Showing papers on "Electronic structure published in 1984"

Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state

Abstract: We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular‐like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrodinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron‐hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.

Electronic Shell Structure and Abundances of Sodium Clusters

Abstract: Mass spectra are presented for sodium clusters of $N$ atoms per cluster ($N=4\ensuremath{-}100$) produced in a supersonic expansion with argon carrier gas. The spectra show large peaks or steps at $N=8, 20, 40, 58, \mathrm{and} 92$. These can be understood in terms of a one-electron shell model in which independent delocalized atomic $3s$ electrons are bound in a spherically symmetric potential well.

Highly conducting polyparaphenylene, polypyrrole, and polythiophene chains: An ab initio study of the geometry and electronic-structure modifications upon doping

Abstract: The effect of charge-transfer doping on the geometric and electronic structures of conjugated polymers has been investigated at the ab initio level with explicit consideration of the doping agents. Three systems were chosen for study as prototypical examples of conjugated polymers with nondegenerate ground states: polyparaphenylene, polypyrrole, and polythiophene. As a result of charge transfer with electron-donating dopants, extra charges appear on the polymer chains and induce strong geometry modifications. The lattice evolves from an aromatic structure towards a quinoid-like structure. Charged defects associated with lattice deformations such as spinless bipolarons are formed. The influence on the electronic structure of the polymer chains is such that with respect to the undoped case, new states appear within the gap. For the maximum doping levels experimentally achieved, band-structure calculations demonstrate that the states in the gap overlap to form bipolaron bands, a few tenths of an electron volt wide. The presence of these bipolaron bands is consistent with optical data as well as with magnetic data which suggest that the charge carriers in the highly conducting regime are spinless.

Electron density, Kohn−Sham frontier orbitals, and Fukui functions

Abstract: In this note we shall show that the ground-state electron density \(\rho ({\mathbf r})\) is a functional of the highest occupied orbital in Kohn–Sham (Phys Rev 140:A1133, 1965, [1]) theory, \(\psi _{\mathrm {max}}\). The functionals \(\rho [\psi _{\mathrm {max}}]\) for an \((M+\delta )\)-electron system are resolved into three cases and connected to three Fukui functions defined by Parr and Yang (J Am Chem Soc, 106(14):4049, 1984, [2]).

Advanced theories and computational approaches to the electronic structure of molecules

Abstract: Chemical Computations on an Attached Processor: Quantum Chemistry Applications.- Considerations in Vectorizing the CI Procedure.- The Method of Self Consistent Electron Pairs. A Matrix Oriented Direct CI.- Evaluation and Processing of Integrals.- Multiconfiguration Wavefunctions for Molecules: Current Approaches.- Internally Contracted Mcscf-Scep Calculations.- Computational Aspects of Direct SCF and Mcscf Methods.- Coupled-Cluster Methods for Molecular Calculations.- State-Specific Theory of Electron Correlation in Excited States.- The Treatment of Electron Correlation: Where do We go from There?.- Computer Technology in Quantum Chemistry.- Problem Limitations and Cost Effectiveness Considerations in Computational Quantum Chemistry.- Algorithmic Considerations in Large Mainframe Computers.- Participants.

Electronic Structure of a "Poisoned" Transition-Metal Surface

Abstract: The authors report self-consistent linearized-augmented-plane-wave calculations of the electronic structure perturbations induced by a catalytic ''poison,'' S, on a Rh(001) surface, focusing particularly on their distance dependence. The S-induced charge density vanishes beyond the immediately adjacent Rh atoms. However, the Fermi-level density of states, which is not screened, and which governs the ability of the surface to respond to the presence of other species, is substantially reduced by the S even at nonadjacent sites.

Visible spectroscopy of jet‐cooled SiC2: Geometry and electronic structure

Abstract: SiC2 has been prepared in a supersonic molecular beam by laser vaporization of a silicon carbide rod within a pulsed supersonic nozzle. Rotational analysis of the 0‐0 band of the well‐known 4980 A band system of this molecule reveals that, contrary to previous assumptions, the molecule is triangular in both the ground and excited electronic states. In both states the molecule is of C2V symmetry with a C–Si–C angle between 40° and 41°. The correct assignment of the spectrum is A ′B2←x ′A1. The carbon–carbon bond length is 1.25 A in the ground state, suggesting that the molecule is best understood as a silicon atom bound to the side of a triply bonded C2 fragment. The optical transition moment is polarized along the b axis of the molecule which is parallel to the carbon–carbon bond axis. In the A 1B2 excited state the carbon–carbon bond opens up to 1.30 A consistent with a π*←π excitation of the carbon–carbon triple bond. The silicon–cargon distance is measured to be 1.81 A in the x 1A1 state, lengtheni...

