Dissociation Of Atoms Research Articles

Influences of Al, Ti and Nb doping on the structure and hydrogen storage property of Mg(BH4)2(001) surface – A theoretical study

First-principles calculations were performed to investigate the influences of Al, Ti and Nb doping on the structure, the hydrogen dissociation energy, the electronic structure and the diffusion path of H atom in Mg(BH4)2(001) surface. The calculated occupation energies indicate that substitution of Mg atom with Ti is the easiest, with Al is a little harder, and with Nb is the most difficult. The doping can reduce the strengths of B–H bonds around the dopants thus favours the dissociation of these H atoms. In comparison, the Nb doping shows the most outstanding effect and the Al doping has the least influence on the dissociation of hydrogen atoms. The minimum energy pathways (MEP) calculations indicate that the substitutions with Ti and Nb can reduce the energy barriers of hydrogen diffusion and thus facilitate H diffusion in the Mg(BH4)2(001) surface, whereas the substitution with Al is not an effective technique for improving the hydrogenation/dehydrogenation performance of Mg(BH4)2.

On dissociation in weakly doped ice

Currently, there is some ambiguity in the problem of decay of a single donor into charged fragments. Thus, in the well-known Ostwald approximation used for semiconductors (ice being one of them) the donor dissociation degree of tends to its maximum value (i.e., unity) as the doping impurity concentration approaches zero. At the same time, the statistical theory of atom reveals within the Thomas–Fermi (or Debye–Hückel) approximation the existence of a thermodynamically equilibrium state of a single multi-electron atom (donor) where charged nucleus keeps the number of counterions just necessary for its neutralization. These scenarios do not show the atom dissociation at all. Discussed in the present paper is the alternative between the full dissociation of a single donor (i.e., dissociation degree equals unity) in a semiconducting media (ice, water, semiconductor) and zero dissociation degree.

Surface analysis by photo-stimulated desorption using tunable VUV radiation

We have demonstrated the photon-stimulated surface spectroscopy using broadband vacuum ultraviolet (VUV) emissions from a laser-produced plasma, in which adsorbed atoms or molecules on material surfaces were to be desorbed and dissociated as a result of absorption of the wavelength-selected VUV photons. Desorbed and dissociated atoms or molecules were then detected by a mass spectrometer. Mass spectra of polyethylene and polyvinyl chloride samples were obtained as a function of the irradiation wavelength. We have found sufficient characteristic differences of the mass spectra in the irradiation wavelength shorter than 200 nm in each sample. The desorption and dissociation of atoms or molecules from material surfaces depended on bond energy or molecular structure.

Symmetry broken and restored coupled-cluster theory: I. Rotational symmetry and angular momentum

We extend coupled-cluster (CC) theory performed on top of a Slater determinant breaking rotational symmetry to allow for the exact restoration of the angular momentum at any truncation order. The main objective relates to the description of near-degenerate finite quantum systems with an open-shell character. As such, the newly developed many-body formalism offers a wealth of potential applications and further extensions dedicated to the ab initio description of, e.g., doubly open-shell atomic nuclei and molecule dissociation. The formalism, which encompasses both single-reference CC theory and projected Hartree–Fock theory as particular cases, permits the computation of usual sets of connected diagrams while consistently incorporating static correlations through the highly non-perturbative restoration of rotational symmetry. Interestingly, the yrast spectroscopy of the system, i.e. the lowest energy associated with each angular momentum, is accessed within a single calculation. A key difficulty presently overcome relates to the necessity to handle generalized energy and norm kernels for which naturally terminating CC expansions could be eventually obtained. The present work focuses on SU(2) but can be extended to any (locally) compact Lie group and to discrete groups, such as most point groups. In particular, the formalism will be soon generalized to U(1) symmetry associated with particle number conservation. This is relevant to Bogoliubov CC theory that was recently applied to singly open-shell nuclei.

Hydrogenation of the CuPL center in silicon

The CuPL center, a complex of four copper atoms in silicon with the zero-phonon photoluminescence line at 1014 meV and the donor level at 0.1 eV above the top of the valence band, is studied in the process of hydrogenation at 380 K. Complexes of a substitutional copper atom (Cus) with one and two hydrogen atoms are observed to form in the hydrogenated region at the expense of CuPL, while no isolated Cus atoms are detected. Our results indicate that the addition of a single hydrogen atom induces the dissociation of all interstitial Cu atoms which decorate the Cus core of the CuPL center.

