Local spin density calculations:KL X-ray transition energies and ionisation potentials

We have calculatedKL X-ray transition energies and ionisation potentials for the firstrow and second-row atoms with the aid of different local spin density approximations to the exchange-correlation energy. The best overall agreement with experiments is obtained by using a generalizedX expression where correlation effects are included (X C approach). In particular, the X-ray energies for the second-row elements are very well described by this approach.     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

XPS study on silica-bismuthate glasses and glass ceramics

X-ray photoelectron spectroscopy (XPS) was used to evidence the effect of the Bi₂O₃ to SiO₂ ratio and of partial crystallisation on the electronic charge density around the atoms entering silica-bismuthate glasses of nominal composition 0.01Fe₂O₃⋅0.99[xSiO₂⋅(100−x)Bi₂O₃] with . The core level spectra show significant composition dependent changes in binding energy, and the full width at half maximum of photoelectron peaks both of cations and of oxygen atoms. The analysis reveals changes in electron density correlated with the ionic and covalent character of the samples. The shift in binding energy suggests charge transfer from silicon and oxygen atoms to bismuth atoms. Contrary to the expected behaviour in conventional silicate oxide systems, the results indicate an increase of ionicity for silicon and of covalency for bismuth atoms. The same evolution of ionicity/covalency is observed after partial crystallisation.     

The structure and stability of Si@Al<Subscript>12</Subscript>H<Subscript>n</Subscript> (n=1−14) clusters

Density functional theory has been employed in order to investigate the structure and stability of Si@Al₁₂H_n () clusters. Hydrogenated Si@Al₁₂ clusters exhibit pronounced stability for even numbers of H atoms. Large binding energy, HOMO-LUMO gaps and increased ionization potentials imply that these clusters should be physically and chemically stable. The analysis of the charge density of the HOMO plot illustrates that a pair of hydrogen atoms prefer to occupy opposing on-top sites for clusters with an even n number. Studies of deformation charge density plots demonstrate that significant charge transfer occurs from the Si@Al₁₂ to the H atoms.     

The influence of oxygen vacancies on the chemical bonding in titanium oxide

A self-consistent LAPW band structure calculation for TiO_0.75 has been performed, assuming long-range order of vacancies on the oxygen sublattice. The calculation is based on a hypothetical model structure which can be described as Ti ₃ ^[4] Ti^[6]O_3o, where _o denotes an oxygen vacancy. In the model structure two types of titanium atoms occur: Ti^[4] atoms which have two vacancies as neighbours and are quadratically surrounded by four oxygen atoms, and Ti^[6] atoms which are octahedrally surrounded by six oxygen atoms. The calculated density of states (DOS) and the local partial densities of states are compared with the respective values for stoichiometric TiO containing no vacancies. A characteristic difference is the appearance of two sharp peaks in the DOS curve below the Fermi energy which are caused by the vacancies. These vacancy states exhibit a considerable amount of charge in the vacancy muffin-tin sphere and are found to be derived from Ti 3d states extending into the vacancy sphere. The introduction of vacancies also leads to a lowering of the Fermi energy indicating a stabilizing effect. The bonding situation in TiO_0.75 as compared to TiO as well as the changes in chemical bonding in the series TiC_0.75–TiN_0.75–TiO_0.75 are discussed on the basis of electron density plots. The loss of Ti^[4]–O bonds (as compared to TiO) is compensated by the formation of Ti^[4]–_o–Ti^[4] bonds across the vacancy and by an increase of the bond strength of the Ti^[4]–O bonds. On the other hand, the Ti^[4]–Ti^[4] and particularly the Ti^[4]–Ti^[6] bonds are weakened by the introduction of vacancies.Dedicated to Professor F. Kohler on the occasion of his 65^th birthday     

Characteristics of a copper bromide laser with flowing Ne-HBr buffer gas

The operating characteristics of a small self-heated copper halide laser (=510.6 and 578.2 nm) are described, where the copper lasant atoms are produced by electric-discharge dissociation of copper bromide that is generatedin situ by flowing Ne-HBr gas mixture over copper pieces in the laser tube. The excitation technique permits fast startup (<1 min to laser oscillation from cold), and rapid and simple control of the CuBr vapour pressure, simultaneously introducing H₂ to increase the efficiency. Specific laser output energies and average powers of 12J cm^–3 and 195 mW cm^–3, respectively, are almost a factor of 2 higher than those previously reported in the literature for multi-kilohertz copper bromide lasers. A maximum power of 7.8 W was obtained from the 40 cm³ active region. At its highest efficiency (0.8%) the laser produced 6.1 W.     

