Showing papers on "Electronic structure published in 1975"

Model for the Electronic Structure of Amorphous Semiconductors

Abstract: It is pointed out that a model which agrees well with the observed properties of semiconducting glasses is an attractive Hubbard model of localized states. Such a model has no gap for two-electron excitations but an energy gap for one-electron ones. The suggested physical model for a two-electron excitation is a new covalent bond in the structure, which is severely localized. It is also proposed that the one-electron excitation spectrum is wholly, or almost wholly, extended, and all observed gaps are identical with the mobility gap.

A theoretical method to determine atomic pseudopotentials for electronic structure calculations of molecules and solids

Abstract: The characteristic features of model potentials, effective potentials and pseudopotentials are carefully investigated. Then we justify our choice to work only with hermitian pseudopotential operators, and we develop a general non-empirical method to determine atomic pseudopotentials. In view of their numerical use for molecular calculations, these pseudopotentials are cast into semi-local forms, and their parameters are obtained by a least-squares process; tables of parameter values are given for the two first rows of the periodic system.

Bound states in the continuum

Abstract: Quantum-mechanical examples have been constructed of local potentials with bound eigenstates embedded in the dense continuum of scattering states. The method employed corrects and extends a procedure invented by von Neumann and Wigner. Cases are cited whereby deformation of the local potential causes the continuum bound state to move downward through the bottom of the continuum, and to connect analytically to a nodeless ground state. A doubly excited model atom is also displayed, with interactions between its two "electrons," having an infinite lifetime (in the Schr\"odinger equation regime). In the light of these examples, attention is focused on quantitative interpretation of real tunneling phenomena, and on the existence of continuum bound states in atoms and molecules.

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

Abstract: Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X 2A1, A 2B2, B 2B1, C 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the A 2B2, B 2B1, and C 2A2 states have all been found to be within 2 eV of the minimum of the X 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 A for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 A and 1.42 A and the inversion barrier is approximately 800 cm−1.

Relativistic molecular calculations in the Dirac–Slater model

Abstract: A new method is presented to calculate binding energies and eigenfunctions for molecules, using the Dirac–Slater Hamiltonian. A numerical basis set of four component wavefunctions is obtained from atom‐like Dirac–Slater wavefunctions. A discrete variational method (DVM) has been applied to generate the binding energies and eigenfunctions for the molecule. Results are given for a series of molecules, including dihydrides H2X (X=O, S, Se, Te), diatomic indium halides InX (X=F, Cl, Br, I), and metal chlorides XCl (X=B, Al, Ga, In, Tl). Comparison is made with results from nonrelativistic calculations using the DVM with numerical Hartree–Fock–Slater‐type wavefunctions and with other types of nonrelativistic calculations. In particular, relativistic level shifts and spin–orbit splitting have been analyzed. The theoretical ionization energies are compared with experimental results. Generally a very good agreement is obtained between the experimental and theoretical binding energies for the valence levels, calcu...

Developments in atomic structure calculations

Abstract: The general availability of electronic computers has resulted in a variety of developments in calculations of the electronic structure of atoms. The starting point is generally the Hartree-Fock approximation, which inadequately accounts for the motion of the electrons. A treatment of electron correlation has been the aim of most of the recent work in this field, and the methods which have been proposed are reviewed. The accuracy to which energies (or at least energy differences) can be evaluated, and calculations of electron affinities and term ratios are considered. The evaluation of oscillator strengths (transition probabilities) and hyperfine structure are also covered. Available atomic structure programs based on the correlation methods are discussed.

The electronic structure of transition metal carbonyl complexes

Abstract: Discrete variational X α calculations are reported on the neutral carbonyl complexes Ni(CO)4, Fe(CO)5 and Cr(CO)6, as well as on the ions Co(CO)4 -, Fe(CO)4 -, Mn(CO)5 -, Mn(CO)6 + and V(CO)6 -. Valence electron ionization potentials are predicted very well by transition-state calculations. The description of complex binding in Ni(CO)4 according to these calculations differs markedly from the one obtained in MSXα and Hartree-Fock calculations, which contradicted each other. The relative magnitude of σ-donation and π-back-donation is found to be roughly equal in the three neutral complexes (0·2–0·3 and 0·5 e respectively). Large changes occur in the ionic complexes, notably in the π-back-donation. These changes are correlated with the observed shifts in C-O stretching frequencies and force constants.