Hydrogen Spectral Line Shape Formation in the SOL of Fusion Reactor Plasmas

The problems related to the spectral line-shape formation in the scrape of layer (SOL) in fusion reactor plasma for typical observation chords are considered. The SOL plasma is characterized by the relatively low electron density (1012–1013 cm−3) and high temperature (from 10 eV up to 1 keV). The main effects responsible for the line-shape formation in the SOL are Doppler and Zeeman effects. The main problem is a correct modeling of the neutral atom velocity distribution function (VDF). The VDF is determined by a number of atomic processes, namely: molecular dissociation, ionization and charge exchange of neutral atoms on plasma ions, electron excitation accompanied by the charge exchange from atomic excited states, and atom reflection from the wall. All the processes take place step by step during atom motion from the wall to the plasma core. In practice, the largest contribution to the neutral atom radiation emission comes from a thin layer near the wall with typical size 10–20 cm, which is small as compared with the minor radius of modern devices including international test experimental reactor ITER (radius 2 m). The important problem is a strongly non-uniform distribution of plasma parameters (electron and ion densities and temperatures). The distributions vary for different observation chords and ITER operation regimes. In the present report, most attention is paid to the problem of the VDF calculations. The most correct method for solving the problem is an application of the Monte Carlo method for atom motion near the wall. However, the method is sometimes too complicated to be combined with other numerical codes for plasma modeling for various regimes of fusion reactor operation. Thus, it is important to develop simpler methods for neutral atom VDF in space coordinates and velocities. The efficiency of such methods has to be tested via a comparison with the Monte Carlo codes for particular plasma conditions. Here a new simplified method for description of neutral atoms penetration into plasma is suggested. The method is based on the ballistic motion of neutrals along the line-of-sight (LoS) in the forward–back approximation. As a result, two-dimensional distribution functions, dependent on the LoS coordinate and the velocity projection on the LoS, and responsible for the Doppler broadening of the line shape, are calculated. A comparison of the method with Monte Carlo calculations allows the evaluation of the accuracy of the ballistic model. The Balmer spectral line shapes are calculated for specific LoS typical for ITER diagnostics.

Confinement induced binding of noble gas atoms.

The stability of Ngn@B12N12 and Ngn@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cages, ab initio simulation reveals that except Ne2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs) at 298 K. The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds at this temperature but at a lower temperature (77 K) it is also found to be kinetically stable. He-He unit undergoes translation, rotation and vibration inside the cavity of B12N12 and B16N16 cages. Electron density analysis shows that the He-He interaction in He2@B16N16 is of closed-shell type whereas for the same in He2@B12N12 there may have some degree of covalent character. In few cases, especially for the heavier Ng atoms, the Ng-N/B bonds are also found to have some degree of covalent character. But the Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. The energy decomposition analysis further shows that the ΔEorb term contributes 40.9% and 37.3% towards the total attraction in the He2 dimers having the same distances as in He2@B12N12 and He2@B16N16, respectively. Therefore, confinement causes some type of orbital interaction between two He atoms, which akins to some degree of covalent character.

Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

Energy level alignment at the porphyrin/cobaltocene interface: From transfer doping to cobalt intercalation

N-type doping of the organic semiconductor zinc tetraphenylporphyrin (ZnTPP) by overlayers of the reducing molecule decamethylcobaltocene (CoCp2∗) is demonstrated using photoelectron spectroscopy. A transfer doping model involving integer charge transfer between molecules reproduces quantitatively all measured level shifts as a function of CoCp2∗ coverage using the ionisation potential of CoCp2∗ and the electron affinity of ZnTPP as sole input parameters. The model yields the experimentally observed limitation of doping to the first monolayer of cobaltocene while further layers remain neutral without the need to resort to special bonding arrangements for the first monolayer. Temperature-dependent studies reveal that doping is still present at room temperature, despite the high vapour pressure of CoCp2∗. Higher annealing temperatures initiate CoCp2∗ molecular dissociation and diffusion of Co atoms into the ZnTPP film. Hence, the nature of doping changes from surface molecular transfer doping to bulk metallic doping as a function of temperature.