An ESCA investigation of some copper complexes

A series of copper complexes have been investigated by ESCA. All complexes were salts of the tetraphenylphosphonium ion. The binding energies of all the atoms in the complexes were determined. From the binding energies of the ligand atoms we estimated the effective charges on these atoms. For this purpose we used linear relations of the formE_b = kq + E_bO which had been established previously within our scheme of C 1s (phenyl) as internal standard. From the data thus obtained, the effective charge on the copper atom was estimated. A linear relation between binding energy and the effective charge on the copper atom was found, i.e.,E_b(Cu) = 1.52q_Cu + 932.2ESCA spectra were recorded for the complexes bis(1,3-diphenyl-1,3-propanediono) copper (II) and bis(3-phenyl-2,4-pentanediono) copper (II). By a combination of the XPS binding energies and IR intensities of the ν_CH vibrations of the phenyl groups in the complexes with empirical relations between these entities and the effective charges of the atoms and groups, a fairly complete mapping of the charge distributions of these complexes has been achieved.     

Feldstärke,Stromdichten und mittlere Energien im Katodenfallraum von Glimmentladungen

By integration of energy distribution functions for the cathode fall of H₂ glow discharges we may show:

• 1) Space charge is almost constant within a large part of the dark space, therefore the field strength is a nearly linearly decreasing function of distance from cathode. Deviations from linearity are quantitatively discussed.
• 2) Composition of space charge: The contribution of atomic ions to the space charge may range from 5 to 10%. and the contribution of electrons from 0,1 to 10%, increasing with growing voltage.
• 3) Relative ion and neutral current densities at the cathode (ion current from the glow edge = 1) are decreasing with growing voltage. Molecular neutral current density equals 3 to 10 times molecular ion current density, in case of atomic particles the relation is from 1 to 5.
• 4) Total current density is increasing from 2.0 · 10^?4 A/cm² torr² at the voltage U₀ = 0,19 kV to 3,7 A/cm² torr² at U₀ = 10 kV; cathode field strength is increasing from 0,53 kV/cm torr to 97 kV/cm torr.
• 5) The mean energy of neutrals is smaller than the mean ion energy by a factor of about 0.45…0.85, and the mean energy of molecules is smaller than the energy of atoms by a about 0.2…0.8. The dependence of the mean energies from voltage is discussed.
• 6) The energy gain of electrons is growing from 15% of total dark space energy at 0.19 kV to 70% at 10 kV; the energy gain of neutrals is decreasing at the same time.

     

X-ray photoelectron investigation of charge distribution in copper(II) phthalocyanine complexes

The charged states of atoms in unsubstituted copper(II) phthalocyanine (CuPcH₁₆) and hexade-cafluorinated copper(II) phthalocyanine (CuPcF₁₆) complexes and in thin films of them deposited on silicon substrates by vacuum thermal evaporation are investigated by X-ray photoelectron spectroscopy (XPS). The C(1s), N(1s), Cu(2p) core level energies and the charged states of atoms in the studied complexes are calculated using the DFT method. The performed experimental study and theoretical calculations show that the introduction of electron acceptor substituents into benzene rings mostly affects the atoms of benzene rings and insignificantly affects the charge state of nitrogen atoms in the pyrrole ring.     

Interatomic bonding in oxygen-stabilized Ti2 Ni-type compounds

X-ray photoelectron-spectroscopic studies of oxygen-stabilized Ti₂ Ni-type compounds Ti₄M₂O_x (M = Fe, Co and Ni;x = 0.5, 1) have revealed some charge transfer from the titanium to the oxygen atoms with no direct involvement of the second metallic constituent M. Limited partial ionicity is deduced from the measured core-electron binding energies, which are intermediate between the respective values for TiO₂ and for the pure elements (Ti⁰, O₂). In particular, approximate estimations of the partial charge residing on Ti are consistent with the charges derived for the oxygen atoms, and the results correlate with the TiO distance in Ti₄M₂O, as compared with corresponding data for other systems. The implications of the interatomic bonding for the hydrogen absorption properties of the oxygen-stabilized compounds are discussed.     

A quantitative approach to ionic adsorption

Binding properties of multi-atomic systems with ionic bonds are calculated by using a recently developed method based on the density functional formalism. The charge density is obtained from a superposition of the respective atomic densities where the charge transfer between these atoms is chosen such that the total energy attains a minimum. The kinetic energy of the electrons can to a very good approximation be calculated by means of a modified Thomas-Fermi-v. Weizsäcker expression. The molecules NaCl and NaW and a NaW₄ cluster have been treated as model systems for ionic interaction. Moreover, we have computed the binding properties of a Na atom adsorbed on a W(100) surface. This particular problem is the primary subject of the present study. The calculations yield binding energies, binding distances, vibrational frequencies, and induced dipole moments.     

Thermodynamical properties and defect energetics of an n-body potential adapted to Cu3Au

By using molecular statics, molecular dynamics, and Monte Carlo techniques we validate a previously developed empirical n-body potential adapted to Cu₃Au. At T=0 K, predicted cohesive energies, lattice parameters, and elastic constants in CuAu and CuAu₃ as well as the formation energy of vacancies in Cu₃Au are in good agreement with experimental data. A satisfactory behavior is also obtained at T 0 K in Cu₃Au, for atomic mean-square displacements and elastic moduli. However, this model underestimates the vacancy migration energy and the order-disorder critical temperature when the latter is evaluated by Monte Carlo including both exchanges between atoms of different species and atomic moves simulating vibrations.     

