Electronic Structure Factors Research Articles

Performance of a versatile instrumentation for inelastic X-ray scattering spectroscopy (IXSS) with synchrotron radiation

The performance and test of an instrument for inelastic X-ray scattering spectroscopy (IXSS) using synchrotron radiation from the HARWI wiggler at DESY/HASYLAB, Hamburg, are described. The instrument can be used both for Compton profile measurements with 0.13 a.u. momentum space resolution using 32 keV X-rays and for measurements of the electronic dynamic structure factor S( q, ω), with 1.6 eV resolution using 13.7 keV X-rays. The fixed-exit monochromator consists of a plane water-cooled Si (511) crystal acting together in a nondispersive setup with a cylindrically bent sagittally focusing second Si (511) crystal. The energy analysis for Compton profile measurement is performed by means of a Cauchois-type cylindrically bent Si crystal and a 1D position sensitive 200-strip Ge detector. The energy analysis for S( q, ω)-measurements is done by using a spherically bent nearly back-reflecting Si crystal in full Rowland geometry, where the advantages of dispersion compensation are fully exploited. Test measurements on Si (Compton profiles) and graphite ( S( q, ω)) are presented.

Investigation of Eu(III) Binding Sites on Datura Innoxia Using Eu(III) Luminescence

A pulsed tunable dye laser has been used to obtain excitation spectra and fluorescence decay curves of solid Eu(III)- Datura innoxia and from a series of Eu(III)-containing complexes. Carboxyl and sulfate groups have been demonstrated to be the dominant functional groups for forming binding sites on the cell wall of Datura innoxia at high (≥4) and low (≤3) pH conditions, respectively. The excitation spectra associated with the 7 F0 → 5 D0 electronic transition of Eu(III) luminescence have been used to provide a measure of the electronic structure factors contributing to the interaction between Eu(III) ions and the binding sites on the cell wall of D. innoxia. The noticeably broadened and asymmetric excitation spectra obtained at high pH conditions are ascribed to multiple binding sites. The corresponding lifetime decay curves exhibited a bi-exponential decay. A pK of 4.5 was determined for the binding of Eu(III) to the cell wall at pH ≥4. Kinetic and thermodynamic studies were also undertaken.

Calculated magnetic properties of the antiferromagnetic UNiSn compound

The electronic structure, magnetic moment, and magnetic form factor of the type-I antiferromagnet UNiSn has been calculated. The calculations include spin-polarization and are fully relativistic, with the spin-orbit interaction included at the variational step. It is found that including relativistic and orbital-polarization terms is important. With these interactions included, fair agreement between theory and experiment is found concerning the sublattice moments as well as the magnetic form factor. The sublattice moment is found to be composed of a large spin and orbital part that are coupled antiparallel with the orbital moment dominating. The present results show that the previous agreement between the spin-only moment and experiment was only fortuitous and indicates the complexity of magnetism in actinide systems.

Exchange and correlation effects in the Visscher-Falicov model for metal intercalated graphite

A simple model for metal intercalated graphite as a layered electron gas treated in the random phase approximation, which was proposed in early work of Visscher and Falicov, is extended to include exchange and short-range intraplane correlations between the electrons. Main attention is focussed on the static screening function of the model system, in relation to effective pair potentials in metal intercalants. The local field factor accounting for exchange and correlation is related to the intraplane electron structure factor and the latter is evaluated in a simple pseudoclassical approach, which is tested against known results for a quantal 2 D electron gas. Screening of an external point charge is shown to be appreciably modified by exchange and correlation in the case when it lies in an electron plane, leading to a deeper main attractive well between two external charges lying in the same electron plane. On the other hand, exchange and correlation have only minor effects when the charges lie at mid distance between two electron planes, at least if reasonable physical requirements are made on acceptable values for the electron screening length in the relevant range of model parameters.

