Showing papers on "Electronic structure published in 1983"

Electronic structure of supported small metal clusters

Abstract: Photoemission, Auger, and x-ray absorption spectroscopy have been used in a systematic study of small metal clusters on a variety of supports. The degree of cluster-support interaction for clusters of group-VIII and noble metals can be divided into two categories: supports with localized $p$ or $d$ orbitals with binding energies overlapping those of the cluster $d$ orbitals and supports without such orbitals. The first type is considered strongly interacting, whereas the latter type is only weakly interacting. For weakly interacting substrates such as carbon, the energy shifts in photoemission, Auger, and x-ray absorption edges, as well as changes in x-ray edge intensities, photoemission valence-orbital intensities and splittings, and photoemission and Auger linewidths all show that initial-state properties are much more sensitive to cluster size than are the final-state properties. The photoemission spectra of small clusters and those of alloys and intermetallic compounds are quantitatively compared. For weakly interacting substrates and host metals, the photoemission spectra of clusters and alloys are virtually identical, depending only on the average coordination number $\overline{n}$. In these systems the net interatomic charge transfer to the substrate or host atoms is very small. However, there is a significant intra-atomic charge transfer, which increases the $d$-electron count with increasing cluster size or alloy concentration. For strongly interacting supports, the cluster binding energy is usually shifted to lower binding energy. This shift can be understood from simple molecular-orbital arguments. The experimental conclusions are supported by calculations with the use of the thermodynamic model of Johansson and M\aa{}rtensson. Their model accurately predicts the observed binding-energy shifts and shows that initial-state effects dominate for weakly interacting systems and that final-state processes are relatively more important for the reactive substrates.

Electronic structure of the ternary chalcopyrite semiconductors CuAls2, CuGaS2, CuInS2, CuAlse2, CuGaSe2, and CuInSe2

Abstract: The electronic structure of six Cu-based ternary chalcopyrite semiconductors is calculated self-consistently for the first time within the density-functional formalism. The chemical trends in the band structures, electronic charge densities, density of states, and chemical bonding are analyzed.

Angle-resolved measurements of the photoemission of electrons in the study of solids

Abstract: Angle-resolved photoemission is shown to be a very versatile technique which probes electronic structure as well as geometric structure. All quantum numbers of an electronic state can be determined. In particular, energy and momentum of electrons in a solid are probed by measuring energy and momentum of photoelectrons. Thus, energy versus momentum band dispersions can be measured. Since photoelectrons have a small but tunable escape depth, it is possible to see surface and adsorbate states as well as bulk states. A completely different application of angle-resolved photoemission concerns the atomic positions and bond orientations at surfaces. Photoelectrons from a well-defined atom serve as source for an electron diffraction experiment which is localized to a surface. Another type of experiment uses the emission pattern from adsorbed molecules to determine their orientation.

Electronic structure of Ni and Pd alloys. I. X-ray photoelectron spectroscopy of the valence bands

Abstract: X-ray photoelectron (XPS) spectra of the valence bands of approximately 60 Ni and Pd alloys and intermetallic compounds with 20 different elements are presented. In alloys with electropositive elements the Ni and Pd $d$-band centroids move to larger binding energies and the density of $d$ states at the Fermi level (${E}_{F}$) is greatly decreased, indicating that the Ni and Pd $d$ bands are being filled. The Ni and Pd $d$ bands become narrower in alloys with electropositive elements. It is shown that in such alloys, the Ni-$M$ or Pd-$M$ interactions give a significant contribution to the Ni and Pd bandwidths. This contribution is larger when the second element $M$ has a large density of states at the energy of the Ni or Pd $d$ bands in the alloy. As this contribution is small in systems that form complicated structural types and glasses we speculate that its absence helps stabilize such structures. The satellite at \ensuremath{\sim}6 eV in the XPS spectrum of Ni is found to weaken and shifts to higher binding energy in alloys with electropositive metals. In the twelve alloys where its intensity is large enough to allow us to identify its energy, the binding energy of the satellite agrees with the two-electron binding energy of the Ni $^{1}G {d}^{8}$ term derived from Auger spectroscopy. The site- and symmetry-selected densities-of-states curves were calculated for 14 of the alloys to show that this filling is largely due to changes in the hybridization of the Ni or Pd $d$ bands with the partner element bands. The actual transfer of Ni and Pd $d$ electrons is probably small. These observations are used to rationalize published results of specific-heat and magnetic measurements on such alloys.

