Showing papers on "Electronic structure published in 1982"

Modern quantum chemistry : introduction to advanced electronic structure theory

Abstract: Graduate-level text explains modern in-depth approaches to the calculation of the electronic structure and properties of molecules Hartree-Fock approximation, electron pair approximation, much more Largely self-contained, only prerequisite is solid course in physical chemistry Over 150 exercises 1989 edition

Strained-layer superlattices from lattice mismatched materials

Abstract: Results are presented from the first theoretical study of the electronic properties of strained‐layer semiconductor superlattices made from lattice mismatched materials. The energy gaps and electronic states of GaAs‐GaAs0.2P0.8 (100) superlattices are studied as a function of layer thicknesses using a tight binding model. The superlattice band gaps are found to depend on the layer thicknesses through quantum mechanical effects and through the strains in the layers.

Concerning the absorption spectra of the ions M(bpy)32+ (M = Fe, Ru, Os; bpy = 2,2'-bipyridine)

Abstract: An electronic structural model, which includes spin-orbit coupling, is developed for the absorption spectra of the ions M(bpy)/sub 3//sup 2 +/ (M = Fe, Ru, Os; bpy = 2,2'-bipyridine). It is found that, even for Os, the excited states can be classified as singlets and triplets although there is considerable mixing between the pure spin states. Consequently, the luminescent excited states of Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ are assigned as being states largely triplet in character. Explicit assignments of the absorption spectra for the complexes are proposed. The implications of the present treatment relative to other theoretical analyses are discussed. 8 figures, 7 tables.

Electron density theory of atoms and molecules

Abstract: This text describes experimental electron density determination in direct and momentum space and develops theories of electronic structure based on electron density with emphasis on systems with a large number of electrons.

Electronic properties of graphite: A unified theoretical study

Abstract: We have calculated the electronic structure of three-dimensional graphite using the modified first-principles Korringa-Kohn-Rostoker technique developed for and applied to the intercalation compound Li${\mathrm{C}}_{6}$. Whereas previous calculations of the electronic band structure of graphite provide explanations either for moderate- to high-energy excitations or for low-energy and Fermi-surface properties, we find excellent agreement between our results and experiments in both regimes. Our analysis of the band structure is based on a comparison with experiments of predicted optical transitions, values for the Slonczewski-Weiss-McClure parameters which we obtain from a fit to our bands, and Fermi-surface properties. We also present a density of states for our band structure and several constant-energy surfaces. Our discussion includes a comparison with other theoretical work.

An empirical approach to K-shell ionisation cross section by electrons

Abstract: The working out of an empirical expression for the cross section for production of atomic K-shell vacancies by electrons is described. The basis is a critical analysis of the existing experimental data. The ranges for which the function is evaluated are 6

Spectral-structural correlations in high-spin cobalt(II) complexes

Abstract: The spectral and magnetic properties of simple high spin cobalt(II) complexes are reviewed with the aim of showing how it is possible to relate the experimental data to the electronic structure and the coordination geometry of the complexes. It is hoped that this may be of help to all researchers who need to characterize cobalt(II) complexes or use cobalt(II) as a spectroscopic probe. The data relative to electronic, EPR, MCD, NMR spectra as well as to magnetic susceptibility measurements are interpreted within an Angular Overlap approach. Energy level diagrams are employed to calculate the electronic spectra and then, applying the relevant perturbation μ, g, A values are calculated for different chromophores in different coordination environments. The underlying theory is sketched paying in every case much attention to show the possibility of calculating the electronic properties in low symmetry environments. An extensive compilation of available experimental data is presented. The chromophores are classified according to the coordination number and according to the donor atom set.

Molecular‐orbital model for metal‐sapphire interfacial strength

Abstract: Self-consistent-field X-Alpha scattered-wave cluster molecular-orbital models have been constructed for transition and noble metals (Fe, Ni, Cu, and Ag) in contact with a sapphire (Al2O3) surface. It is found that a chemical bond is established between the metal d-orbital electrons and the nonbonding 2p-orbital electrons of the oxygen anions on the Al2O3 surface. An increasing number of occupied metal-sapphire antibonding molecular orbitals explains qualitatively the observed decrease of contact shear strength through the series Fe, Ni, Cu, and Ag.

