Showing papers on "Valence (chemistry) published in 1974"

X-ray photoemission studies of diamond, graphite, and glassy carbon valence bands

Abstract: The high-resolution x-ray photoemission spectra (XPS) of the total valence bands of atomically clean diamond, graphite, and glassy carbon, obtained with monochromatized Al K ..cap alpha.. radiation, are reported and discussed. By comparing valence-band and carbon-1s photoelectron kinetic energies, the XPS valence-band spectra I'E) of diamond and graphite were rigorously affixed to the same energy scale as earlier K x-ray emission spectra i(E). The two spectra - I'(E) and i(E) - have very different energy dependencies of intensity because selection rules and cross-section ratios render i(E) sensitive only to 2p character and I'(E) far more sensitive to 2s character. Taken together, I'(E) and i(E) show that the fractional p character in the diamond valence band increases from approx. 16% at the bottom of the band to approx. 92% at the top, with an average hybridization of approx. s/sup 1/ /sup 2/p/sup 2/ /sup 8/. The spectra agree well with the density of states of Painter et al, but indicate a valence-band width of 24.2(10) eV rather than their 20.8 eV. The C(1s) binding energy of 284.68(20) eV in graphite agrees well with a recent theoretical estimate of 284.4(3) eV by Davis and Shirley. Analysis of I'(E) i(E) for graphite resolvesmore » the valence bands cleanly into sigma and ..pi.. bands, with the spectrum I'(E) of the former resembling that of diamond, but with a stronger 2s admixture (sp/sup 2/ vs sp/sup 3/). The XPS cross section of the (p/sub z/)..pi.. bands was very low, as expected by symmetry. The bandwidth of 24(1) eV somewhat exceeded Painter and Ellis's calculated value of 19.3 eV. Glassy carbon showed an I'(E) between that of diamond and graphite, consistent with an amorphous lattice containing both trigonal and tetrahedral bonds. 8 figures, 3 tables.« less

Electronic structure of silicon

Abstract: It is shown that a purely local-pseudopotential calculation is able to accurately reproduce the major optical gaps and cyclotron masses. However, deviations from the experimental results become manifest in photoemission and x-ray charge-density results as we extend our calculations to the lower valence bands. These deviations indicate the necessity of an energy-dependent nonlocal $s$-well potential, a conclusion which is also supported by an analysis of the Heine-Abarenkov pseudopotential scheme. A detailed comparison is made between experimental results obtained from optical, photoemission, x-ray, and cyclotron-resonance measurements, and the results of both the local calculation and an energy-dependent nonlocal calculation. Yang and Coppens's recent determination of the valence charge density in silicon makes it possible to assess the accuracy of the pseudocharge densities for the first time.

Quantum theory of chemical valence concepts

Abstract: Electron density operators and projection operators are fundamental quantities in the general physical interpretation of quantum theory In this series of papers the aim is to explore their relevance to the definition and use of chemical valence concepts Here, the problems involved in defining the charge on an atom in a molecule are discussed, and a new approach is formulated in terms of these operators The approach depends on the result that, if a projection operator P is formed representing some subspace of a molecular Hilbert space, then the probability of occupancy of that subspace is Tr DP, where D is an appropriate electron density operator In particular, the molecular one-electron Hilbert space is considered, and projection operators for atomic orbitals, atoms, pairs of atoms, atoms in threes, and so on, are found The above result allows for the definition of corresponding occupation numbers From these follow definitions of the charge on an atom in a molecule and of occupation numbers for elec

Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

Abstract: The pressure-volume relationships for EuSe, EuS, YbSe, and YbS have been obtained from high-pressure x-ray-diffraction studies to nearly 300 kbar. Like EuTe, EuSe and EuS exhibit normal compression and undergo the NaCl-to-CsCl-type transition at about 145 and 215 kbar. The compression curves of YbSe and YbS are anomalous in the 150-200-kbar region, which we believe is because of a change in the valence state of Yb from ${2}^{+}$ towards the ${3}^{+}$ state. Among the Eu monochalcogenides EuO is the only substance that shows a valence transformation (\ensuremath{\sim}300 kbar) followed by a NaCl-to-CsCl-type transition (\ensuremath{\sim}400 kbar). However, all the Yb monochalcogenides undergo the valence transformation continuously with pressure. The bulk moduli evaluated from the experimental $P\ensuremath{-}V$ data and the measured pressure coefficients of the energy gap between the $4f$ level and the conduction-band edge from optical-absorption studies of divalent rare-earth monochalcogenides are compared. These data are used to rationalize the occurrence or nonoccurrence of the valence transitions in this series of compounds.