Self-consistent impurity calculations in the atomic-spheres approximation

Abstract: We have developed a method for calculating self-consistently the electronic structure around an impurity atom, or an impurity cluster, in a crystalline host. Our method is a Green-function matrix technique based on the linear muffin-tin orbital method in the atomic-spheres approximation. The calculation of the host Green function is extremely efficient and involves diagonalization of a small Hamiltonian matrix for the band structure and subsequent Hilbert transforms. The method is tested for the calculation of one-electron spectra and total energies on systems for which (essentially) exact solutions are known: the Hulth\'en potential, a free H atom, a H atom in jellium, and a Li atom in jellium. The accuracy is better and the computational speed significantly higher than that obtained with the standard Korringa-Kohn-Rostoker Green-function technique.

A self‐consistent eikonal treatment of electronic transitions in molecular collisions

Abstract: We develop an eikonal treatment of electronic transitions in many‐atom collisions, in which classical nuclear trajectories are self‐consistently coupled to quantal electronic transitions. The treatment starts with a discussion of the electronic representations required to assure that Hamiltonian matrices are Hermitian. The amplitudes of wave functions are found to satisfy coupled equations which are expanded in powers of a local de Broglie wavelength. Time‐dependent equations are transformed to derive a Hamiltonian formalism that couples nuclear positions and momenta with electronic amplitudes. Cross sections are obtained from flux conservation and also from T‐matrix elements.

Second and third derivatives of variational energy expressions: Application to multiconfigurational self‐consistent field wave functions

Abstract: General analytical expressions are given for the second and third derivatives of constrained variational energy expressions. It is pointed out that variational energy expressions and odd‐order derivatives have a distinct advantage over nonvariational (e.g., perturbative) energy expressions and even‐order derivatives. In particular, the first‐order wave function suffices to determine the derivatives of the variational energy up to third order. The coupled‐perturbed multiconfigurational SCF (MC‐SCF) equations, obtained from the general results, are equivalent, with minor corrections, to the ones very recently presented by Osamura, Yamaguchi, and Schaefer. Explicit expressions are given for the second and third derivatives of the MC‐SCF energy. Computational implementation is briefly discussed.

Nonadiabatic representations of the 1Σ+u and 1Πu states of the N2 molecule

Abstract: The vibronic bands in the dipole‐allowed absorption spectrum of N2 associated with the lowest three electronic 1Σ+u and the lowest three electronic 1Πu states are represented in a basis of electronically coupled diabatic states as well as in the basis of nuclear‐momentum coupled adiabatic states. Parameters defining the diabatic states and their electronic coupling energies are first evaluated by fitting the eigenvalues of a vibronic interaction matrix to the observations. The coupled‐oscillator equations are then solved directly by Johnson’s numerical integration method and the diabatic representation is redetermined via the matrix method and coupled equations iteratively. The fit of the experimental vibronic terms, B values, and absorption intensities achieved with R‐independent electronic coupling energies in a diabatic basis of valence and Rydberg‐type states (b′+c′+e′)1Σ+u and (b+c+o)1Πu is satisfactory. Comparison with the corresponding adiabatic representation shows that the nonadiabatic perturbati...