Total energies of diamond (111) surface reconstructions by a linear combination of atomic orbitals method

Abstract: An ab initio linear combination of atomic orbitals approach to local-density theory, capable of handling complex structural geometries, is presented. It incorporates a self-consistent treatment of interatomic charge transfer, which allows an accurate calculation of total energies. The method is applied to study a variety of possible 1 x 1 and reconstructed 2 x 1 models of the diamond (111) surface. Among the many models suggested, only the Pandey ..pi..-bonded chain model has a lower energy than that of the 1 x 1 surface. A minimum-energy structure is obtained for this model after extensive consideration of relaxations. No dimerization of the surface chain is found to occur.

Photoionization spectra and electronic structure of small iron clusters

Abstract: Laser photoionization spectra over the range 4.5–6.5 eV have been taken for iron clusters of from 2 to 25 atoms. From the observed ionization thresholds, the cluster ionization potentials are determined and trends in I.P. vs cluster size are examined. In the case of the iron dimer, a sharp, vertical threshold is seen which places the I.P. of Fe2 at 6.30±0.01 eV. Finally, SCF Xα scattered wave molecular orbital calculations have been carried out on the iron dimer and iron trimer. These results show the importance of ferromagnetic spin polarization in the electronic structure of Fe2 and Fe3.

Theory of collective excitations in semiconductor superlattice structures

Abstract: In this paper electronic collective excitations of type-I and -II superlattices are examined in detail. Type-I superlattices consist of quasi-two-dimensional layers of electrons, while type-II superlattices consist of alternating quasi-two-dimensional layers of electrons and holes. We use a simple model of the electronic structure and linear-response theory to calculate the density response of the system to an external perturbation. From this, we obtain an expression for the dielectric tensor, the zeros of which yield the dispersion relations of the collective modes. The theory is such that one can take into account many-body effects (depolarization and excitonic shifts), magnetic fields, and electron-phonon coupling in a simple way. A rich spectrum of excitations is found: quasi-two-dimensional plasmons, intersubband plasmons, magnetoplasmons, phonon-plasmon modes, and so on. Some interesting features of the excitations are examined, and their relevance to experiment is discussed.

Local‐density Hartree–Fock theory of electronic states of molecules with self‐interaction correction

Abstract: A scheme for incorporating the self‐interaction correction (SIC) to the local density approximation of the Hartree–Fock theory of electronic structure of molecules is presented. This method is applied to the N2 molecule and the resulting orbital energies and total energy are in good agreement with the Hartree–Fock values.

Spectroscopic studies of the jet‐cooled nickel dimer

Abstract: We report the first gas‐phase electronic spectrum of nickel dimer. Ni2 is produced by laser vaporization of metallic nickel in the throat of a supersonic nozzle. Using resonant two photon ionization, bands previously observed in inert matrices and attributed to Ni2 are conspicuous in their absence. Further to the red, an abrupt onset of complicated spectral structure indicates rapid predissociation above 16 680 cm−1. We argue that this represents the true dissociation limit, and places D0=2.068±0.01 eV. A congested pattern of spectral features from 6000 to 9000 A confirms theoretical predictions of a large number of low‐lying electronic states in nickel dimer. Rotationally resolved bands near 8500 A are indicative of a ΔΩ=+1 transition, with Ω″=4, Ω′=5. Rotational analysis of these bands yields a bond length of 2.200±0.007 A for the ground state of Ni2, which must be of either 1Γg or 3Γu electronic symmetry species. Both the long bond length of 2.20 A and the high value of Ω″ are in agreement with theoretical predictions, and confirm that no substantial 3d participation contributes to the chemical bonding of Ni2.

Electronic structure and magnetic properties of YM2 compounds (M=Mn, Fe, Co and Ni)

Abstract: The electronic structures of itinerant de electrons in the intermetallic compounds YMn2, YFe2, YCo2 and YNi2 with cubic Laves-phase structure are calculated using both the recursion method and the standard method of the tight-binding approximation (TBA). Using the calculated density-of-states curves in the TBA and taking the effect of spin fluctuations into account, the temperature dependences of the paramagnetic spin susceptibility in these compounds are calculated. The field dependence of the induced magnetic moment in YCo2 at OK is also calculated. Good agreement between the calculated and observed results can be obtained by allowing the number of d electrons to be an adjustable parameter. Furthermore, the densities of states for magnetised YCo2 and ferromagnetic YFe2 are calculated. It is shown that the values of the local magnetic moments on Co and Fe are smaller than those in pure metals and the local magnetic moments on Y become negative due to the hybridisation between the d bands of Y and Co or Fe.