Electronic structure factors of silicon-hydrogen bond activation by transition metals. Valence photoelectron spectra of (.eta.5-C5H4CH3)Mn(CO)(PMe3)HSiCl3 and (.eta.5-C5H4CH3)Mn(CO)(PMe3)HSiHPh2 (Me = CH3, Ph = C6H5)

The observations indicate that the electronic structure of the Si-H interaction with the metal in this complex is in the initial stages of Si-H bond addition to the metal, before oxidative addition has become prevalent

Electronic structure factors of Ge–H bond activation by transition metals. Photoelectron spectra of [Mn(η5-C5H5)(CO)2(HGePh3)], [Mn(η5-C5H4Me)(CO)2(HGePh3)], and [Mn(η5-C5Me5)(CO)2(HGePh3)

The He I photoelectron spectra of [Mn(η5-C5H5)(CO)2(HGePh3)], [Mn(η5-C5H4Me)(CO)2-(HGePh3)], and [Mn(η5-C5Me5)(CO)2(HGePh3)] have been obtained to measure the nature and extent of Ge–H bond interaction with the transition metal centre in these complexes. The principal electronic structure factors contributing to the addition of the Ge–H bond to the transition metal involve the interaction of the Ge–H σ and σ* orbitals with the metal. The shape and splitting pattern of the metal-based ionisation band indicates the extent of Ge–H σ* interaction. The electron distribution between the Ge–H bond and the metal is indicated by the relative stabilities of the metal- and ligand-based ionisations. The electron charge-density shift from the metal to the ligand is negligible in these three complexes and the metal ionisations reflect the formal d6 electron count at the metal centre. The electronic structure of the Ge–H interaction with the metal is in the initial stages of Ge–H bond addition to the metal, before oxidative addition has become prevalent. The mechanism of interaction of the Ge–H bond with the manganese centre is predominantly through interaction of the filled Ge–H σ-bonding orbital with the empty metal orbitals. It is concluded that the magnitude of Ge–H σ interaction with the metal centre is similar to that of the corresponding Si–H σ interaction.

Factors influencing the presence and detection of compositional grading at semiconductor hetero-epitaxial interfaces

The importance of sharp interfaces in semiconductor hetero-structures has long been recognized. In this paper, a simple growth model, appropriate to gas-phase epitaxy, is used to deduce how composition, lattice parameter, electron structure factor and energy-gap vary across interfaces between InP and lattice-matched GaInAs. It is shown that highly strained regions may be formed at the interfaces. The effect of these strained interfaces on X-ray diffractometry, TEM and optical investigations is discussed. It is shown that X-ray diffraction is significantly more sensitive to the strained interfacial regions than conventional TEM methods.

Electronic structure factors of silicon-hydrogen bond activation by transition metals. The valence photoelectron spectrum of silylmanganese complex (.eta.5-C5H5)Mn(CO)2HSiCl3

Electronic structure factors of silicon-hydrogen bond activation by transition metals. The valence photoelectron spectrum of silylmanganese complex (.eta.5-C5H5)Mn(CO)2HSiCl3

Accurate structure-factor phase determination by electron diffraction in noncentrosymmetric crystals.

A solution to the phase problem is described which gives the phase invariant $\ensuremath{\Psi}=2{\ensuremath{\varphi}}_{h}\ensuremath{-}{\ensuremath{\varphi}}_{g}$ for electron structure factors with phases ${\ensuremath{\varphi}}_{h}$ and ${\ensuremath{\varphi}}_{g}$ in noncentric crystals. The method exploits the high-voltage dependence of a minimum in convergent-beam transmission-electron-diffraction patterns in the systematics three-beam geometry. For CdS, with $h=(00\overline{2})$ and $g=(00\overline{4})$ we find $\ensuremath{\Psi}=54.4\ifmmode^\circ\else\textdegree\fi{}+0.9\ifmmode^\circ\else\textdegree\fi{}$. The error in the derived ($00\overline{2}$) x-ray structure-factor phase is 0.069\ifmmode^\circ\else\textdegree\fi{}. The method is accurate enough to provide information on the bonding charge distribution in noncentric crystals.