Strain effect on the adsorption, diffusion, and molecular dissociation of hydrogen on Mg (0001) surface

The adsorption, diffusion, and molecular dissociation of hydrogen on the biaxially strained Mg (0001) surface have been systematically investigated by the first principle calculations based on density functional theory. When the strain changes from the compressive to tensile state, the adsorption energy of H atom linearly increases while its diffusion barrier linearly decreases oppositely. The dissociation barrier of H2 molecule linearly reduces in the tensile strain region. Through the chemical bonding analysis including the charge density difference, the projected density of states and the Mulliken population, the mechanism of the strain effect on the adsorption of H atom and the dissociation of H2 molecule has been elucidated by an s-p charge transfer model. With the reduction of the orbital overlap between the surface Mg atoms upon the lattice expansion, the charge transfers from p to s states of Mg atoms, which enhances the hybridization of H s and Mg s orbitals. Therefore, the bonding interaction of H with Mg surface is strengthened and then the atomic diffusion and molecular dissociation barriers of hydrogen decrease accordingly. Our works will be helpful to understand and to estimate the influence of the lattice deformation on the performance of Mg-containing hydrogen storage materials.

Direct Synthesis of Graphene Meshes and Semipermanent Electrical Doping

Here we describe a new method for the direct patterned synthesis of graphene meshes on Cu foils that use self-assembled silica sphere arrays as growth masks. Structural analyses based on electron microscopy and Raman spectroscopy showed that the graphene meshes are mostly single- or double-layer necks with empty holes that have abrupt edges. On the basis of experimental observations, we proposed the model illustrating the dissociation of carbon atoms at the Cu/silica interface through catalytic hydrogenation of the graphene lattice. Moreover, our approach can minimize problems associated with the graphene etching process, including contamination and exposure to reactive plasma. This enables stable electronic doping through covalent C–N bonds at the edges of graphene meshes.

Synthesis, Structure, and Reactions of a (η3-α-silabenzyl)molybdenum Complex: A Synthetic Equivalent of a Coordinatively Unsaturated Silyl Complex

The (η3-α-silabenzyl)molybdenum complex Cp*Mo(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1; Cp* = η5-C5Me5, p-Tol = p-C6H4Me) was synthesized in high yield by removal of 4-(dimethylamino)pyridine (DMAP) from the (arylsilyl)(DMAP)molybdenum complex Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2) with BPh3. The precursor, complex 2, was readily prepared by reaction of the (DMAP)(methyl)molybdenum complex Cp*Mo(CO)2(DMAP)Me (3) with tri-p-tolylsilane (HSi(p-Tol)3) through methane elimination. Study on the reactivity of 1 toward DMAP and nitrile revealed that complex 1 serves as a synthetic equivalent of the 16-electron silyl complex Cp*Mo(CO)2{Si(p-Tol)3}. Thus, complex 1 reacted with DMAP quantitatively at room temperature to reproduce arylsilyl complex 2 through dissociation of the arene carbon atoms coordinated to molybdenum. Complex 1 also reacted with acetonitrile at room temperature to give the N-silyliminoacyl complex Cp*Mo(CO)2{η2(C,N)-C(Me)═NSi(p-Tol)3} (4) exclusively via cleavage of the Mo–C(arene) bonds followed by insertion...

A Classical Trajectory Study of the Dissociation and Isomerization of C2H5

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.

The role of an oxometallic complex in OH dissociation during water oxidation: a microscopic insight from DFT study

The uncatalyzed atomic dissociation of water requires breaking of a strong O–H bond with an enthalpy of 494 kJ mol−1, which necessitates the understanding and designing of appropriate catalysts. Here we employ transition state theory within quantum chemical density functional theory to understand the role of metal-oxide inorganic complexes in the OH → O + H process, the most important reaction in water oxidation. We study the effect of (a) chemical bonding in different M4O4 (M = Mn, Co) cubane complexes, (b) heterocubane geometry containing Ca, in addition to a transition metal ion, (c) dimensionality by considering both three-dimensional and two-dimensional geometry of the oxometallic unit, and (d) connectivity between two oxometallic cubane units, corner shared versus edge shared geometry. Analysis of our density functional theory based calculations singles out a robust microscopic quantity among various plausible and competing factors, which elucidates the important role of metal–oxygen covalency at the oxidized site. The M–O bonding strength inversely determines the strength of the O–H bond, and thus the energy required for OH dissociation. This provides one with an important microscopic design principle for a metal-oxide complex catalyst responsible for water oxidation.

Structure of nanoscale copper precipitates in neutron-irradiated Fe-Cu-C alloys

The structure of copper nanoclusters/precipitates formed under neutron irradiation in Fe-0.3 wt. % Cu-C alloy is studied by the internal friction and positron annihilation experiments of postirradiation annealed alloys. The appearance of a carbon-relaxation peak during the first recovery stage at about 723 K reveals the fact that complex carbon-vacancy-copper clusters have formed during the irradiation-mediated copper precipitation process. The stability of Cu-C-vacancy clusters is confirmed by ab initio calculations. The existence of a structural phase transition of copper precipitates is observed in the alloys that are annealed at temperatures between the first and second recovery stages, which indicates that the dissociation of vacancies and carbon atoms from the vacancy-copper-carbon clusters is accompanied by the crystallization of Cu precipitates.