Vinylidene: Potential energy surface and unimolecular reaction dynamics

Abstract: New quantum chemistry calculations (with a triple zeta plus polarization basis set, and a single and double configuration interaction) have been carried out to determine the equilibrium points and the transition state for the vinylidene (H2C=C:)→acetylene (HC≡CH) isomerization. A classical barrier height (i.e., with no zero point energy effects) of 6.3 kcal/mol is obtained, and application of the Davidson correction for unlinked clusters reduces this to 5.4 kcal/mol. Our best estimate is that the true classical barrier lies in the range 2–4 kcal/mol. The dynamics of the vinylidene/acetylene isomerization is described with the framework of the reaction path Hamiltonian. The lifetime of vinylidene (in its ground vibrational state) with respect to this process is calculated to be 0.24 to 4.6 ps for a classical barrier of 2 to 4 kcal/mol. This lifetime decreases by a factor of ∼2 if one quantum of the CH2 scissors mode of vinylidene is excited, but is predicted to increase somewhat if a quantum of the C–C str...

Half‐metallic ferromagnets and their magneto‐optical properties (invited)

Abstract: We have calculated the electronic structure of PtMnSb in order to explain the very high magneto‐optical Kerr effect (over 2.5° at 720 nm at room‐temperature) of this compound. It is shown that this behavior is related to the unusual electronic properties of PtMnSb: it is a half‐metallic ferromagnet like NiMnSb. The extreme asymmetry in the electronic structure of these compounds—metallic behavior for one spin direction and at the same time semiconducting behavior for the other spin direction—is responsible for the unusual magneto‐optical properties.

Alloying Effect on the Electronic Structure of Ni 3 Al (γ

Abstract: The influence of a number of alloying transition elements on the electronic structure of Ni 3 Al ( γ') has been studied by the DV-X α cluster method The d orbital levels of the alloying elements appear above the Fermi level, and these change monotonously with atomic number, electronegativity and metallic radius of the element These level structures associated with d electrons characterize the alloying elements as whether they are elements partitioning to γ phase, to γ' phase or to grainboundary The substitution of alloying elements for either Ni or Al site was also discussed in terms of electronic structure Further, the ionicity of each element, and the bond order between elements in γ' phase have been calculated, which provide basic informations for understanding bond nature in the alloy These informations were found to be useful for developing better nickel base superalloys with a high performance at elevated temperatures

Self-consistent cluster calculations with correct embedding for 3d, 4d, and some sp impurities in copper

Abstract: Self-consistent calculations are presented for the electronic structure of $3d$, $4d$, and some $\mathrm{sp}$ impurities in Cu. The calculations are based on density-functional theory in the local-spin-density approximation and on the Korringa-Kohn-Rostoker Green's-function method. The muffin-tin potentials of the impurity and of the neighboring atoms are calculated self-consistently. The use of the proper host Green's functions guarantees the correct embedding of this cluster of 13 muffin-tin potentials in the ideal Cu host. One shell of perturbed neighbor potentials is sufficient for a good description of the electronic properties. While the results considerably improve the previous singlesite calculations, they nevertheless confirm for most impurities the qualitative results obtained in these calculations. Charge-transfer effects are most pronounced for some $4d$ impurities and the vacancy. The magnetic moments of the $3d$ impurities are only slightly changed compared to the single-site calculations.

Changes in the electronic structure and vibrational potential of hydrogen fluoride upon dimerization: A well‐correlated (HF)2 potential energy surface

Abstract: A large basis, coupled‐cluster level treatment of the electronic structure of the hydrogen fluoride dimer has been used to develop an accurate potential energy surface and dipole moment surface. The vibrational motion of the HF subunits was analyzed using a number of different methods for comparison with recent experimental results. Molecular properties averaged over the vibrational motion were also calculated. The vibrational transition frequencies for the two HF stretches in the dimer agree with experiment to 1%, while the shifts in these frequencies agree with experiment to 10 cm−1. The importance of higher order correlation effects and treatment of vibrational anharmonicities is made clear by these results. Transition moments were calculated and show enhancements for both HF stretches in the dimer.

Clarification of the electronic asymmetry in Π‐state Λ doublets with some implications for molecular collisions

Abstract: The asymmetry of the orbital part of the electronic wave functions and electronic charge distributions in 1Π, 2Π, and 3Π Λ doublets is carefully examined, to clear up considerable past confusion on this subject. The results are: (1) For 1Π and 3ΠΩ=1 states the electronic wave function in the e Λ‐doublet levels is symmetric with respect to reflection in the plane of rotation of the molecule and, in the f levels, antisymmetric. (2) For 2Π and 3Π0,2 states, in the Hund’s case (a) limit the electronic distributions in both Λ‐doublet levels are cylindrically symmetric. (3) As the case (b) limit is approached, the F1 e and F2 f wave functions of a 2Π state acquire an increasing degree of symmetric character with respect to reflection in the plane of rotation, while the F1 f and F2 e levels acquire antisymmetric character. In a 2Σ+–2Π radiative transition, the main branch P and R lines probe 2Π levels which are symmetric with respect to reflection in the plane of rotation while the main branch Q lines probe leve...