Electronic structure of trigonal and amorphous Se and Te

Abstract: The electronic structure of trigonal and amorphous Se and Te is investigated using the empirical pseudopotential method (EPM), charge-density calculations, and simple tight-binding models. Band structures and electronic densities of states are obtained which are in excellent agreement with recent photoemission measurements. The tight-binding models are used to obtain analytic expressions for the energy bands and to interpret the EPM band structures in terms of real-space orbital-orbital interactions. Charge-density calculations obtained as a function of energy and evaluated within specific energy intervals are used to interpret various structure in the density of states. Specifically certain easily resolvable peaks in the experimental photoemission spectra are associated with intrachain and interchain localized states, respectively. By taking only short-wavelength components of the charge density, a bonding charge can be defined which gives an estimate of the intrachain vs interchain bonding strengths. The trigonal results along with model calculations to investigate the effects of bond-angle variations on chains and the presence of eight- and six-fold rings of bonds are used to interpret the changes observed in the experimental spectra of amorphous Se and Te. A new model of amorphous Se is proposed.

Electronic structure of PbSe and PbTe. I. Band structures, densities of states, and effective masses

Abstract: We present new improved pseudopotential calculations for PbSe and PbTe using several non-local corrections in addition to the local empirical pseudopotential. We discuss results foreffect~ve masses, Knight shift measurements . and recent photoemission measurements. In addition to .· the optical properties in an energy range from 0 to 20 eV (which will be discussed in a subsequent the . paper), al~ above experimental results can for the . first time be explained consistently using one band structure model. Introduction A large number of band structures of the lead chalcogenides have already been calculated, using various methods. 1 The OPW method has been employed by F. Hermann et al., the APW method by several authors 2 ' 3 ' 4 and the KKR method H. by11 over•hof and V. Rossle1•. 5 Two different versions ol the pseudopotential method have also been published; one of ·. .d:) 00 ou oa 4+'~ 2l uu // ~JJ 94 6~1 -2them inciuding a strong non local s-like potential6 ' 7 of the Lin-Kleinman form and one being purely local -the empirical pseudopotential method (EPM). 8 None of these calculations was able to give an overall coherent picture of the physical properties of PbSe and PbTe. In particular, only Bernick and Kleinman~·were able to reproduce the effective masses, whereas only the EPM8 results yielded optical results in agreement with experiment. Furthermore~ the appearance of recent XPS and UPS measurements 9 ' 10 revealed general disagreements with all published band structures. We have thus reopened this problem in an attempt to obtain acceptable agreement with all known experimental measurements. To perform the calculations we have chosen the EPM which uses a local empirica,l. pseudopotential. This local potential had to be modified by adding an effective mass to the kinetic energy operator and by adding a full nonlocal d-potential. A detailed description of this procedure is given in Section I. The resulting band structures are presented in Section II together with a.justification of the form of the potential used and a discussion of the parameters involved. In Section III we compare the physical properties near the fundamental absorption edge to experimental results. Section IV is devoted to the calculation of the density of states of the valence bands and to a comparison of these results with experimental photoemission data. The study of the optical properties in an energy

Nature of the 5f electrons in the actinide series

Abstract: Theoretical and experimental work on the nature of the 5f electrons in the actinide series is reviewed briefly. It is suggested that intra-atomic coulomb repulsion between 5f electrons is of great importance in the actinides and quantitative support is given to the itinerant picture of the 5f states already found in band structure calculations. The effective intra-atomic interaction is calculated by the same method as applied to cerium in earlier work. The question of valencies in the actinides is discussed; the appearance of superconductivity is correlated with the f-bandwidth. 19 references.

APW--LCAO band model for A15 compounds

Abstract: The results of nonrelativistic augmented-plane-wave (APW) band-structure calculations at symmetry points in the cubic Brillouin zone for V$sub 3$Si, V$sub 3$Ge, Nb$sub 3$Al, and Nb$sub 3$Sn are fit using the Slater-Koster linear- combination-of-atomic-orbitals (LCAO) interpolation scheme. This LCAO model involves Bloch sums formed from 30 A-atom (A = V, Nb) d orbitals and eight B-atom (B = Si, Ge, Al, Sn) s-p orbitals. In its simplest form, this LCAO model fits 73 APW energy eigenvalues at GAMMA, X, M, and R with an rms error of 0.020 to 0.023 Ry by means of 21 two-center parameters. Improved accuracy is achieved, particularly for states near E/sub F/, by weighting these more heavily in the LCAO fit, partially relaxing the two-center approximation, and increasing the number of LCAO parameters to 39. The results of this APW-LCAO model are applied to evaluate the accuracy of the Labbe-Friedel (LF) linear-chain and the Weger- Goldberg (WG) coupled-chain band models for the A15 compounds. It is concluded that (a) corrections to the LF and WG models (about 3 and 1 eV, respectively) are sufficient to wash out fine structure in the density of states on a meV energy scale; (b) there is no evidence for describingmore » the electronic structure of these A15 as one-dimensional or quasi-one-dimensional in character; (c) the density-of- states peak near E/sub F/ involves primarily A-atom d states with delta$sub 1$(x$sup 2$ - y$sup 2$) symmetry, in contrast to the predictions of the LF (delta$sub 1$ + delta$sub 2$) and WG (delta$sub 2$) models; (d) the accuracy of the present LCAO model is insufficient for predicting the precise shape of the density of states near E/sub F/ from first principles. (auth)« less