Structure factor of weakly pinned charge-density waves in sub-threshold electric field

Abstract The electric field dependence of the static electronic structure factor S(x) of a one-dimensional charge-density wave (CDW) is investigated theoretically within the Fukuyama-Lee-Rice Hamiltonian in the limit of weak pinning and for sub-threshold electric fields. By a perturbation treatment of a Fokker-Planck equation with respect to the electric field it is found that the CDW splits into a closely spaced series of partial waves. The dominating phase correlation length is found to decrease slightly until the threshold electric field is reached. Partial Q-shifts in S(x) are also obtained and they are related to the capacitance of the CDW and the associated phenomenon of charging.

Quantum chemical study of electronic structure and receptor binding in opiates.

A Quantitative Structure-Activity Relationship study for a group of opiates interacting with the mu receptor was carried out. The study consisted in searching a relationship between the drug-receptor equilibrium constant and steric/ electronic structure factors. Several equations were obtained and analyzed. It is possible to conclude from this study that if all the experimental values correspond only to the interaction with the mu receptor, then the hypothesis of a common skeleton interacting with the mu receptor cannot be supported.

Small angle scattering experiments from Al-4wt.% Cu single crystals containing G.P.I. zones

The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al–Cu, Al–Mg, Al–Zn, and Al–Si alloys, we carry out cluster expansions of ab initio calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems.

Electronic structure of piano-stool dimers—VII. Interrelationships among the electronic structure, reactivity and conformational preferences of hydrocarbyl-bridged piano-stool dimerst

The electronic structure of a number of hydrocarbyl-bridged piano-stool dimers has been calculated by using the non-empirical Fenske-Hall molecular orbital method. It is shown that there is a close connection between a number of electronic structural factors (e.g. the number of available metal-based orbitals, the number and energies of available orbitals on the hydrocarbyl bridge, and the electronic competition between the hydrocarbyl bridge and the ancillary ligands) and the reactivities and conformational preferences of the dimers. Examples are chosen from both low-valent dimers with π-acid ancillary ligands and high-valent dimers with predominantly σ-donor ligands.

Lattice-induced double peak in the dielectric response of Be metal.

Measuring the inelastic scattering of synchrotron x rays by Be metal we found a pronounced double peak in the electronic dynamic structure factor S(q,\ensuremath{\omega}) for q parallel [110]. We attribute the double peak to the existence of an excitation gap for final states on the (110) Bragg plane. For q=0.77 a.u. a zone-boundary collective state could be identified by means of a Kramers-Kronig--deduced zero of Re\ensuremath{\epsilon}(q,\ensuremath{\omega}). Applications to band analysis are discussed.

Momentum Dependence of the Dynamic Structure Factor of Core Electrons in Metallic Lithium

The dynamic structure factor of the core electrons in metallic lithium, S c ( q , ω), is calculated in a unified way on the basis of both the one-electron and the one-site approximation. The aim is to furnish the first theoretical data which covers the whole range of q and ω. The results are analysed from several angles. It is found that the Compton-to-Raman transition occurs gradually with decreasing q on the interval \(1.5{\gtrsim}qa{\gtrsim}0.2\), a being the orbital radius of core electrons. Experimental studies of S c ( q , ω) in this transition region are expected to provide us with new information on the excitation of core electrons.

Scanning Tunneing Microscope Study of Cadmiun Telluride

ABSTRACTLayered samples of cadmium telluride grown epitaxially on gallium arsenide substrates have been investigated by means of scanning tunneling microscopy (STM). The surface geometric and electronic structures are both of interest. Techniques were developed to remove the native oxide by etching, or to create a fresh surface by cleaving, and to protect them from oxidation by employing mineral or paraffin oil media. STM studies were conducted within the protective medium. These techniques were adapted and tested for both etched and cleaved samples of Si, CdTe and HgTe. The currentvoltage characteristics of the CdTe surface were investigated during dynamic changes of the tunnel current and barrier height while the probe-to-sample separation (gap) was static. Recorded values are shown for current versus bias for several constant gap values. A range of bias values has been employed to test a possible solution for the problem of interference between the contributions from geometric and electronic structure factors.