Photoemission study of the electronic structure of Mo and Mo oxides

Abstract: The electronic structure of Mo and Mo oxides has been studied using ultraviolet and X-ray photoelectron spectroscopy. The valence band spectrum of Mo(100) is compared with calculations of the density of states (DOS). A fairly good agreement of the characteristic peak positions with the DOS maxima is obtained. The Mo 3d core lines indicate well-defined MoO2 and MoO3 oxidation states with binding energies respectively 0.9 to 5.0 eV higher than that of the Mo metal. By comparing the atomic photoionisation cross sections and electron state occupation numbers with the experimental band intensities the highly ionic character of MoO2 has been estimated. The XPS valence band of MoO2 can be qualitatively described using the molecular orbital model based on a distorted rutile structure. The MoO3 valence band is composed of the O 2p derived band with a characteristic structure around 5 eV reflecting the existence of cationic d interactions. A small peak at about 1 eV below EF is observed in the photoemission of blue substoichiometric MoO3-x (in contrast to transparent MoO3). This is assigned to a small occupied defect band of trapped electrons in oxygen vacancies. Ion bombardment of transparent MoO3 results in a drastic change of the valence band indicating the MoO3 to MoO2 surface modification due to oxygen depletion.

Molecular properties from MCSCF‐SCEP wave functions. I. Accurate dipole moment functions of OH, OH−, and OH+

Abstract: Potential energy and dipole moment functions for the ground states of OH, OH−, and OH+ have been calculated from MCSCF, MCSCF‐SCEP, and SCEP‐CEPA electronic wave functions. The stability of the dipole moments with respect to the number of configurations (up to 598) and orbitals (up to 14) simultaneously optimized in the MCSCF procedure and the number of reference configurations (up to 11) in the MCSCF‐SCEP wave functions (up to 69 830 configurations) has been investigated. The dipole moment functions obtained from the best electronic wave functions are more accurate than all previously calculated ones. The deficiencies of the former calculations have been critically analyzed. The OH− and OH+ ions are predicted to be stronger IR emitters than the neutral OH radical. The rotationless rates of spontaneous emission A10 for the fundamental transitions are calculated to be 12.2, 137, and 263 s−1 for OH, OH−, and OH+, respectively. The calculated dipole moments in the vibrational ground states are 1.65, 1.04, an...

Corundum Structure Oxides Studied by XPS

Abstract: The electronic structure behaviour of Me2O3 oxides (Me = Ti, V, Cr) has been determined from XPS valence band and core level measurements. From Ti to Cr sesquioxide the relative intensity of the 3d band increases systematically which can be explained by the (atomic) photoelectric cross section and the number of 3d electrons. Simultaneously the 3d-2p band separation decreases indicating a more stronger band overlap equivalent an enhanced covalent bonding component. The different electrical properties among the corundum structure oxides are determined by the trigonal field splitted a1g and eg orbitals, their occupation and relative energy position at the Fermi level. The observations confirm the change from itinerant to more localized behaviour of d electrons caused by crystal structure parameter change (c/a ratio).

Ab initio SCF calculations on electrostatically solvated molecules using a deformable three axes ellipsoidal cavity

Abstract: The analytical expression of the free energy of solvation of a molecule interacting with a dielectric continuum through a three axes ellipsoidal cavity is used to derive the SCF equations of such a solvated molecule. In this paper, the shape of the cavity is defined after the principal values of the electronic polarizability tensor. Applied to two sets of rotational isomers (trans and gauche 1,2 difluoroethane, and E and Z N methyl formamide), this method confirms that the electronic structure, the molecular geometry, and the equilibrium constant are influenced by the solvent. The energy of solvation depends strongly on the shape of the cavity although the electronic structure appears to be less influenced by a modification of the geometrical characteristics of the boundary surface at constant volume of cavity. Therefore, the computation of the electronic wave function of a molecule interacting with a solvent by electrostatic and induction forces appears to be quite feasible.

Recent investigations on the electronic structure of the fourth and fifth group transition metal monocarbides, mononitrides, and monoxides

Abstract: An overview is given here of band structure calculations on the fourth and fifth group transition metal monocarbides, mononitrides, and monoxides, published since the review article by Calais [J.-L. Calais, Adv. Phys. 26, 847 (1977)]. Furthermore, the relations of three categories of experimental properties, which allow insight into the electronic structure of the above mentioned compounds, and the results of band structure calculations are discussed. Theoretical predictions are compared with experimental findings. The considered experimental properties are valence band photoemission spectra, valence band x-ray emission spectra, and optical properties.