Atomic Ionization and Molecular Dissociation in a Hydrogen Gas within the Physical Picture

AbstractWe study a hydrogen gas at low densities within the physical picture. Recombination processes leading to the formation of atoms and molecules are properly taken into account via the well‐known Ebeling function and a new four‐body partition function. Our method provides a reliable equation of state which covers the plasma, atomic and molecular phases (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancing Light Emission of Nanostructured Vertical Light‐Emitting Diodes by Minimizing Total Internal Reflection

AbstractNanostructured vertical light‐emitting diodes (V‐LEDs) with a very dense forest of vertically aligned ZnO nanowires on the surface of N‐face n‐type GaN are reported with a dramatic improvement in light extraction efficiency (∼3.0×). The structural transformation (i.e., dissociation of the surface nitrogen atoms) at the nanolevel by the UV radiation and Ozone treatments contributes significantly to the initial nucleation for the nanowires growth due to the interdiffusion of Zn into GaN, evident by the scanning photoemission microscopy (SPEM), high‐resolution transmission electron microscopy (HR‐TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. This enables the growth of densely aligned ZnO nanowires on N‐face n‐type GaN. This approach shows an extreme enhancement in light extraction efficiency (>2.8×) compared to flat V‐LEDs, in good agreement with the simulation expectations (∼3.01×) obtained from 3D finite‐difference time‐domain (FDTD) tools, explained by the wave‐guiding effect. The further increase (∼30%) in light extraction efficiency is also observed by optimized design of nanogeometry (i.e., MgO layer on ZnO nanorods).

Two-site double-core-hole states formed when fast protons capture electrons from aligned N2

We report on an experimental investigation of 1.04 MeV H++N2 electron transfer collisions. The fast protons were stored in the electron-cooler ion-storage ring, CRYRING, and the molecular nitrogen target was provided with a supersonic gas jet. We report momentum distributions of atomic nitrogen dissociation products Nq + with charge states q + (q = 1, 2, 3) which are detected in coincidence with neutralized projectiles. Further, we investigate the influence of the angle between the direction of the incoming projectile beam and the target molecular axis. The orientation of the latter is determined from the momentum vector of one emitted atomic nitrogen fragment ion. We find significantly higher total yields, dominated by N+, of charged atomic dissociation products when the N2 molecular axis is perpendicular to the incoming H+-beam. The relative contributions from N2 +- and N3 +-fragments, however, are strongest when the N2 axis is aligned—or close to aligned—with the ion beam. This, we suggest, is due to increased probabilities for the formation of two-site double-core-hole states.

Sculpturing Effect of Sodium Thiosulfate in Shape Transformation of Silver Nanoparticles from Triangular Nanoprisms to Hexagonal Nanoplates

Sodium thiosulphate (Na2S2O3) solution was used to etch silver nanoparticles (NPs) and transform them from triangular nanoprisms to hexagonal nanoplates. UV-Vis spectroscopy and transmission electron microscopy were used to monitor the evolution of the hexagonal Ag nanoplates. S2O3(2-) etched the corners of the triangular Ag nanoprisms, and the Ag atoms that were removed aggregated into small clusters. The facet-selective etching effect of S2O3(2-) can be mainly attributed to the surface energy difference of each face of the nanoplate. The mechanism of S2O3(2-) etching also involves formation of Ag2S2O3 on the nanoprisms vertices, which is followed by its hydrolysis and subsequent dissociation of Ag atoms to form stable Ag2S. The hexagonal Ag nanoplates showed higher surface-enhanced Raman scattering activity than the original nanoprisms.

On the interaction between radiation-induced defects and foreign interstitial atoms in α-iron

Interaction between Frenkel pair (FP) defects and nitrogen atoms in α-iron has been investigated by means of resistivity recovery method. Both FP defects are attracted to nitrogen atoms. Dissociation of self-interstitial atoms from nitrogen atoms is observed at 165 K while that of vacancies at 250 K. The binding energies of FP defects with nitrogen are both about 0.1–0.15 eV. A weak RR stage is observed at 340 K assigned to the presence of carbon in concentration of about 1 appm. Analysis of its features leads to a conclusion on a dissociation (binding) energy of vacancy-carbon atom pairs of about 0.9 (0.35) eV.