier show that these clusters are all reluctant reacts with the ground state O₂. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

Investigation of composition and chemical state of elements in iron boride by the method of X-ray photoelectron spectroscopy

The composition and chemical state of iron and boron in the surface layer of iron boride under different kinds of pretreatment of samples have been investigated by the method of X-ray photo-electron spectroscopy. It has been found that in the initial sample there is oxygen chemically combined with iron and boron atoms. Upon heating (450°C) in hydrogen, in argon, and in vacuo there occurs removal of oxygen only from iron atoms (no pure iron was found to be formed). Boron oxidizes and there probably appears a new surface combination of boron with oxygen in which the bonding energy of 1s electrons is higher than that in B₂O₃. Treatment of the iron boride surface with argon ions and with protons ensures uniform removal of oxygen from iron and boron atoms. It has been found that thermal treatment of iron boride leads to depletion of iron atoms from the sample surface layer, and pickling with argon ions and with protons leads to strong enrichment. Iron boride samples subjected to Ar⁺ and H⁺ bombardment tend to undergo significant oxidation when subsequently exposed to air at room temperature.     

Diatomic potential well depths from analyses of high-resolution electron energy spectra for autoionizing collision complexes

High-resolution energy spectra of electrons released in Penning ionization collisions of metastable rare gas atoms Rg*(ns) (Rg = He, Ne, Ar, Kr, Xe) with several open-shell and closed-shell atoms are analyzed to determine the well depth of the potential energy curve which describes the respective autoionizing collision complex. We thereby elucidate trends in the chemical interaction of Rg* with closed-shell target atoms A(ns ²) and establish a basis for detailed comparison with the respective interactions involving open-shell, ground state alkali atoms A(ns). From electron energy spectra due to␣associative ionization (RgH⁺ formation) in Rg* + H(1s) collisions, we determine binding energies for the RgH⁺(¹Σ) ground state potential (Rg = Ne, Ar, Kr, Xe) with uncertainties around 0.03 eV. Received: 30 June 1998 / Accepted: 5 August 1998 / Published online: 28 October 1998     

Development of resonance‐enhanced multiphoton ionization SNMS for state‐selective detection of sputtered atoms under low‐energy ion irradiation

An instrument for a sputtered neutral mass spectrometry with a quadrupole mass spectrometer (QMS) by resonance‐enhanced multiphton ionization method is developed to study sputtered neutrals emission phenomena under ion irradiation in a low‐energy region. We have prepared a pulsed primary ion beam and an ion counting system, and have optimized the operation parameter including a sample bias, energy analyzer voltages, pulsed timing of laser and ion beam, etc. A yield ratio of the lowest‐lying excited state a⁵S₂ to the ground state a⁷S₃ for sputtered Cr atoms has been measured as a function of incident energy of Ar⁺ and O₂⁺ down to 600 eV using a polycrystalline Cr sample. The yield ratio has become a constant value for the Ar⁺ incidence, while it has exponentially increased below 1 keV for the O₂⁺ incidence. It is found that the internal energy distribution of sputtered Cr atoms has been significantly influenced by oxygen density at the surface. Copyright © 2011 John Wiley & Sons, Ltd.     

Formation of Superoxo Species by Interaction of O2 with Na Atoms Deposited on MgO Powders: A Combined Continuous‐Wave EPR (CW‐EPR), Hyperfine Sublevel Correlation (HYSCORE) and DFT Study

The formation of O₂^? radical anions by contact of O₂ molecules with a Na pre‐covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O₂. The typical EPR signal of isolated Na atoms disappears when the reaction with O₂ takes place and new paramagnetic species are observed, which are attributed to different surface‐stabilised O₂^? radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O₂^?Na⁺ species to be determined, demonstrating the direct coordination of the O₂^? adsorbate to surface Na⁺ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix‐isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface‐stabilised Na⁺? O₂^? complexes.     

Kinetic study of N2(B3Πg, υ) quenching by laser-induced fluorescence

The quenching kinetics of N₂(B³Π_g, υ) have been determined in a low-pressure flow system in the absence of N atoms. Individual vibrational levels of the B state were populated by laser pumping (first-positive bands) of N₂(A³Σ_u⁺) molecules. The latter were created upstream by energy transfer from Ar metastable atoms.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

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.     