Electronic Hamiltonian, wave functions, and energies, and derivative coupling between Born–Oppenheimer states in the vicinity of a conical intersection

Abstract: We derive the most general form of the first terms of the power‐series expansion of the electronic Hamiltonian in the neighborhood of the conical intersection at the equilateral triangle configuration of the homonuclear triatomic system M3. Previous treatments of this problem had assumed that the derivative coupling between Born–Oppenheimer states could be transformed away by choosing a strictly diabatic basis of electronic states. It has recently been pointed out, however, that this is not possible, in general. Making full use of the symmetry of the problem, and also taking account of the molecular Aharonov–Bohm effect, we obtain explicitly the leading terms of the expansion for electronic energies and wave functions, and of the derivative coupling. In terms of the expansion parameter r, a measure of the distance from the equilateral triangle configuration, the derivative coupling can be transformed away through the first order, but in the second order, nonremovable terms appear which are expected to be important in some problems.

Electronic properties of alkali trimers

Abstract: The electronic properties of the alkali trimers Li3, Na3, and K3 are studied using the pseudopotential and the local‐spin‐density approximations. More than 100 configurations were calculated for each trimer in order to obtain a complete picture of the adiabatic Born–Oppenheimer surfaces. The equilibrium geometry of the trimers are Jahn–Teller distortions of an equilateral triangle. Although the three surfaces are quite similar, Li3 is more affected than Na3 or K3 by the dynamical character of the Jahn–Teller distortion. The calculated ionization potentials agree very well with the experimental values and the qualitative features of the Born–Oppenheimer surface are confirmed by recent ESR experiments.

Valence effective Hamiltonian technique for nitrogen containing polymers: Electronic structure of polypyrrole, pyrolized polyacrylonitrile, paracyanogen, polymethineimine, and derivatives

Abstract: We present the parametrization of nitrogen for use in the valence effective Hamiltonian (VEH) technique. We describe five different parametrization schemes and discuss the quality of the results each of them affords. The VEH method is then applied to the study of the electronic structure of nitrogen containing polymers of interest with regard to the conducting polymers area. These polymers include polypyrrole, poly‐β‐dimethylpyrrole, poly‐N‐methylpyrrole, pyrolized polyacrylonitrile, paracyanogen (polypyrazinopyrazine), and polymethineimine [CH=N]x. Parameters related to the conductivity properties upon doping such as ionization potentials, bandgaps, and bandwidths, are discussed in detail.

Bulk and surface electron states in WO3 and tungsten bronzes

Abstract: Calculations of the electronic structure of WO3 suggest that the detailed crystallographic arrangement determines rather sensitively the size of the semiconducting energy gap. This increases from 1.6 eV in the perovskite-like cubic approximation to the structure, as used in the only previous study, to 2.4 eV in the full monoclinic distorted structure. Valence-band widths and density-of-states peaks are consistent with experimental photoemission spectra. The extra electrons introduced by alkali-metal atoms in the bronzes NaxWO3 enter the conduction band of W t2g states, behaving rather like a rigid-band model. Surface states are predicted to be absent from the semiconducting energy gap of the defect-free (001) surface of WO3.

