Valence Electron Distribution Research Articles

AES investigation of the chemical structure of silicon oxy-nitride films

The silicon-L2,3VV, nitrogen-KVV, and oxygen-KVV Auger signals have been measured for a series of chemical-vapor-deposited silicon nitride films in which the oxygen content varied from 2 to 67 at. %. Uncorrected derivative spectra and background-corrected, loss-deconvoluted integral lines for silicon nitride films, for clean Si, and for SiO2 were compared. The silicon spectra indicate that a high-energy Si-L2,3VV feature from the nitride, previously identified with elementally bound Si, actually indicates an oxygen impurity. The N-KVV lineshape is found to be insensitive to oxygen incorporation while the three-peaked oxygen line becomes broader with increasing oxygen content. The valence-electron distribution width in the nitride is more than twice that of the distribution in elemental Si, but the width does not change appreciably with changing oxygen content. These Auger data indicate that oxygen incorporation into silicon nitride results in the formation of oxynitrides for ≲ 200 ppm of oxygen in the reactor. Beyond this concentration, silicon oxide is largely formed.

Ab initio LCAO MO SCF CI Calculations on the Electronic Structure of the Cyclopropenyl Cation

Abstract Ab initio LCAO MO SCF and limited CI calculations are carried out on the electronic structure of the C3H3+ ion. Two kinds of basis sets are used, namely the minimal STO’s and the minimal STO’s plus 3d orbitals on each carbon atom. With the latter basis, the equilibrium C–C distance is calculated to be 1.389 Å. It is predicted that the C–C bond distance of C3H3+ is shorter than that of C6H6. The calculated C–C stretching force constant is 7.92 mdyn/Å and is close to the experimental value 6.59 mdyn/Å. Contour diagrams of the charge distribution of valence electrons are drawn and it is revealed that the calculated C–C bonds are bent and have two maxima in charge density. In terms of population analysis of localized MO’s, it is shown that the C–H bond in this molecule has a higher s-character than the C–H bonds in C2H4 and C6H6 and is close to an sp hybrid. This is in accord with a very large NMR coupling constant J13C–H. From CI calculations of low lying excited states, singlet σ-π* states (1A1″, 1E″, 1A2″) are calculated to be lower than the singlet π-π* state (1E′).

Theoretical study of lithium graphite. II. Spatial distribution of valence electrons

Based on band-structure calculations presented in part I, the valence charge density of Li${\mathrm{C}}_{6}$ is evaluated and presented in contour diagrams and iinear plots. The hybridization of the Li $2s$ states with the bands of graphite is studied and quantified in terms of the Li contact density. It is found that the Li contact density of 0.15 electrons/${\mathrm{a}.\mathrm{u}.}^{3}$ is mainly contributed by the bonding C $\ensuremath{\pi}$ band and only 6% by the weakly antibonding Fermi-level band, as is consistent with the experimental value of the $^{7}\mathrm{Li}$ Knight shift.

Structural information from directional compton profile measurements on Si and Ge

Compton profile measurements on single crystals can reveal effects on the valence electron distribution arising both from local atomic interactions and from the long-range ordering of the atoms in the crystal. In order to separate those features relating primarily to the structure, we have analysed the long-range autocorrelations of the electronic wave functions obtained directly from the Fourier transform of the Compton profile. New experimental results for Si and Ge, measured along 〈110〉, clearly illustrate the expansion of the valence electron wave functions which follows the 4% change in the lattice constant. In particular, the autocorrelations display zero passages which arise from the lattice translational symmetry, and these features provide a basic test of the quality of experimental data.

3d‐Orbital Participation in Sulfur Diimides

An electron decet is often invoked to explain unusual properties of sulfur compounds. The PE and electronic spectra of (1) and (2) have now been compared. They are almost superposable, suggesting analogus distributions of valence electrons. Tetravalent sulfur does not appear to be present.

Valence electron distribution and “atomic size” in crystals

Valence electron distributions are analyzed for crystals of various types. It is found that for a given element the distance from the center of an atom to the maximum (minimum) along the “bond” the valence electron density remains roughly the same in crystals of different structures. A correlation is shown to exist between the distance mentioned above and the radius of the external valence orbital of the free atom.