Deposition of silicon nitride thin films by reaction of SiH4 in a nitrogen post-discharge

Silicon nitride thin films are deposited on silicon wafers at room temperature when silane gas is injected in a nitrogen flowing post-discharge. Reactive processes involving siane molecules and long-lifetime nitrogen species are studied, pointing out the nonreactivity of the N₂(A³ _u ⁺ ) metastable state, the low contribution of the vibrationally excited nitrogen ground-state molecules, and the high reactivity of N(⁴S) atoms. Spectroscopic observations performed in the reaction region are correlated with thin-film characteristics.     

La-O小团簇上超氧物种与过氧物种间的连接途径

采用密度泛函理论方法考察了La-O团簇上超氧物种与过氧物种间转化的连接途径. 单重态下, 团簇上单个超氧物种可通过一系列臭氧物种转化为过氧物种, 且转化能垒较高;三重态下, 单个超氧物种则并无与过氧物种间连接的途径. 然而, La-O团簇上两超氧物种间的相互作用及其转化也具单重态和三重态两条途径. 三重态下, 超氧物种可很容易地转化为过氧物种(O₂^- + O₂^-↔O₂^2- + O₂), 超氧物种与过氧物种处于快速的交换状态之中;单重态下, 超氧物种转化为过氧物种则需较高的活化能垒, 表明在单重态下这些氧物种具有较高的稳定性.     

Microwave spectra of meta-chloroaniline

The microwave spectra of the normal, NHD and ND₂ molecules of meta-chloroaniline for both isotopic species ³⁵C? and ³⁷C? have been studied. We have identified 12 rotators corresponding to the ground state (O⁺) and the first excited vibrational state (O^?) associated with the inversion motion of amino group. The influence of the deuteration of the large amplitude motion of the amino group is estimated. The limiting geometries of the NH₂ and ND₂ meta-chloroaniline are proposed.     

Water-rich molybdotellurates: Preparation and crystal structure of Li6[TeMo6O24] · 18 H2O and Li6[TeMo6O24] · Te(OH)6 · 18 H2O

Li₆[TeMo₆O₂₄] · 18 H₂O is triclinic (space group P1 , a = 1 041.7(1), b = 1 058.6(1), c = 1 070.8(1) pm, α = 61.08(1), β = 60.44(1), γ = 73.95(1)°). Single crystal X-ray structure analysis (Z = 1, 295 K, 317 parameters, 3 973 reflections, R_g = 0.0250) revealed an infinite branched chain of edge-sharing Li coordination polyhedra to be the prominent structural feature. One of the four crystallographically independent Li⁺ is coordinated octahedrally. The coordination polyhedra of the remaining Li⁺ are distorted trigonal bipyramids. Only three unique oxygen atoms (O(9), O(10), O(12)) of the centrosymmetric [TeMo₆O₂₄]^6? anion are bound to Li⁺. The further positions in the coordination spheres of the Li⁺ are occupied by water molecules. Intermolecular hydrogen bonds involve mainly oxygen atoms of the [TeMo₆O₂₄]^6? anion as nearly equivalent proton acceptors without regard to their different bonding modes to Te and Mo, respectively. Li₆[TeMo₆O₂₄] · Te(OH)₆ · 18 H₂O crystallizes monoclinically in space group P2₁/n with Z = 4, a = 994.1(3), b = 2 344.8(10), c = 1 764.9(4) pm, and β = 91.36(4)°. Single crystal structure analysis with least squares refinement of 627 parameters (5 900 reflections, 295 K) converged to R_g = 0.0324. There are six unique Li⁺ cations. The coordination polyhedra of Li(1), Li(2), Li(3), and Li(4) are linked by common edges to yield an eight membered centrosymmetric strand. The coordination polyhedra of the remaining two Li⁺ sites (Li(5), Li(6)) are connected to a dimeric unit via a common corner. All oxygen atoms of the Te(OH)₆ molecule are involved in the coordination of Li⁺. However, only three oxygen atoms (O(13), O(18), O(23)) of the [TeMo₆O₂₄]^6? anion which lacks crystallographic symmetry are involved in the coordination of Li⁺. The oxygen atoms of the anion act as proton acceptors in hydrogen bonds of predominantly medium strength. Te(OH)₆ molecules and [TeMo₆O₂₄]^6? anions connected by strong hydrogen bonds form an infinite chain.