A semiempirical calculation of electronic structure of multiple CdS-based clusters

The electronic structure of (Cd _x S _y ) _n clusters, wherex, y16 andn8, is calculated using the Extended Hückel method with charge and configuration selfconsistence. Variations of charge distribution, energy level structure, stability, etc. as the result of coupling of single Cd _x S _y , clusters with different intercluster distance are analysed. This coupling has a pronounced effect upon orbital energies and HOMO-LUMO difference at small intercluster distances. In contrast, atomic charges are close for double and single clusters. The coupling effects quickly fall down with intercluster distance.The work was performed under partial support of Fundamental Research Foundation of Belarus. The author thanks the referee for the valuable remark on a possible source of weak cluster-cluster interaction included in Discussion of the revised version.     

Structural, electronic and vibrational properties of indium oxide clusters

Geometric, electronic and vibrational properties of the most stable and energetically favourable configurations of indium oxide clusters In_mO_n (1≤m, n≤4) are investigated using density functional theory. The lowest energy geometries prefer the planar arrangement of the constituent atoms with a trend to maximize the number of ionic In-O bonds. Due to the charge transfer from In to O atoms, the electrostatic repulsion occurs between the atoms with the same kind of charge. The minimization of electrostatic repulsion and the maximization of In-O bond number compete between each other and determine the location of the isometric total energy. The most stable linear In-O-In-O structure of In₂O₂ cluster is attributed to the reduced electrostatic repulsive energy at the expense of In-O bond number, while the lowest energy rhombus-like structure of In₂O₃ cluster reflects the maximized number of In-O bonds. Furthermore, the vibrational frequencies of the lowest energy clusters are calculated and compared with the available experimental results. The energy gap and the charge density distribution for clusters with varying oxygen/indium ratio are also discussed.     

Influence of the nearest environment symmetry on the character of the <Emphasis Type="Italic">p-d</Emphasis> interaction in binary copper sulfides

The energy structure of copper sulfides CuS and CuS₂ has been theoretically investigated by the modified method of associated plane waves (WIEN2k program). CuS is considered in two different cubic modifications of the sphalerite and NaCl (hypothetical) types, in which Cu atoms are in different coordinations. The density distribution of the electronic p states of sulfur and d states of copper are calculated taking into account the separation into the e _g and t _2g states. The specific features of these distributions are interpreted.     

Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations (H₂O)_nH⁺, 4 n 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 E _coll 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, E _coll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of 35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H₂O)₂H⁺ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.     

New boron barrelenes and tubulenes

The structure of a new class of boron nanostructures—barrelenes and tubulenes—based on a boron atomic lattice constructed by the alternating B-atomic polygons with central atoms and without them has been proposed and their properties have been described. Ab initio density functional calculations have been performed for the energy and electronic structure of the fullerene-barrelene-nanotube series based on the lowest energy fullerene B₈₀. It has been shown that the energy and band gap of a barrelene are lower than the respective quantities of the corresponding fullerene and tend to the respective values for nanotubes in the infinite limit. It has been shown that there are isomers of nanotubes of the same type that are significantly different in symmetry and electronic properties: a semiconductor (C _5v symmetry) and a metal (D _5h symmetry).     

Performance of the constrained minimization of the total energy in density functional approximations: the electron repulsion density and potential

In the constrained minimization method of Gidopoulos and Lathiotakis [N.I. Gidopoulos, N.N. Lathiotakis, J. Chem. Phys. 136, 224109 (2012)], the Hartree exchange and correlation Kohn-Sham potential of a finite N-electron system is replaced by the electrostatic potential of an effective charge density that is everywhere positive and integrates to a charge of N ? 1 electrons. The optimal effective charge density (electron repulsion density, ρ_rep) and the corresponding optimal effective potential (electron repulsion potential v_rep) are obtained by minimizing the electronic total energy in any density functional approximation. The two constraints are sufficient to remove the self-interaction errors from v_rep, correcting its asymptotic behavior at large distances from the system. In the present work, we describe, in complete detail, the constrained minimization method, including recent refinements. We also assess its performance in removing the self-interaction errors for three popular density functional approximations, namely LDA, PBE and B3LYP, by comparing the obtained ionization energies to their experimental values for an extended set of molecules. We show that the results of the constrained minimizations are almost independent of the specific approximation with average percentage errors 15%, 14%, 13% for the above DFAs respectively. These errors are substantially smaller than the corresponding errors of the plain (unconstrained) Kohn-Sham calculations at 38%, 39% and 27% respectively. Finally, we showed that this method correctly predicts negative values for the HOMO energies of several anions.     

The role of the distribution of nucleons in finite nuclei

The total energy of many-nucleon system is expressed as a functional E[ _p(r), _n(r)] of the proton and neutron densities _p(r) and _n(r), respectively. The distribution(r) of nucleons in the nucleus, which is essential to determine the energy functional, is chosen. The energy density formalism is applied to finite nuclei, and then the binding energies per nucleon together with the mean square radii, for some medium and heavy nuclei, are obtained. Finally the achieved results are compared with the corresponding experimental values.