Additions and Corrections. Electronic Structure Factors of Carbon-Hydrogen Bond Activation. The Photoelectron Spectroscopy of (Cyclohexenyl) manganese Tricarbonyl

Additions and Corrections. Electronic Structure Factors of Carbon-Hydrogen Bond Activation. The Photoelectron Spectroscopy of (Cyclohexenyl) manganese Tricarbonyl

Pseudoclassical approach to electron and ion density correlations in simple liquid metals

Electron-electron and electron-ion structural correlations in simple liquid metals are treated by using effective pair potentials to incorporate quantal effects into a pseudoclassical description of the electron fluid. An effective pair potential between simultaneous electron density fluctuations is first constructed from known properties of the degenerate jellium model, which are the plasmon sum rule, the Kimball-Niklasson relation and Yasuhara’s values of the electron pair distribution function at contact. An analytic expression is theraby obtained in the Debye-Huckel approximation for the electronic structure factor in jellium over a range of density appropriate to metals, with results which compare favourably with those of fully quantal evaluations. A simple pseudoclassical model is then set up for a liquid metal: this involves a model of charged hard spheres for the ion-ion potential and an empty-core model for the electron-ion potential, the Coulombic tails being scaled as required by the relation between the long-wave-length partial structure factors and the isothermal compressibility of the metal. The model is solved analytically by a pseudoclassical linear response treatment of the electron-ion coupling and numerical results are reported for partial structure factors in liquid sodium and liquid beryllium. Contact is made for the latter system with data on the electron-electron structure factor in the crystal from inelastic X-ray scattering experiments of Eisenberger Marra and Brown.

Electronic structure factors of carbon-hydrogen bond activation. The photoelectron spectroscopy of (cyclohexenyl)manganese tricarbonyl

The He I and He II ionizations for (cyclohexenyl)manganese tricarbonyl, a molecule which exhibits an activated C-H bond, are reported. Electronic structure factors contributing to the initial activation of the C-H bond are discussed in terms of two limiting descriptions that have been presented in the literature. These descriptions are labeled sigma activation, involving the donation of C-H sigma bonding orbital electron density into the empty metal orbitals, and sigma* activation, involving electron density flow in the opposite direction, i.e., from filled metal levels into the empty C-H sigma* antibonding level. Both processes are shown to be possible based only on geometry, symmetry, and overlap considerations. The experimental data demonstrate that the principal electronic structure mechanism for the early stages of C-H bond lengthening and interaction with the metal exhibited in (cyclohexenyl)manganese tricarbonyl is sigma activation. The data are also related to variable-temperature NMR studies of this complex which show that fluxionality proceeding through a 16 e/sup -/ intermediate is more favorable than that through an 18 e/sup -/ intermediate with a full metal-hydrogen bond. Both bond strength and ionization energy data show that the C-H activation of (cyclohexenyl)manganese tricarbonyl stops at the agostic stage because there is no netmore » gain in carbon-carbon or metal-carbon bonding to compensate the loss of the C-H bond.« less

Stresses in semiconductors: Ab initio calculations on Si, Ge, and GaAs.

Explicit formulas for the calculation of stress are presented based on the stress theorem and the local-density-functional approximation. Norm-conserving pseudopotentials are applied in a plane-wave basis for calculations on the semiconductors Si, Ge, and GaAs. Besides the lattice constants and bulk moduli, complete sets of elastic constants are given, together with the optical \ensuremath{\Gamma} phonon frequencies and internal-strain parameter \ensuremath{\zeta}. Electronic charge density structure factors, deformation potentials, and strain-induced splittings of phonons are given, as well as the nonlinear third-order elastic constants. Good agreement with experiment is found throughout, except for persistent deviations from the x-ray diffraction values for \ensuremath{\zeta}.