Neutral Carbon Atoms Research Articles

Large Negative-Ion Formation in Grazing Scattering of Neutral Carbon Atoms from a CsI(100) Surface

Large Negative-Ion Formation in Grazing Scattering of Neutral Carbon Atoms from a CsI(100) Surface

Observation of two-photon excitation-induced fluorescence of ground-state neutral carbon atoms in vacuum generated by laser ablation of graphite

We demonstrated a practical gas production method for the ground-state carbon atoms in vacuum by pulsed laser ablation of graphite. The observation of $^{3}P\text{\ensuremath{-}}^{3}D$ two-photon excitation-induced fluorescence at 165.7 nm in the absence of a visible ablation plume proved the production of ground-state carbon atoms and enabled the evaluation of the velocity distribution and atomic density of the produced gas. With a low ablation fluence of 0.18 $\mathrm{J}/{\mathrm{cm}}^{2}$ at 266 nm, the number of atoms produced was $1.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ per pulse. The temperature of the gas was approximately 5600 K. Spectral measurements of the plumes at higher excitation fluences of 8, 25, and 50 $\mathrm{J}/{\mathrm{cm}}^{2}$ indicated that thermal sublimation at a low excitation fluence was essential to produce ground-state carbon atom gases. This method provides a promising source of atomic gas of carbon for laser cooling and precision spectroscopy when combined with precooling, such as using buffer-gas cooling.

Review on Heteroatom Doping Carbonaceous Materials Toward Electrocatalytic Carbon Dioxide Reduction

AbstractCarbon dioxide (CO2) reduction into chemicals or fuels by electrocatalysis can effectively reduce greenhouse gas emissions and alleviate the energy crisis. Currently, CO2 electrocatalytic reduction (CO2RR) has been considered as an ideal way to achieve “carbon neutrality.” In CO2RR, the characteristics and properties of catalysts directly determine the reaction activity and selectivity of the catalytic process. Much attention has been paid to carbon-based catalysts because of their diversity, low cost, high availability, and high throughput. However, electrically neutral carbon atoms have no catalytic activity. Incorporating heteroatoms has become an effective strategy to control the catalytic activity of carbon-based materials. The doped carbon-based catalysts reported at present show excellent catalytic performance and application potential in CO2RR. Based on the type and quantity of heteroatoms doped into carbon-based catalysts, this review summarizes the performances and catalytic mechanisms of carbon-based materials doped with a single atom (including metal and without metal) and multiatoms (including metal and without metal) in CO2RR and reveals prospects for developing CO2 electroreduction in future.

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.

Modelling dust transport in DIII-D with DTOKS-Upgrade

Comprehensive upgrades to the dust transport code Dust in TOKamaks (DTOKS) that extend the plasma-dust interaction model are presented and compared with recent measurements of dust transport in DIII-D. Simulations incorporating variation in physical properties of graphite dust with temperature and size in a stationary plasma background suggest a substantial decrease in lifetimes due principally to thermal expansion. The trajectories of 53 dust grains identified from analysis of visible camera data taken across two similar shots were used to measure the dust particle velocity distributions. Dust tracks terminated mostly at the outer divertor strike point having a mean observation time of 2.1 ± 0.4 ms. Stochastic modelling of 200 graphite dust particles in the DIII-D tokamak performed with DTOKS-U using plasma simulations generated by OEDGE found similar behaviour, with particles ablating rapidly after acquiring a positive charge in the region close to the outer strike point, creating an acute source of neutral carbon atoms. The simulated mean lifetime, 11 ± 2 ms, showed approximate agreement with experimental observation when corrected by accounting for dust visibility and ignoring the longest trajectories 1.5 ± 0.2 ms. Synthetic diagnostic data generated from coupling the results of DTOKS-U with the visualisation software Calcam offers a powerful new tool for validation of simulations and predictive calculations of dust dynamics.

Carbones and Carbon Atom as Ligands in Transition Metal Complexes.

This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL2 as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and π) donation to the metal atom [M]⇇CL2. The second part of this review reports complexes which have a neutral carbon atom C as ligand. Carbido complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR3 → [M]-CR2 → [M]-CR → [M]-C. This review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands.

Low-energy collisions between carbon atoms and oxygen molecules in a magnetic trap

Trapping of atoms and molecules in electrostatic, magnetic and optical traps has enabled studying atomic and molecular interactions on a timescale of many seconds, allowing observations of ultra-cold collisions and reactions. Here we report the first magnetic deceleration and trapping of neutral carbon atoms in a static magnetic trap. When co-trapping the carbon atoms with oxygen molecules in a superconducting trap, the carbon signal decays in a non-exponential manner, consistent with the decay model describing losses resulting from atom-molecule collisions. Our findings pave the way to studying both elastic and inelastic collisions of species that cannot be laser cooled, and specifically may facilitate the observation of reactions at low temperatures, such as C + O2 → CO + O, which is important in interstellar chemistry.

Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization

X-ray free-electron lasers have, over the past decade, opened up the possibility of understanding the ultrafast response of matter to intense X-ray pulses. In earlier research on atoms and small molecules, new aspects of this response were uncovered, such as rapid sequences of inner-shell photoionization and Auger ionization. Here, we studied a larger molecule, buckminsterfullerene (C60), exposed to 640 eV X-rays, and examined the role of chemical effects, such as chemical bonds and charge transfer, on the fragmentation following multiple ionization of the molecule. To provide time resolution, we performed femtosecond-resolved X-ray pump/X-ray probe measurements, which were accompanied by advanced simulations. The simulations and experiment reveal that despite substantial ionization induced by the ultrashort (20 fs) X-ray pump pulse, the fragmentation of C60 is considerably delayed. This work uncovers the persistence of the molecular structure of C60, which hinders fragmentation over a timescale of hundreds of femtoseconds. Furthermore, we demonstrate that a substantial fraction of the ejected fragments are neutral carbon atoms. These findings provide insights into X-ray free-electron laser-induced radiation damage in large molecules, including biomolecules. X-ray pump–probe experiments reveal that the molecular structure of C60 molecules substantially delays their fragmentation following photoionization. This may help to understand X-ray laser-induced radiation damage on molecules.

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy.

Fine-structure populations and collision–induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state 3Pj spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H2, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(3Pj) by H2 or D2 are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(3P0) + D2 → C(3Pj) + D2 (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm−1, respectively). C-atoms were produced mainly in their ground spin-orbit state, 3P0, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D2 beam from a second valve. The state-to-state cross-sections were derived from the C(3Pj) (j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(3P1) or C(3P2) when C(3P0) collides with ortho-D2 or normal-D2. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H2 collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.

Energy spectrum and electrical conductivity of graphene with a nitrogen impurity

The electronic structure of graphene with a nitrogen impurity has been studied based on the model of tight binding using exchange-correlation potentials in the density-functional theory. Wave functions of 2s and 2p states of neutral noninteracting carbon atoms have been chosen as the basis. When studying the matrix elements of the Hamiltonian, the first three coordination shells have been taken into account. It has been established that the hybridization of electron-energy bands leads to the splitting of the electron energy spectrum near the Fermi level. Due to the overlap of the energy bands, the arising gap behaves as a quasi-gap, in which the density of the electron levels is much lower than in the rest of the spectrum. It has been established that the conductivity of graphene decreases with increasing nitrogen concentration. Since the increase in the nitrogen concentration leads to an increase in the density of states at the Fermi level, the decrease in the conductivity is due to a sharper decrease in the time of relaxation of the electron sates.

Електронна структура та електропровідність графену з домішкою азоту

On the basis of the tight-binding model with the use of exchange-correlation potentials, the electron structure and the electric conductivity of graphene with a nitrogen impurity have been studied in the framework of density functional theory. The wave functions of 2s and 2p states of neutral noninteracting carbon atoms are selected as the basis ones. Band hybridization was found to result in the splitting of the electron energy spectrum near the Fermi level. In the nitrogen-doped graphene, owing to the overlapping of 2p energy bands, the mentioned gap is realized as a quasi-gap, in which the electron density of states has a much lower value in comparison with the other spectral region. It is found that an increase in the nitrogen concentration reduces the electric conductivity of graphene, although the density of states at the Fermi level grows at that. Hence, the reduction of the electric conductivity is associated with a sharper decrease in the relaxation time for electron states.

DIVIMP Modeling of Impurity Transport in EAST

Simulations of carbon impurity transport in SOL/divertor plasmas with Ohmic heating on EAST tokamak were performed using the two-dimensional (2D) Monte Carlo impurity transport code DIVIMP. The background plasmas for DIVIMP simulations were externally taken from B2.5/Eirene calculation. Besides the basic output of DIVIMP, the 2D density distributions of the carbon impurity with different ionization states and neutral carbon atoms were obtained, the 2D distributions of CII and CIII emissivities from C+1 and C+2 radiation respectively were also calculated. Comparison between the measured and calculated CIII emissivities showed favorable agreement, indicating that the impurity physics transport models, as implemented in the DIVIMP code, are suitable for the EAST tokamak plasma condition.

Effect of Element Diffusion Through Metallic Networks During Oxidation of Type 321 Stainless Steel

A detailed study was conducted on localized oxidation on Type 321 stainless steel (321ss) using synchrotron x-ray nanobeam analysis along with Raman microscopy. The results showed the presence of metallic nanonetworks in the oxide scales, which plays an important role in the continued oxidation of the alloy at 750 °C. A mechanism is proposed to explain the rapid oxidation of 321ss in complex gaseous environments at elevated temperature. Neutral metal atoms could diffuse outward, and carbon atoms could diffuse inward through the metallic nanonetworks in oxide layers. Alternately, diffusion tunnels can dramatically affect the phase composition of the oxide scales. Since the diffusion rate of neutral metal and carbon atoms through the metallic nanonetworks can be much faster than the diffusion of cations through Cr2O3, the metallic nanonetwork provides a path through the protective Cr2O3 layer for the rapid outward diffusion of metallic chromium and iron atoms to the nonprotective spinel layer. This diffusion process affects the solid-state reaction near the alloy-oxide boundary, and a dense Cr2O3 protective layer does not form. The classic stable structure of the oxide scales, with a dense Cr2O3 layer at the bottom, is damaged by the rapid diffusion through the tunnel at the reaction front, resulting in locally accelerated oxidation. This process can subsequently lead to “breakaway” oxidation and catastrophic failure of the alloy.

