Neutral Nitrogen Atoms Research Articles

A diagnostic method for real-time measurements of the density of nitrogen atoms in the postglow of an Ar–N2 discharge using a catalytic probe

We determined the density of neutral nitrogen atoms in an Ar–N2 postglow using a fiber-optics catalytic probe. The probe, which had a catalyst made of nitrided iron, was calibrated with a NO titration. The recombination coefficient for the heterogeneous recombination of N atoms on the nitrided iron surface was determined by comparing the probe signal with the NO titration. Within the limits of experimental error the coefficient was found to be independent of the catalyst temperature between 400 and 650K and had a value of 0.21±0.04. Real-time measurements of the N-atom density were performed at a nitrogen flow of 600SCCM (standard cubic centimeter per minute) for several discharge powers between 80 and 300W, and for argon flow rates between 200 and 3000SCCM. With increasing discharge power the N-atom density increased monotonously; with increasing Ar flow the N-atom density at first increased, reaching a broad maximum at about 1.8×1021m−3 for an Ar flow of 2000SCCM, after which the N-atom density decreased with any further increase of Ar flow.

The activation of tertiary carboxamides in metal complexes: an experimental and theoretical study on the methanolysis of acylated bispicolylamine copper(II) complexes.

It is a well-established concept that the C-N bond cleavage of carboxamide functions is facilitated by the coordination of a metal ion to the carbonyl oxygen atom. In contrast, the alternative C-N bond activation by coordination of a neutral tertiary carboxamide nitrogen atom has not been studied. We present the first results on the effect of nitrogen pyramidalization in N-coordinated metal complexes on the methanolysis of tertiary carboxamide groups. An analysis of the reactions products obtained from the methanol cleavage of [(N-Acyl-bpa)Cu]2+ (bpa = N,N-bispicolylamine) complexes is presented together with experimental and high-level theoretically calculated structures. The strong effect of different anions on the amide pyramidalization and subsequent C-N-bond cleavage is evaluated. We show that dichloro complexes [(N-Acyl-bpa)CuCl2] have much less activated amide groups than the corresponding triflate species. They should therefore be less reactive. However, [(N-Acyl-bpa)CuCl2] complexes dissociate in solution to give cationic monochloro complexes [(N-Acyl-bpa)Cu(S)Cl]+ (S = solvent molecule). Theoretical calculations show that the amide pyramidalization in the monochloro complexes is equal to that in the corresponding CF3SO3- salts. Consequently, chloro and triflato complexes are cleaved with similar rates and efficiencies. Parallels to and differences in the reactivity of purely organic distorted amides are discussed.

A quantum Monte Carlo and density functional theory study of the electronic structure of peroxynitrite anion

Single point calculations of the ground state electronic structure of peroxynitrite anion have been performed at the optimized cis geometry using the restricted Hartree–Fock (RHF), Møller Plesset second order perturbation theory (MP2), generalized gradient approximation density functional theory (GGA DFT) in the B3LYP form and two quantum Monte Carlo (QMC) methods, variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC). These calculations reveal differences in atomization energies estimated by B3LYP (287.03 kcal/mol), MP2 (290.84 kcal/mol), and DMC, 307.4(1.9) kcal/mol, as compared to experiment, 313(1) kcal/mol. The error associated with MP2 and B3LYP methods is attributed largely to differential recovery of correlation energies for neutral nitrogen and oxygen atoms relative to the oxygen and peroxynitrite anions. In addition, basis set studies were carried out to determine potential sources of error in MP2 and B3LYP valence energy values. Our studies indicate that MP2 and B3LYP valence energies are strongly dependent on the presence of diffuse functions for the negative ions O− and ONOO−.

Direct Verification of the Core−Shell Structure of Shell Cross-Linked Micelles in the Solid State Using X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is used to probe the nanostructure of shell cross-linked micelles in the solid state. A novel ABC triblock copolymer was synthesized via atom transfer radical polymerization (ATRP) comprising quaternized nitrogen atoms in the relatively hydrophilic “A” block, cross-linkable hydroxy groups in the middle “B” block, and neutral nitrogen atoms in the relatively hydrophobic “C” block. Two types of micelles were prepared: conventional micelles with the C block in the core in aqueous media and inverted micelles with the A block in the core in nonaqueous media. Efficient shell cross-linking of both types of micelles was achieved using divinyl sulfone (DVS), which reacts with the hydroxy groups in the central B block at room temperature. Although the XPS sampling depth of 2−10 nm is comparable to the dimensions of the micelle layers, and despite the probability of some micelle deformation in the absence of solvent, analysis of the high-resolution N 1s spectra provided the first direct evidence for the expected core−shell nature of SCL micelles in the solid state.