A simple refinement of density distributions of bonding electrons. IV. The distribution of valence electrons in cyanuric acid

A simple refinement of density distributions of bonding electrons. IV. The distribution of valence electrons in cyanuric acid

Electron-density studies. III. A re-evaluation of the electron distribution in crystalline silicon

Highly accurate absolute measurements of the X-ray structure factors of silicon [Aldred & Hart, Proc. R. Soc. London, Ser. A, (1973). 332, 223-238] have been used to analyse a number of models for the electron distribution. Initially, the valence-electron distribution (with the neon core assumed to be unmodified from that of the isolated silicon atoms) was built up with a radial basis of the form rn exp (-ζr) and non-sphericity was allowed for by inclusion of octupole and hexadecapole terms. Improved representation was achieved with related models in which deformations from the total isolated-atom electron density were refined instead. The exact shape of the deformation electron density in the region of the bond was sensitively dependent on the monopole deformation term. The anomalous-dispersion contributions (Δf') to the scattering factors were refined and found to be in agreement with recent interferometric measurements, but not with recent calculations. The octupole density term is slightly sharper at 293.2 than at 92.2 K, and the structure factor for the 222 reflection is predicted to be larger at the higher temperature. These effects may be due to a failure of the convolution approximation or to uncertainties in the anharmonic corrections to the structure-factor data.

A theory of energy loss of channeled protons

Abstract The methods due to Bloch and Pines have been used to calculate the energy loss of channeled protons. The contribution from the various atomic shells to the stopping power is assumed to be additive. The collective effects in a solid plasma produced by the valence electrons are calculated disregarding the band structure. The nonuniform distribution of valence electrons across the channel is taken into account. In contrast to Robinson and Dettmann, a detailed summation is performed over the spectrum of oscillator strengths when calculating the energy loss due to inner-shell electrons. It has been found that, contrary to the common belief, the energy loss due to inner-shell electrons is considerable even for wide channels, whereas the energy loss due to the valence electrons turns out to be lower. The numerical calculations for silicon and germanium have yielded results that agree well with experiment. The energy loss at relativistic velocities has also been calculated. The obtained results may be used for quantitative predictions of energy loss in channeling experiments.

Valence electron momentum distributions for acetylene

The momentum distributions of valence electrons for acetylene molecules were determined by the (e, 2e) technique. This technique involves knock-out of each electron of interest by electron impact with complete determination of the momenta of the incident and out going electrons.(AIP)

A dielectric calculation of energy loss to valence electrons of channelled protons in silicon

The Bonsignori-Desalvo theory for calculating the energy loss of protons to valence electrons under channelling conditions through the non-diagonal components of the response function has been applied to silicon. Analytical approximations to the components of the response function are developed and their range of validity discussed. Local stopping power maps at 100 keV are obtained for the (110), (112) and (111) channels and the link with the distribution of valence electrons in a covalent semiconductor is analysed. By using a Monte Carlo computer model to simulate ion paths it is possible to make a significant comparison with experiments by different authors, obtaining a good agreement.

X-ray photoemission spectra of valence electrons in V 3X and Nb 3X compounds

The X-ray photoemission spectra (XPS) of the A15 type compounds V 3Au, Nb 3Os, Nb 3Ir, Nb 3Pt and Nb 3Au have been studied. The inner level binding energies of the different components and the valence electron distribution were measured. The Nb4 d and the X5 d energy bands of the Nb 3 X compounds appear to be more and more separate with increasing atomic number of the X component. The comparison between the results from X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) of corresponding V 3 X and Nb 3 X compounds points out the similarity of their electronic structures.

Theory for the anisotropic hyperfine coupling with fluorine: the ·CF3radical

A method has been developed to estimate the dipolar hyperfine interaction for 19F in fluorinated radicals based on INDO theory. The dipolar integrals have been calculated using Slater-type orbitals. Careful selection of effective nuclear charges, different from those obtained by Slater's rules, and adjustment for the valence electron distribution proved necessary in order to obtain agreement between theory and experiment. The method has been applied to a prototype system, the ·CF3 radical. The results confirm a pyramidal structure with an F-C-F bond angle θ = 109·5° - 112·0° and a C-F bond length of 1·30-1·35 A. With θ=111·1° the dipolar tensor is calculated to be (122·2, -55·5, -66·7) G, close to the experimental result (120·0, -56·5, -63·5) G. An ambiguity in the determination of the sign of the angle α' between the 120·0 G component and the C3v symmetry axis was found to occur experimentally but not theoretically. According to experiment α′ = ± 17·8°, where the negative sign corresponds to an axis perp...