Model studies of chemisorption. Interaction between atomic hydrogen and beryllium clusters

Abstract: The interaction between hydrogen atoms and Be metal clusters has been studied by ab initio electronic structure theory. Self‐consistent‐field (SCF) calculations have been carried out using both minimum and larger basis sets of contracted Gaussian functions. Both spatially restricted and unrestricted SCF methods were used, and different results were obtained in several cases. Reasons for the choice of this particular model system are discussed. Clusters as large as ten Be atoms have been considered, as have four different sites for the approach of the H atom. The electronic structure is discussed on the basis of predicted orbital energies and Mulliken atomic populations.

Exact results for a three-dimensional alloy with site diagonal disorder: comparison with the coherent potential approximation

Abstract: Densities of states and spectral functions are obtained for large (\ensuremath{\sim}8000 atoms) three-dimensional alloy models with cell localized disorder and are compared with the single-site coherent-potential approximation (CPA). We find that the CPA agrees remarkably well with the exact numerical results. In particular, the overall structure of the electronic spectrum and the transition from one- to two-band behavior are well described. There is, however, some substructure which is not reproduced by the single-site approximation.

Short-range correlations and the structure factor and momentum distribution of electrons

Abstract: It is shown that for any system of non-relativistic electrons and nuclei the large-wavevector limit of the probability of finding an electron with momentum h(cross)k is proportional to 1/k8, and the electronic structure factor is proportional to 1/k4 Furthermore, the coefficient of the 1/k4 term in the structure factor is proportional to the zero separation electron-electron correlation The coefficient of the 1/k8 term in the momentum distribution is proportional to the sum of zero-range electron-electron and electron-nuclear correlations

Electronic structure of the alkaline-earth fluorides studied by photoelectron spectroscopy

Abstract: Precise values have been determined for the binding and separation energies of the outer electronic bands of the alkaline-earth fluorides using 40.81-eV ultraviolet and Al $K\ensuremath{\alpha}$ (1486.6 eV) x-ray photoelectron spectrometry. Excellent agreement is found between the experimental and Born-model theoretical values for the energy separation between the outermost levels of the alkaline-earth and fluorine ions when the experimental values are corrected for relaxation (polarization) effects and when the Madelung energy alone is taken to represent the electron-lattice interaction. Very good agreement also occurs between experimental and theoretical values for the absolute binding energies. These results are accounted for by the fact that the substantial repulsive energy existing in the alkaline-earth fluorides is stored predominantly as interionic elastic potential energy and has little effect on the electronic energy levels on the ions. Comparison of the bandwidths of the alkaline-earth fluorides from the present work and alkali-metal fluorides from previous work shows that the ${\mathrm{F}}^{\ensuremath{-}}$ $2p$ valence-band width is a function of the nearest-neighbor distance and independent of the particular compound.

Molecular photoelectron spectroscopy at 132.3 eV. The second-row hydrides

Abstract: Photoemission spectra of the second‐row hydrides CH4, NH3, H2O, and HF, as well as Ne, obtained with ultrasoft (132.3 eV) x rays from the yttrium Mζ line and with soft x rays are compared and discussed. The 2s‐derived 2a1 or 2σ orbitals show large relaxation energies, as do the 2s orbitals in the free atoms. The high binding energies of the 2a1 orbitals in CH4 and NH3 indicate that much of the bond energy resides in these orbitals. Bond energies estimated from changes in the average valence‐electron binding energies from atoms to hydrides show rough agreement with literature values. Relative molecular orbital peak intensities show dramatic changes from 132.2 eV to 1253.6 eV photon energies, with the atomic cross‐section ratio 2p/2s near unity at 132.3 eV and near 0.1 at 1253.6 eV. This difference allows peaks to be assigned to molecular orbitals in some cases by visual inspection, on the basis of atomic orbital composition. Comparison with theoretical intensities based on plane‐wave or OPW continum final ...