Photoelectron spectra of acenes. Electronic structure and substituent effects

Abstract: The valence-electronic structure of the first three members of the acene series, benzene, naphthalene and anthracene, seems to be confirmed now by their photoelectron (PE) spectra. This especially holds for the r-ionizations which, according to their specific behaviour, can be assigned to different ionization modes. The vibrational fine structure of the observed systems is discussed. The effects of a series of substituents on the electronic structure of the parent molecules were traced in their PE spectra and intercompared. The results for methyl, chloro, methoxy and cyano substitution are described in detail. The effect is most pronounced for the two lowest energy -ionizations (i.e. HOMO and SHOMO in the Koopmans' picture) and can be described by the shift of the mean energy ("center of gravity") of HOMO and SHOMO, and the additional splitting compared to the unsubstituted molecule. Both parameters show good linear dependence for the three series of compounds, thus allowing to formulate general substituent parameters. A linear dependence of the lowest ionization energy with Hammett constants is found to exist as well. INTRODUCTION Acenes are a series of molecules whose properties often can be described as a monotonic function of the number of benzene rings, e.g. the similarities in position and intensity of bands in electronic spectra (1,2), first ionization energy (3), electron affinity (4-7), as well as reduction and oxidation potentials (8-10). It is known that these properties depend mainly on the characteristics of the outer, occupied and unoccupied, molecular orbitals, rather than on the total electronic configuration. Ionization energies of the valence electrons measured by molecular photoelectron (PE) spectroscopy provides, within the limitations of Koopmans' theorem, a valuable method for the determination of energies of the outer occupied molecular orbitals. In combination with quantum chemical calculations other characteristics as symmetry, space distribution and "origin" are obtained, all of them yielding a picture which is usually described as the electronic structure of a molecule. In this study the electronic structures of a number of monosubstituted benzenes, naphthalenes and meso-substituted anthracenes are compared with that of the non-substituted parent molecules in order to determine common effects of substituents which would enable to predict these effects in a quantitative way. Such an approach has been used several times as yet, but these studies have concerned mainly the effect of substituents on one of the molecules mentioned above. Pioneer work in the comparison of these effects can be found in the approach of Heilbronner (11) who indicated the parallelism of substituent effects on certain energy levels of benzene, naphthalene and anthracene. However, it is important to notice that until recently even the location of all n-ionizations in the PE spectra of naphthalene and anthracene was based mainly on considerably diverging calculations and thus was rather uncertain. Now consistent experimental evidence exists for the assigment of ir -ionizations which certainly is necessary to rely on such correlations and makes the intended comparison more than worthwhile. EXPERIMENTAL Compounds: All compounds were of high purity, redistilled or recrystallized before use and checked by mass spectrometry. The 9-Iand 9-CF3-anthracene were synthesized by Dr. M. Mintas. PE spectra: All spectra were recorded on a Vacuum Generators UV-G3 spectrometer (12) using He! (FWHM of 35 meV and 15 meV for fine structure determinations) and He!! (FWHM of 50 meV) excitation. The inlet system was heated if necessary and the spectra calibrated by addition of argon and xenon to the sample gas flow. Calculations: Standard PPP calculations of the n -electron levels were performed for most of the compounds. The correlation with assigned lower n -ionizations up to 15 eV is reasonably good. 289 RESULTS AND DISCUSSION ELECTRONIC STRUCTURE OF THE PARENT MOLECULES: BENZENE, NAPHTHALENE AND ANTHRACENE For the first three members of the acene series, benzene, naphthalene and anthracene, the valencr electronic structure can now be considered as well established; different techniques of obtaining their PE spectra were of great help in the assignment. Thus, a comparison of Hel and Hell spectra (fig. 1)) shows that cross sections of T-ionizations are relatively enhanced under Hell radiation (13,14). On the other hand, in the perfluorinated derivatives TI-ionizations change their energy much less than G -ionizations which are shifted to higher energy by 2-3 eV (15). Especially under Penning ionization the -ionizations observed are pronounced (16). All this in combination with quantum chemical calculations yielded the following iT-electronic structures (fig. 2); the listed energies correspond to vertical ionizations and the wave numbers to observed vibrational modes. E/eV 7 (44o 1) 1812, !32b9cII !)::i b29 b19 -•22_bl(1O4O 1ol82SJj I11) b9 b3.QOJ b3f 290 L. KLASINC et

Relationships between atomic chemical potentials, electrostatic potentials, and covalent radii