Densities of valence states of amorphous and crystalline III-V and II-VI semiconductors

Abstract: The densities of valence states (DOVS) of the amorphous and crystalline forms of GaP, GaAs, GaSb, InP, InAs, InSb, AlSb, ZnTe, and CdTe have been determined from the energy-distribution spectra of photoelectrons emitted by high-energy photons (16.9, 21.2, 40.8, and 1486.6 eV). In general the DOVS of the amorphous forms can be represented by a broadened version of those of the corresponding crystalline forms. Fine structure which appears in the upper valence bands of the crystalline materials, due to critical points at $L$, $X$, and $W$, is completely washed out in the amorphous phase. The core-level spectra have nearly the same positions and widths in the amorphous as in the crystalline modifications. This fact indicates that the fluctuations in the Coulombic environment about each type of atom are small, suggesting that the structure is homogeneous and contains an insignificant number of odd-membered rings. The plasma frequencies, determined from the plasma-loss spectra associated with core levels, are the same in the amorphous as in the crystalline phases to within 3%. This fact enables us to conclude that the densities of both modification differ by less than 6%. We present a simple bond-charge model which can simulate realistically the density of valence states of germanium and zinc-blende-type semiconductors. The valence bands at any point of the Brillouin zone are obtained in this model as the solution of a 4\ifmmode\times\else\texttimes\fi{}4 secular equation. Within this model, the structure of the top $p$-like valence bands depends primarily on overlap between second-neighbor bonds. Thus fluctuations in the position of second neighbors can be invoked to explain the smearing of the fine structure of these bands in the amorphous modifications. A simple model which relates the chemical shifts of the compounds to their ionicity is also discussed.

Ab initio configuration interaction studies of the π-electron states of benzene

Abstract: Ab initio configuration interaction studies of the π-electron states of benzene were carried out with a double-ζ basis set of contracted Gaussian functions augmented by two diffuse π functions on each carbon atom. The core potential of the σ electrons was obtained from an all-electron SCF calculation on the ground state. Vertical excitation energies of 5.00, 7.64, and 8.34 eV were obtained for the 1B2u, 1B1u, and 1E1u states, respectively, corresponding to the e1g→e2u excitation; the first two of these levels have a valencelike electron distribution, but the 1E1u state was found to have a diffuse 1e2u natural orbital, with = 38 bohr2. The analogous set of triplet states, all of which are valencelike, have calculated vertical excitation energies of 3.83 (3B1u), 4.98 (3E1u), and 7.00 eV (3B2u). The low-lying valence states 1E2g (8.33 eV) and 3E2g (7.28 eV) were found to have substantial double-excitation character, but no additional valence E2g states were obtained. Many other states, including quinte...

Photoemission spectra and band structures of d -band metals. IV. X-ray photoemission spectra and densities of states in Rh, Pd, Ag, Ir, Pt, and Au

Abstract: X-ray photoemission spectroscopy (XPS) data for the valence bands of Rh, Pd, Ag, Ir, Pt, and Au are compared with densities of states calculated from interpolated band structures. The parameters of the interpolation scheme were obtained in an earlier paper by fitting first-principles augmented-plane-wave calculations, and then making adjustments, where necessary, to the width of the $d$ bands in order to improve agreement with ultraviolet photoemission spectroscopy (UPS) data. In the $4d$ metals, Rh, Pd, and Ag, there is excellent agreement between the energy positions of peaks in the occupied density of states and the XPS valence bands. In the $5d$ metals, Ir, Pt, and Au, the agreement is reasonable, but inferior to that for the $4d$ metals particularly in the middle of the $d$-band region. It is argued that the main discrepancies in peak locations are not significant and are most likely due to the approximations in the interpolated band structures, particularly those involved in the spin-orbit splitting and hybridization. The relative intensity of the XPS data in the lower-energy region of the $d$ bands is consistently lower than that in the density of states, indicating appreciable modulation of the spectra by optical-transition strengths.