Diatomic Molecule Electronic Structure beyond Simple Molecular Constants

Abstract: Molecules with partly filled 3d or 4f shells have a surprisingly large number of low lying electronic states, resulting in incredibly complex spectra. This paper attempts to show that, although the spectra are complicated, the explanation of the global electronic structure is quite simple. Ligand Field Theory displays all of the important relationship between the dN or fN structures in the free-atomic-ion and the molecule. The splitting of the core into numerous substates causes a gratuitous repetition and intermingling of the usual ϱ, π-valence structures which are so familiar for molecules with filled-shell cores. The 4f electron is shown to be a useful probe of the unknown structures of more diffuse valence orbitals. A semi-empirical theory is described which allows important features of the molecular level diagram to be predicted using atomic energy level tables and a scrap of paper. The same theory provides a basis for detailed numerical parametrization and quantitative predictions of energy levels, perturbation matrix elements, hfs splittings, and transition intensities.

Surface electronic structure

Abstract: The theory of the electronic structure of clean metal and semiconductor surfaces is reviewed, starting from an effective one-electron Schrodinger equation. Methods for solving the Schrodinger equation at surfaces are briefly described, and the effects of the surface on the electronic wavefunctions are discussed using simple models. The results of detailed calculations of the surface electronic structure of s-p bonded metals, transition metals and semiconductors are reviewed, with an emphasis on the effect of the local environment on the density of states. Properties like the work function and surface energy depend on the surface electronic structure, and their variation with material and surface is discussed; the surface energy contains an important contribution from the interaction between electrons, and this will be considered in some detail. The change in electronic structure compared with the bulk leads to changes in atomic structure, with surface reconstruction on semiconductor and some metal surfaces, and this is also discussed. The interplay between theory and experiment is very important in surface studies, and the theoretical surface energy bands reviewed compare well with experimental photoemission results; this comparison has proved particularly useful for understanding surface reconstructions.

Temporary negative ions in the chloromethanes CHCl2F and CCl2F2: Characterization of the σ* orbitals

Abstract: The electron transmission spectra of chlorometers CHCl2F and CCl2F2 are presented. The electron affinities are evaluted compared with thoses calculated using self‐consistent field methods. (AIP)

Electronic Structure of Black Phosphorus in Self-Consistent Pseudopotential Approach

Abstract: The energy band structure of black phosphorus is calculated by using self-consistent pseudopotential method. The resulting band structure has the direct minimum gap at the point Z in the Brillouin zone in agreement with the result of the tight-binding approach. Effective electron and hole masses and the level shift of the band edge by pressure are calculated from the bands obtained. The pressure dependence of the energy gap is in good agreement with experiment, but the anisotropy of the effective masses contradicts that of the electrical conductivity measured for the single crystal. The nature of the optical absorption edge is discussed in terms of the calculated band structure.

X-ray form factors and the electronic structure of graphite

Abstract: The bonding and spectroscopic properties of graphite are investigated by carrying out first-principles, self-consistent electronic structure calculations, and by comparing the results with high-resolution data from recent x-ray diffraction and angle-resolved photoemission measurements. The theoretical valence-charge density is in excellent agreement with values derived from experimental x-ray from factors. Unlike other group-IV covalent materials, the bonding charge exhibits a prominent double-humped structure due to the lack of $p$ core states. The energy band structure is also in good agreement with experimental measurements and previous calculations.

Band structure, intercalation, and interlayer interactions of transition-metal dichalcogenides: Ti S 2 and LiTi S 2

Abstract: The layered structure dichalcogenide Ti${\mathrm{S}}_{2}$ was studied in thin-film and bulk models, with the use of the self-consistent linearized augmented-plane-wave (LAPW) scheme. Band-structure calculations were performed on the single sandwich (S-Ti-S), the double sandwich ${(\mathrm{S}\ensuremath{-}\mathrm{T}\mathrm{i}\ensuremath{-}\mathrm{S})}_{2}$, and the bulk to reveal the strength and effect of interlayer interactions on the electronic structure. Comparisons are made with existing bulk band models. The fully intercalated LiTi${\mathrm{S}}_{2}$ compound was studied by the same LAPW methods. Changes in the electronic density of states and charge density induced by the alkali intercalate are described.

Excited states of polar negative ions

Abstract: The spectra of electronically excited states of strongly polar negative ions are discussed in terms of general features that may be predicted for such systems. The general properties are then studied through a systematic treatment of lithium halide and lithium hydride anions. Only one or two excited electronic levels exist for these systems, and the binding energies are so low that a limited number of bound rotational levels are associated with each of the excited states. For the states of lowest binding energies, abnormal rotational level spacings are demonstrated. Also, a discussion is given of the implictions for electron scattering and photodetachment studies, of the higher lying dipole states which cross over into the continuum.