Abstract: The chemical potential μ of a many‐electron system equals its total electrostatic potential V(r) at any point r at which δT/δρ =−δ(eX+eC)/δρ, where ρ is the electronic density and T, eX, and eC are, respectively, the kinetic, exchange, and correlation energy functionals. The Thomas–Fermi–Dirac theory predicts that this relationship is satisfied at all points at which ρ=0.008 72. This prediction has been tested for 25 ground‐state atoms and has been found to give unsatisfactory results; the values of V(r) at the points in question are not in good agreement with μ, as approximated by −0.5(I+A), I and A being the atomic ionization potentials and electron affinities. However, an investigation of the radial distances rμ at which V(r) does equal μ shows that these are very close to the standard covalent radii of the atoms. (This supports an early electronegativity formulation by Gordy.) It is also shown that there is a very good correlation between μ and VQ, the electrostatic potential created at rμ by the nucl...

A general method for approximating the electronic correlation energy in molecules and solids

Abstract: A detailed analysis of a method proposed earlier for calculating a good approximation to the electronic correlation energy is presented here The principal advantages of this method compared to the usual CI techniques are discussed

Electronic structure of tin dioxide surfaces

Abstract: We present the results of a theoretical study of the surface electronic structure of the ideal (110), (001), and (100) faces of tin dioxide, a semiconductor crystallizing with the bulk rutile structure. The surface electronic structure is determined using a scattering-theoretic method in which the bulk electronic structure is described by a tight-binding Hamiltonian including first- and second-nearest-neighbor interactions. The results are obtained in terms of surface bound states, surface resonances, and surface densities of states, which are wavevector, layer, atom, and orbital resolved. All this information allows a detailed discussion of the origin, the nature, and the localization of the obtained surface features. Common trends are found for the three faces. The optical band gap is found to be nearly free of surface states. The dominant features are backbond states in the lower valence-band region and Sn $s$-derived states in the lower conduction-band region. We find one of each for the (001) and (100) faces and two of each for the (110) face which contains two tin atoms per surface unit cell. The upper valence-band region leads only to weak, O $p$-derived resonances. All these features are governed by the relatively strong ionicity of Sn${\mathrm{O}}_{2}$. The differences in the results obtained for the three faces are interpreted in terms of the coordination of the surface cations.

Electronic structure of semiconductors with doping superlattices

Abstract: The dynamically two-dimensional electronic subband structure and the effective energy gap are tunable quantities in semiconductors with a doping superlattice. We present self-consistent calculations of the electronic states, in the framework of the local-density approximation, as a function of the charge-carrier concentration. A discussion of several superlattices differing in their design parameters exemplifies the wide range of electronic subband structures which may be realized in this type of system.

Interactions between neutral dissociation and ionization continua in N2O

Abstract: Autoionized Rydberg states converging to the A 2Σ+ and B 2Π states of N2O+ are shown to be predissociated into neutral fragments. The decay to excited neutral fragments is observed by their fluorescence in the visible and ultraviolet regions of the spectrum. These decay channels are in competition with autoionization, which was shown in a previous study to yield unexpectedly large numbers of low energy electrons. On the basis of the striking similarity between the fluorescence excitation and the threshold photoelectron spectra, models are presented which connect dissociative channels to the production of threshold electrons. Ionization of dissociating neutral states and Rydberg states converging to the ground electronic state of N2O+(X 2Π) are suggested as being responsible for the production of low energy electrons. The distribution of photoelectron energies resulting from such autoionization channels are calculated.

Electronic structure and properties of hydrogen in metals

Abstract: This book reports on research on the basic interactions among hydrogen, host metal atoms, intrinsic lattice defects, and electrons. It covers such subject areas as phase diagrams, spinodals, order-disorder transformations, thermodynamics, neutron scattering, elastic interactions, electronic structure, magnetic and hyperfine properties, superconductivity, diffusion, interaction with lattice defects, chemisorption, and catalysis. It discusses a variety of experimental and theoretical techniques and emphasizes the use of these techniques in probing the electronic structure and properties of metal-hydrogen systems. The book constitutes the proceedings of the ''International Symposium on the Electronic Structure and Properties of Hydrogen in Metals'' held in Virginia in 1982.