Ionization of carbon, nitrogen, and oxygen by electron impact

Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that (a) some target atoms used in an experiment were in metastable states close to the ground state, (b) excitation-autoionization of ${2s2p}^{m}$ exited states may be substantial, and (c) ions produced in experiments may be in excited, low-lying metastable states. The binary-encounter Bethe (BEB) model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. 64, 032713 (2001)] are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.

Sub-μm scale photoluminescence images of wide bandgap semiconductors by cryogenic scanning optical microscope

Photoluminescence images with a sub-μm spatial resolution were obtained at 15 K using a newly developed cryogenic scanning microscope with both high throughput and simplicity of measurement procedure. The objective was put in a sample chamber to reduce thermal conduction from the objective to the sample. The spatial resolution at 15 K was measured to be 300 nm at a wavelength of 488 nm, which is almost equal to the diffraction limit. The microscope demonstrated μm-scale defective regions emitting a PL signal at 3.40 eV in GaN grown on SiC. At an elongated surface defect in a 4H–SiC homoepitaxial layer, the PL image ascribed to excitons bound to neutral nitrogen atoms showed a dark line coincident with the defect, indicating that non-radiative recombination dominates over the excitonic emission at the surface defect.

Pulsed laser deposition of ZnSe:N epilayers assisted by active atomic nitrogen beams

Abstract Epilayers of ZnSe doped with nitrogen have been grown on polished GaAs(1 0 0) and Si(1 0 0) substrates by pulsed laser deposition (PLD) assisted by atomic nitrogen beams. An arc-heated beam source was used to dissociate N 2 and produced neutral nitrogen atom beams with concentration of 10 17 –10 19 at. sr −1 s −1 , which intersected with laser ablated plumes and substrate surfaces in the process of deposition. X-ray diffraction (XRD) results indicate that the ZnSe film grown on GaAs(1 0 0) at 2×10 −3 Torr is a single crystalline epitaxial layer. XRD analysis gives that the crystallographic quality of PLAD-grown ZnSe thin films largely depends on ambient pressure and lattice match. X-ray photoelectron spectroscopy (XPS) demonstrates that Zn and Se atoms in the PLD-grown ZnSe epilayers on GaAs(1 0 0) at 2×10 −3 Torr bonded each other, and besides 11% [N] and 5% [O] no other impurity exist. The concentration of doped nitrogen for the best ZnSe thin film grown on GaAs(1 0 0) was estimated to be over 10 21 cm −3 .

Ab initio and molecular mechanics (MM3) calculations of protonated-neutral diamine hydrogen bonds.

Ab initio calculations of cation-neutral diamine complexes have been carried out at the MP2/6-311+G** level. The geometry and energetics of the charge-reinforced hydrogen bond are analyzed with respect to the alkyl substitution of both the protonated and neutral nitrogen atoms, and these results have been used to improve the quality of the MM3(2000) force field. In addition, specialized hydrogen bond parameters optimized for MM3(2000) are presented. These parameters allow very accurate gas-phase modeling of the charge-neutral diamine environment. Molecular mechanics calculations can model effectively protonated amine-neutral amine hydrogen bonds in the gas phase and solution (continuum dielectric) through a combination of charge-dipole interactions and explicit hydrogen-bonding terms.

Preparation of Nitrogen-Doped ZnSe Thin Films by Radical-Assisted MOCVD

Radical-assisted metalorganic chemical vapor deposition of ZnSe thin films was made using nitrogen trifluoride (NF3) as co-reactant and source of nitrogen atoms. The secondary ion mass spectroscopy analysis indicated that the concentration profile of nitrogen in a ZnSe film was not uniform in regard to the thickness and was the order of magnitude of 10 25 -10 26 m -3 . The photoluminescence (PL) spectra of all the films showed weak excitonic emission bound due to the neutral nitrogen atom pair (r N-N t ) and intense donor-acceptor pair emission (DAP). The net acceptor concentration obtained from capacitance-voltage measurements was 1.3 x 10 23 m -3 at maximum, which increased to 1.3 x 10 24 m -3 after heat-treatment.

The Neutral Nitrogen Spectrum: Term Populations, Multiplet Intensities, Diagnostic Diagrams, and Comparisons with Observations

A spectroscopic model of the neutral nitrogen atom is constructed using published radiative rates and derived approximate collisional excitation rates, giving term populations and allowed and forbidden multiplet intensities for a range of electron temperatures and densities. Figures illustrate the predicted variations of the stronger UV and IR multiplet intensities, which are compared with laboratory- and solar-observed UV line intensities and with IR line intensities observed in a number of astrophysical sources. Reasonable agreement is found for all of the compared UV and IR lines, except for one anomalous solar line. Diagnostic diagrams of allowed/allowed, allowed/forbidden, and forbidden/forbidden line ratios in nitrogen are provided and are applied also to other published observations of nitrogen UV and IR lines.