Electron‐impact excitation of H2O and D2O at various scattering angles and impact energies in the energy‐loss range 4.2–12 eV

Abstract: The electron‐impact excitation of H2O and D2O has been studied at electron energies close to threshold and at large scattering angles in order to enhance spin and/or symmetry forbidden electronic transitions; and at energies far from threshold and at small scattering angles to enhance optically allowed transitions. The energy‐loss range covered is 4.2–12 eV. From a comparison of the present measurements and recent, accurate ab initio calculations, several new assignments of electronic transitions in both H2O and D2O have been made or suggested. Also suggested are future works which could be carried out in order to unravel the complex Rydberg spectra above 11 eV.

Electronic structure of the oxides of lead. Part 2.—An XPS study of bulk rhombic PbO, tetragonal PbO, β-PbO2 and Pb3O4

Abstract: XPS studies of the bulk oxides of lead reveal no clear evidence for the presence of the expected two oxidation states of lead in Pb3O4. They also show that the Pb 4ƒ binding energies of the two crystallographic modifications of lead monoxides are the same, within experimental error, but are greater than those for β-PbO2. These results are discussed in the light of lattice potential calculations.

Model calculation of the electronic structure of a (111) surface in a diamond-structure solid?

Abstract: The authors present exact results for a model calculation of the surface electronic structure of a (111) face of a diamond-type solid The model contains only one s-state per atom, but allows for surface rearrangement both at the outer layer and at the outer bounds Two kinds of surface states are found The first kind are localized at the outer bounds and split off the bulk bands as the bond potential increases beyond a critical value The second kind are intrinsic surface states and exist independently of the surface rearrangement but only in a restricted domain in the Brillouin zone where structure factors are smaller than 1 These intrinsic states have not been discussed before; they exist also in more realistic hybrid models of diamond-structure solids

Electronic structure of TiFe

Abstract: In the last few years a number of self-consistent band calculations appeared in the literature. Many of them were dealing with pure elements' and several others with compounds. In the area of transition-metal compounds the calculations which have been reported until recently were for materials in which the one component is a transition metal and the other C, N, and O. We now present a selfconsistent calculation by the augmented-plane-wave (APW) method on a compound whose components, namely, Ti and Fe, are both transition metals. While this work was in progress, Yamashita and Asano3 published the results of a similar calculation on TiFe, performed by the Green's-function Korringa-Kohn-Rostoker (KKR) method. We have already reported on non-self-consistent calculations of TiFe, and have given a preliminary account' of the present results. In this work we have calculated the energy bands, Fermi surfaces, densities of states, x-ray spectra, the electron-phonon coupling constant, and the i.somer shift. The results are compared with the calculation of Yamashita and Asano, with the Fermisurface experiments of Kamm, ' and with electronicspecific-heat, isomer-shift, and x-ray measurements. The intermetallic compound TiFe is a material of considerable technological interest due to its properties of being brittle and hard. Despite the presence of Fe, it is not ferromagnetic. At 50-50 composition, TiFe has an ordered phase and the CsCl structure. '

Formation of Molecular Lines in Stellar Atmospheres

Abstract: Statistical equilibrium of electronic states of diatomic molecules in stellar atmospheres is examined. Atmospheres discussed are representative of the sun, Arcturus (K-giant) and Betelgeuse (M-supergiant). A comparison of the relative collisional and radiative contributions to the equilibrium of the ground electronic state shows that this state is collisionally controlled and that the line source function for vibration-rotation transitions within this state is equivalent to the Planck function. Examination of the equilibrium for excited electronic states demonstrates that the exchange between these states and the ground electronic state is most probably determined by radiative excitation. This result implies that scattering rather than pure absorption is the appropriate mechanism for the formation of lines belonging to these electronic transitions. The scattering hypothesis is given a preliminary check against solar observations. Areas for future investigations are outlined.

The electronic structure of dialkylbenzenes

Abstract: A new CNDO model parametrization, differing from earlier ones primarily by its embodiment of large orbital exponents for conjugated carbon atoms, is proposed for the description of the electronic spectra of aromatic hydrocarbons. In contrast to its predecessors, the new model is shown to describe both the ultraviolet absorption and the valence electron eigenvalue spectra of methyl substituted benzenes. The predictions of this model are compared with those of earlier molecular‐orbital calculations (Hartree–Fock, CNDO/2, and SPINDO) and with the ultraviolet‐photoemission and ultraviolet‐absorption spectra of benzene, p‐xylene, m‐xylene, and 1,4‐bis(trifluoromethyl) benzene. The model also correctly predicts the symmetry of the radical anion ESR spectra of those compounds for which they have been measured.