An (e,2e) study of ammonia: Binding energies and momentum distributions of valence electrons

The valence shell electronic structure of NH 3 is studied in an (e,2e) experiment with symmetric non-coplanar geometry. The momentum distributions obtained for the separate orbitals are compared with those calculated from several approximate wavefunctions. The 3a 1 distribution is found to be particularly sensitive to the form of the wavefunction.

The interpretation of the 119 mSn Mössbauer data for some complexes of tin(II)

A model for the interpretation of the 119 m Sn Mössbauer data for some tin(II) compounds in terms of the distribution of the tin valence electrons is suggested. It accounts for the positive signs of the quadrupole coupling constants ( eQV zz) observed for tin(II) compounds and for the variations in the Mössbauer parameters of triligandstannates(II) as the cation is changed. Although there is no general relationship between the chemical isomer shift and the quadrupole splitting parameters for tin(II) materials, the model does explain why approximately linear relationships are found between these parameters for particular series of triligandstannates(II).

Elastic constants of some MAl2 single crystals

Elastic constants of single crystals of the cubic Laves phases CaAl2, YAl2, LaAl2, and GdAl2 have been measured over the temperature range 4.2–300 K. The ferromagnetic transition in GdAl2 near 170 K is evident by changes in slope in plots of the three independently measured elastic constants of GdAl2 as functions of temperature. Debye temperatures at 0 K were evaluated as 474 K for CaAl2, 484 K for YAl2, 374 K for LaAl2, 399 K for magnetically disordered GdAl2, and 406 K for magnetically ordered GdAl2. It was inferred from the data that the valence electron distributions in nonsuperconducting YAl2 and GdAl2 are much alike, but the valence electron distribution in superconducting LaAl2 is significantly different. Thus the results support a theoretical calculation indicating an admixture of 4f character into the valence bands of LaAl2.

Pseudopotential Calculation of the Piezoelectric Constant of GaAs

AbstractA theory of the piezoelectricity of zincblende type lattices is presented. The piezoelectric polarization in a uniformly strained crystal is produced by the internal displacements of the two core ion sublattices and the distortion of the valence electron distribution. The latter is calculated applying first order perturbation theory to the empirical pseudopotential method. The computational work has been carried out for GaAs. Satisfactory agreement between theoretical and experimental values of the piezoelectric constant is achieved if the deformable ion model is applied to empirical pseudopotentials. As a test for the accuracy of the numerical calculation the transverse effective charge is evaluated.

Centrosymmetric and asymmetric bichloride ion in crystals and in solution: Comparison of electron populations from nuclear shielding and quadrupole coupling

A simple model relates the hydrogen nuclear shielding in a bihalide ion to the geometry and the valence electron distribution. Applied to symmetric bichloride ion in solution it gives the same electron populations as are found by nuclear quadrupole resonance in the solid. It also yields reasonable electron distributions for the presumed asymmetric ion in solutions of triethylammonium bichloride.

Theory of cohesion in hcp metals

A general expression is obtained, based on the statistical approximation of the quantum theory of solids, for the cohesive energy of a metal, including the correlation of valence electron distributions over the lattice with its symmetry. The general nature of these functions is verified by calculating the free lattices of zinc and cadmium.

Electron distribution in BN

Abstract The Compton profiles of cubic and hexagonal BN have been measured with AgKα X-rays on polycrystalline samples and reveal a somewhat broader distribution in electron momenta for the cubic form. The structure factors for cubic BN were measured on a polycrystalline sample with monochromatic MoKα and CuKα. The values of the 111 and 200 structure factors (the latter is absent in diamond) reveal a significant change in the valence electron distribution as compared to a superposition of free-atom charge densities. Solid state theoretical structure factors are available for comparison in the following paper (Euwema, Surratt, Wilhite and Wepfer 1974). The Debye-Waller factor for cubic BN indicates an effective Debye temperature only about one half that of diamond (750°K compared to 1500°K) but this is consistent with specific heat measurements for diamond and cubic BN. The magnitude of the (420) structure factor suggests that the thermal mean-square displacement of the boron atoms is larger than that of the nitrogen atoms.