A microwave plasma source of neutral nitrogen atoms

A microwave source of nitrogen atoms is described. Atom production in the discharge bulk ( mixtures) and atom losses in the flowing afterglow are analysed. The rotational temperature is measured by emission spectroscopy along the discharge (0 - 0 band of the second positive system) and the afterglow (first positive or first negative system depending on the afterglow type). Depending on the experimental conditions ( concentration), two types of flowing afterglow can exist. At high percentage, a `pink' afterglow occurs upstream of the classical `Lewis - Rayleigh' afterglow. An NO titration method provides the atom concentration downstream of the discharge. Then, the N atom transport is explained from a simple model accounting for the energy characteristics of the plasma (obtained by the microwave interferometry methods) and the kinetics.

Efficient doping of nitrogen with high activation ratio into ZnSe using a high-power plasma source

We have developed a high-power (5 kW) rf plasma source for nitrogen doping in ZnSe molecular beam epitaxy. Optical emission spectroscopy shows dominant atomiclike emissions around 800 nm due to excited neutral nitrogen atoms in the high power region and their intensities rapidly increase with increasing the rf power from 1 to 3 kW. The high net acceptor concentration (NA–ND) of 1.2×1018 cm−3 was achieved at the growth temperature of 220 °C and the activation ratio [(NA–ND)/N] as high as 60%, which is the highest value so far obtained for NA–ND∼1018 cm−3. Consequently, the PL spectrum showed well-resolved deep donor-acceptor pair emissions even with high NA–ND.

Growth of N-doped ZnSe thin films by radical-assisted metalorganic chemical vapor deposition.

Radical-assisted metal organic chemical vapor deposition of ZnSe thin films was performed using nitrogen trifluoride (NF3) as the co-reactant and source of nitrogen atoms. The growth rate showed a gradual increase with an increase of NF3 flux. The photoluminescence (PL) spectra of all the films showed the intense donor-acceptor pair (DAP) emission and the weak excitonic emission bound to neutral nitrogen atom pair (I1N-N). The results gave a direct evidence of N-doping in ZnSe thin films by radical-assisted MOCVD. The I1N-N emission intensity was dependent upon NF3 flux, indicating that non-radiative recombination centers were introduced at high NF3 flux. In case of the growth at high NF3 flux, the additional PL emission was observed at 2.4-2.5eV, the intensity of which decreased after the heat-treatment. The result suggests that the heat-treatment might be effective for quality enhancement through the decrease of undesirable defects and nonradiative recombination centers.

Knock-on subplantation-induced formation of nanocrystalline c-BN with r.f. magnetron sputtering and r.f. argon ion plating

Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating at an argon gas pressure of 8 × 10−4 mbar. The content of cubic boron nitride (c-BN) is very high, as determined by infrared absorption spectroscopy (IR, 84% c-BN), factor analysis of Auger electron spectroscopy (AES, approximately 100% sp3) and X-ray reflectivity (approximately 100% sp3). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM).The film-forming particles (fluxΦn) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (fluxΦi) are argon ions. The current density is about 2.25 mA cm−2, as determined from energy and mass analysis. The energy dependence due to r.f. substrate bias shows a maximum in the c-BN content at 137 eV at an arrival ratioΦi/Φn= 13 where AFM investigations show a minimum of the area roughness of 0.2 nm. Increasing the arrival ratio (to 20 and 66), the optimal c-BN formation is shifted to lower energies (87 eV and 62 eV respectively) which is in agreement with the subplantation model. Stress reduction experiments, such as UV-assisted deposition and post-annealing, are discussed.

Energy barrier to reorientation of the substitutional nitrogen in diamond.

A first-principles method using the local spin-density approximation to density-functional theory is used to investigate the neutral substitutional nitrogen atom in diamond. The length of the unique extended nitrogen-carbon bond in the ${\mathit{C}}_{3\mathit{v}}$ symmetry equilibrium structure is found to be 33% longer than the bulk diamond bond length. The energy barrier to the reorientation of this extended bond between the four possible nitrogen-carbon bonds is determined to be 0.7 eV, in excellent agreement with experimental estimates. \textcopyright{} 1996 The American Physical Society.

Evidence for the formation of ions in ion - surface collisions

Slow and ions are scattered from partially caesiated W(110) surfaces under grazing incidence. The collision-induced emission of electrons (due to inter- and intra-atomic Auger processes) is studied. We provide evidence that the metastable negative nitrogen ion state is formed in a two-step capture process: in the first step neutral nitrogen atoms are formed in excited multiplet states of the configuration. In the second step an additional surface electron is attached to the excited parent neutral atoms. The excitation energy of with respect to the nitrogen neutral ground state would be 1.4 eV; its lifetime must be shorter than 200 ns.