Strong nonadiabatic effects and conical intersections in molecular spectroscopy and unimolecular decay: C2H4+

Abstract: The importance of nonadiabatic effects in small polyatomic molecules is discussed. It is pointed out that the interaction between different molecular electronic states can in general not be described in terms of a single vibrational mode. Rather, totally symmetric modes which modulate the electronic energy separation must also be taken into account. The inclusion of these modes leads to a multidimensional intersection of the adiabatic potential energy surfaces and to a dramatic enhancement of the nonadiabatic effects. In the presence of several totally symmetric modes, those modes that have a minor influence on the vibronic coupling problem by themselves can still strongly enhance the nonadiabatic effects. The importance of the multimode effects is demonstrated for the second band in the photoelectron (PE) spectrum of ethylene. This band is well separated energetically (2 eV and more) from all other bands in the spectrum. It is found that in this band none of the ∼1000 calculated lines can be understood within the adiabatic approximation. The line structure is highly erratic and cannot be explained by any decoupling of the modes nor by a ’’broadening’’ of the adiabatic vibrational levels. It is concluded that strong nonadiabatic effects constitute a more common phenomenon than is usually believed.

Laser excitation spectra and lifetimes of Pb2 and Sn2 produced by YAG laser vaporization

Abstract: A new and very general technique, combining pulsed laser vaporization with laser induced fluorescence is described. The gas phase electronic spectra of Pb2 cooled to near 77 °K by liquid N2 are very simple and yield the vibrational constants for the ground state and two low lying electronic states. Some lifetime information is also obtained and preliminary observations of Sn2 spectra are reported.

Structure and bonding in crystalline boron and B12C3

Abstract: The first complete set of electronic structure calculations are reported for elemental boron in its alpha -rhombohedral (12 atoms per unit cell), beta -rhombohedral (105 atoms) and suggested alpha -tetragonal (50 atoms) crystalline forms. The results show that a band picture provides an accurate description of the bonding in these solids. The alpha -rhombohedral structure is found to produce semiconducting properties, with an indirect gap of 1.7 eV. The ordering of bands can be qualitatively interpreted in terms of the internal molecular orbitals of a B12 icosahedron and the two-centre and three-centre external bonds linking neighbouring icosahedra. Replacement of the electron-deficient bonds by linear C3 units gives the much stronger B12C3 structure, in which the calculated gap increases to a direct 3.8 eV. In the more complicated beta -rhombohedral elemental structure, a forbidden gap approximately 2.7 eV occurs in the spectrum of electron states. Some degree of defect- or impurity-induced disorder seems essential to stabilise the structure, since the valence band of the 'ideal' structure can accommodate 320 electrons per unit cell, compared with the 315 available. In the suggested alpha -tetragonal modification of pure boron, the electron deficit would be so severe that this structure probably occurs only for compounds such as B50C2 and B50N2.

Electronic structure of vinoxy radical ch2cho

Abstract: Ab initio multiconfiguration Hartree–Fock (MCHF) wave functions have been used to describe the electronic structure of vinoxy radical CH2CHO. The energy difference between the ground 2A\ state and the first excited 2A\ state is found to be 3.22 eV using a double zeta quality basis set, in good agreement with the observed transition at 3.57 eV. In addition, the calculations predict a near infrared electronic transition.

Isolated ultracold porphyrins in supersonic expansions. III. Free‐base porphine

Abstract: The vibrational level structure of the S0→S1x transition (the Qx band) and of the S0→S1y transition (the Qy band) of free‐base porphine in supersonic expansions of He was interrogated by laser‐induced fluorescence excitation spectroscopy. Electronic relaxation in the S1x manifold was explored by time‐resolved spectroscopy, revealing a constant value of the decay lifetime τ=9.5±1.0 ns for excess vibrational energies in the range Ev=0–5000 cm−1. The line broadening (FWHM) Δ=1.0–1.5 cm−1 of the electronic origin and of low‐lying vibrational excitations in the S1x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ=11.9 cm−1 of the electronic origin of the S1y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S1y was found to be Lorentzian, providing a quantitative determination of the lifetime τ=5×10−13 s for interstate S1y–S1x electronic relaxation within a bound level structure of...