Absolute detection efficiencies for keV energy atoms incident on a microchannel plate detector

The absolute detection efficiencies of a microchannel plate detector (MCP) were determined experimentally for neutral hydrogen, carbon, and tungsten atoms with impact energies of 0.5–4.5keV. We measured detection efficiencies using our recently developed method, which uses coincidence counting between neutralized incident ions and ionized target atoms in single-electron capture collisions. The obtained detection efficiencies showed similar impact-energy and mass dependence to those of rare gas atoms (Ne, Ar, Kr, and Xe), measured previously using our method. The detection efficiencies increased with increasing impact energy and converged to the open area ratio (∼50%) of the MCP used. The efficiencies at fixed energies decreased as the mass of the incident atom increased. The absolute detection efficiencies obtained for H, C, W, Ne, Ar, Kr, and Xe atoms could be scaled according to the average electron emission yield estimated using the formulas for electronic and nuclear stopping powers.

A Scheme for Calculating Atomic Structures beyond the Spherical Approximation

We present a scheme for calculating atomic single-particle wave functions and spectra with taking into ac-count the nonspherical effect explicitly. The actual calculation is also performed for the neutral carbon atom within the Hartree-Fock-Slater approximation. As compared with the conventional atomic structure of the spherical approximation, the degenerate energy levels are split partially. The ground state values of the total orbital and spin angular momenta are estimated to be both about unity, which corresponds to the term P3PP in the LS-multiplet theory. This means that the nonspherical effect may play an essential role on the description of the magnetization caused by the orbital polarization.

Unusual Reactions Between Aromatic Carbon Supernucleophiles and 1,2‐Diazabuta‐1,3‐dienes: Useful Routes to New Pyrazolone and Cinnoline Derivatives

AbstractReactions between 1,3,5‐tris(dialkylamino)benzenes and 1,2‐diazabuta‐1,3‐dienes produce semicarbazone derivatives through attack of the supernucleophile aromatic carbon atom at the terminal carbon atom of the heterodiene reagent. The strong activation of the aromatic ring due to the presence of the three amino groups in a symmetrical relationship promotes electrophilic aromatic substitution by the neutral carbon atom of the electrophile. The resulting semicarbazones, in methanol in the presence of sodium methoxide, mainly afford pyrazolone derivatives, whereas in THF in the presence of sodium methoxide an unusual cyclization reaction produces cinnoline derivatives as the major products. Both cyclization reaction mechanisms are discussed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

溅射制备C膜的内压力分析

Intrinsic stresses of carbon films deposited by direct current (DC) magnetron sputtering were investigated. The bombardments of energetic particles during the growth of films were considered to be the main reason for compressive intrinsic stresses. The values of intrinsic stresses were determined by measuring the radius of curvature of substrates before and after film deposition. By varying argon pressure and target-substrate distance, energies of neutral carbon atoms impinging on the growing films were optimized to control the intrinsic stresses level. The stress evolution in carbon films as a function of film thickness was investigated and a void-related stress relief mechanism was proposed to interpret this evolution.

Potential routes to carbon inclusion in apatite minerals: a DFT study

We have conducted a computational study to investigate a number of possible routes for the incorporation of carbon into apatites. Using density functional theory (DFT) we have calculated geometry optimised structures for fluor- and hydroxy-apatites with and without various substitutions. We have studied several different carbonate substitutions, pure carbonate and pure formate apatites, neutral carbon atoms occupying interstices, and carbon dioxide and acetylene absorbed in oxyapatite.

Development of Tandem Reactions Based on Structural Features of Cyclic Phosphorus and Sulfur Ylides

AbstractFor Abstract see ChemInform Abstract in Full Text.

環状リン，硫黄イリドの構造特性を利用したタンデム型反応の開発

Cyclic phosphorus or sulfur ylides have a carbon atom adjacent to the phosphorus or sulfur atom other than the ylide carbon atom. The neutral carbon atom could be activated as a carbanion or another ylide after the initial reaction of the ylide. Novel synthetic methods using the two reactive positions in the cyclic ylides were developed. (Z)-Alkenylphosphine oxides generated by the Wittig reaction of a five-membered cyclic phosphonium ylide were converted to (Z)-γ, δ-unsaturated ketones via the Horner-Wittig reaction of the phosphinoyl carbanions. The reaction of the cyclic ylide with α, β-unsaturated ketones, esters, and thioesters provided cycloheptene derivatives via tandem Michael-Wittig reactions in which the ylide was regenerated on the other carbon atom. The similar tandem reactions of a five-membered cyclic oxosulfonium ylide gave cycloheptene oxide derivatives. Their novel tandem reactions of the cyclic ylides proceeded via rigid bicyclo [3.2.1] octane to result in a highly diastereoselective synthesis of the products.