Radio frequency ion plating-induced phase transition from h-BN to nanocrystalline c-BN

Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target and by r.f. ion plating at low gas pressures. The content of cubic boron nitride (c-BN) is determined by IR spectroscopy to be higher than 90%. The grain size derived from X-ray diffraction (XRD) is in the region of 5 nm. Furthermore, the films are characterized by Auger electron spectroscopy (AES) and stress measurements. The deposition of the cubic phase is only possible in a small range of boron to nitrogen ratio ( 0.9 < B N < 1.1 ). The film-forming particles (flux Φ n ) are mainly neutral boron and nitrogen atoms with typical sputtering energies of a few electronvolts, and the plating particles (flux gF i ) are argon and krypton ions with a well defined energy up to 140 eV. The current density is more than 2 mA cm −2 , as determined from energy and mass analysis. The arrival ratio Φ i Φ n is approximately 20. The high ion current density causes a substrate temperature of about 510 K, and external substrate heating allows us to raise the temperature to 800 K. The c-BN formation depends mostly on the r.f. substrate bias and slightly on the substrate temperature and the rare gas masses. The results are compared with trim calculations. The film properties are discussed on the basis of the subplantation model.

One- and two-color two-photon resonance enhanced multiphoton ionization spectroscopy of the d 1Σ+ state of NH

The ionization and dissociation processes occurring after two-photon excitation of the d 1Σ+ state of NH have been investigated using one- and two-color multiphoton ionization spectroscopy in combination with mass-resolved ion and kinetic energy-resolved electron detection. The photoelectron spectra obtained for ionization of the molecule via the d 1Σ+ state (v′=0,1) reveal a one-photon ionization process to all energetically accessible vibrational levels of the X 2Π ground ionic state, at variance with a one-configuration description of the two states. Moreover, electrons are observed that derive from a one-photon ionization of excited neutral nitrogen and hydrogen atoms. Two-color excitation experiments of the d 1Σ+ state show that the influence of discrete states at the overall three-photon level on the dissociation dynamics is insignificant. Using the results of ab initio calculations it is concluded that these experimental observations can be interpreted consistently if two competing processes are assumed to take place from the d 1Σ+ excited state: a molecular one-photon ionization which is forbidden in zeroth order, and a two-photon nonresonance enhanced excitation to neutral (pre)dissociative states.

Recovery process of photochromism of H2 and H3 centres in diamond

To elucidate the mechanism of photochromism of H2 and H3 centres, i.e. H2( H3 −) + X + + h ̵ ω → H3 + X 0 , a study has been made of the recovery process of optical absorption and the photosensitive electron spin resonance (ESR) and thermoluminescence. The ESR measurement indicates that the neutral state concentration of nitrogen is increased by illumination. This fact gives evidence for one of the unknown traps X + to be ascribed to ionized nitrogen, N +. The glow curve in thermoluminescence after photoexcitation denotes the existence of at least four traps besides N +. The recovery appears logarithmic with respect to time and is faster when the concentration of remaining neutral nitrogen is larger. The recovery rate is almost independent of temperature below 420 K, while it increases with increasing temperature above 420 K. As a result, it has been concluded that the dominant process of the recovery arises via the tunnelling of an electron from a neutral nitrogen atom to an H3 centre rather than via the thermal release of a trapped electron.

Lightning spectra in the 850‐ to 1400‐nm near‐infrared region

Lightning spectra in the 850‐ to 1400‐nm near‐infrared region have been recorded with 200‐ to 300‐ms time resolution using a slitless spectrometer with a lead sulfide detector. Except for the wire portion of triggered discharge channels, rocket triggered and natural return stroke spectra are very similar. The following neutral atomic nitrogen (NI) and oxygen (OI) multiplet emissions have been identified (the wavelength, in nanometers, of the brightest line in each group is shown in parentheses): NI(2) (821.6), OI(4) (844.6), NI(1) (868.0), NI(15) (906.1), OI(8) (926.6), NI(7) (939.3), NI(19) (986.2), NI(18) (1011.3), NI(28) (1053.9), and NI(36) (1246.8). Continuum emissions with peak intensities at least an order of magnitude less than the strongest line emissions were detected. A laboratory arc simulation of a return stroke discharge produced a near‐IR spectrum containing all the features emitted by lightning. Additional NI radiation peaks at 1131.4 nm (NI(17)) and 1358.1 nm on the arc spectra overlapped water vapor absorption bands and were not visible on lightning spectra recorded at 2.2‐km range. A time‐averaged lightning channel temperature of about 16,000°K was calculated from the ratio of relative intensities of the NI(1) and NI(18) multiplets.