Electronic and magnetic structure of BCC Fe-Co alloys from band theory

Abstract: The connection between electronic structure and ferromagnetism is studied for the Fe-Co alloy system by means of self-consistent local spin-density functional calculations. The local environment of these BCC-substitutional alloys is modelled by different ordered compounds: besides BCC Co and Fe these are Co3Fe and Fe3Co in the Fe3Al structure and FeCo in the CsCl and NaTl structures. FeCo in the CsCl structure is studied in great detail giving the ferromagnetic band structure, decomposed state densities, Wigner delay times and the spatial form of the spin density. The role of the nearest-neighbour coordination in determining the value of the magnetic moments is discussed for all the compounds studied. The calculated and measured magnetic moments are compared and trends are analysed and explained.

Electronic structure of ideal TiO 2 (110), TiO 2 (001), and TiO 2 (100) surfaces

Abstract: We report theoretical investigations on the electronic structure of ideal, defect-free surfaces of rutile (${\mathrm{TiO}}_{2}$). The (110), (001), and (100) faces are studied, and for each face, several models are examined, which correspond to different surface atomic compositions. The bulk electronic structure is described by a tight-binding Hamiltonian including interactions up to second-nearest neighbors. From this, the surface electronic structure is determined by making use of the scattering-theoretic method. The results are obtained in terms of surface bound states, surface resonances, and surface densities of states, which are wave-vector, layer, atom, and orbital resolved. This allows a detailed discussion of the origin, the nature, and the localization of the various surface features obtained. For all surface models examined, no occupied states are found in the gap, in accordance with experimental results. The main effect of the surface is to induce O $p$-derived resonances in the valenceband region and Ti $d$-derived resonances in the conduction-band region. These trends are correlated with the relatively strong ionicity of ${\mathrm{TiO}}_{2}$. The nature and the strength of the surface states and resonances, as well as their variation from one face to the other are interpreted in terms of the coordination of the surface cations, the removal of anion second-nearest neighbors, and the reduced screening of cation-cation interactions due to the removal of surface anions.

The bond length and electronic structure of V2

Abstract: We report of the first gas‐phase electronic spectrum of V2. The dimer was generated in an expansion cooled molecular beam by laser vaporization of vanadium metal. Using the technique of resonant two photon ionization spectroscopy a strong band system with origin near 7000 A was observed. Rotationally resolved spectra of this band system conclusively demonstrate that the transition is of the type 3Πu(a)←3Σ−g(a), and we assign the 3Σ−g state as the ground electronic state of V2. The short ground state bond length re″=1.77 A and high vibrational frequency ωe″=535 cm−1 are indicative of extensive 3d‐orbital participation in the bonding of this molecule. The unusual case (a) coupling and large second‐order spin‐orbit splitting observed in the 3Σ−g state implies that the ground state electronic configuration contains a half‐filled π(3d) or δ (3d) orbital. The most plausible valence configuration consistent with the data is σg(3d)2πu(3d)+σg(4s)2δg(3d)2. In the excited state there is strong evidence of predissoci...

Temperature Dependence of the Exchange Splitting of Fe by Spin-Resolved Photoemission Spectroscopy with Synchrotron Radiation

Abstract: Temperature-induced changes in the electronic structure of Fe(100) have been investigated by spin- and angle-resolved photoemission for temperatures between room temperature and the Curie temperature ${T}_{\mathrm{C}}$. States nearly stationary in energy (${\ensuremath{\Gamma}}_{25}^{\ensuremath{'}\ensuremath{\uparrow}}$, ${\ensuremath{\Gamma}}_{12}^{\ensuremath{\uparrow}}$) have been observed for photon energy $h\ensuremath{ u}=60$ eV. However, from a strong increase in minority-spin intensity for $h\ensuremath{ u}=31 \mathrm{and} 21$ eV, a downwards shift of the ${\ensuremath{\Delta}}_{5}^{\ensuremath{\downarrow}}$ band is inferred to occur upon heating towards ${T}_{\mathrm{C}}$ for large $k$ vectors.

Classical barrier height for H+H2→H2+H

Abstract: Ab initio configuration interaction calculations on H3 and H2 show that the classical barrier for H+H2→H2+H must be less than 9.86 kcal/mol and most likely lies between 9.53 and 9.65 kcal/mol.