Valence Electronic Structure and Charge Transfer in Tetrathiofulvalinium Tetracyanoquinodimethane (TTF-TCNQ) from Photoemission Spectroscopy

Abstract: Comparison of ultraviolet-photoemission, valence-orbital spectra (as well as high-resolution, x-ray photoemission, N-core-level spectra) of TTF-TCNQ, neutral TCNQ, and ${\mathrm{K}}^{+}$ ${(\mathrm{TCNQ})}^{\ensuremath{-}}$ indicates a large amount of charge transfer in TTF-TCNQ. The spectra for this complex show strong coupling between states near the Fermi energy and molecular vibrations due to localization of the mobile electrons on individual molecules, at least on the time scale of bond vibrations.

Electrostatic binding in the first-row AH and A2 diatomic molecules

Abstract: The charge migration that converts two overlapping spherical atoms into a bound molecule produces attractive Hellmann-Feynman fields at the two nuclear positions, offsetting the repulsive field that each nucleus encounters on penetrating inside the charge cloud of its neighbour. The magnitudes of these fields correlate positively with the molecular dissociation energies. The contribution of the σ valence orbitals to the field at a first-row nucleus varies regularly from binding in lithium to anti-binding in fluorine. The electrostatic effect of these orbitals is weakened by reversed polarization inside the 2s nodal sphere and is further opposed by a tiny but effective exchange polarization of the core. The major binding role in the first row thus falls to the π orbitals, which, like the σ orbital in hydrogen, have no radial node to inhibit a uniformly binding forward polarization. The net electrostatic interaction of two spherical atoms, derived with neglect of antisymmetry, is always binding. Especially ...

Photoemission spectroscopy using synchrotron radiation. I. Overviews of valence-band structure for Ge, GaAs, GaP, InSb, ZnSe, CdTe, and Agl

Abstract: We determine bandwidths and critical-point positions with respect to the valence-band edge for the valence bands of several extensively studied semiconductors (with an accuracy of about 0.3 eV) by using photoemission densities of states derived from photoemission spectra obtained in the 24-78-eV photon energy range. These photoemission spectra were obtained using synchrotron radiation from an electron storage ring; a double-pass, electrostatic, electron energy analyzer; and samples cleaved and measured in situ in ultrahigh vacuum. We give a detailed description of the data-reduction techniques by which electronic state densities and certain valence-band feature positions are determined from photoemission spectra. This description includes a discussion of the effect of various phenomena such as optical-transition-probability variations and Auger-emission peaks. Tables are presented which compare our valence-band-position values with the results of both empirical calculations fit to optical data as well as ab initio calculations. We find systematic and significant differences between experiment and calculations fit only to optical data, these differences increasing with crystal ionicity, while ab initio calculations generally give a better fit to experiment.

CNDO model and interpretation of the photoelectron spectrum of CO chemisorbed on Ni

Abstract: CNDO calculations for CO chemisorbed on a cluster of 10 Ni atoms produce a pattern of valence orbital energy levels that agrees well with the experimental photoelectron spectrum of CO adsorbed on Ni; thus allowing the experimental energies to be assigned to specific orbitals. The bonding of the carbon atom to the Ni atom largely involves nickel s and p orbitals, with little contribution from the d orbitals.

Measurement and calculation of polarization and potential-energy effects on core-electron binding energies in solids: X-ray photoemission of rare gases implanted in noble metals

Abstract: The binding energies of core electrons on Ne, Ar, Kr, and Xe implanted in Cu, Ag, and Au have been measured by x-ray photoemission and are found to be 2-4 eV smaller in magnitude than the corresponding binding energies obtained from gas-phase measurements. All implanted data have been referenced to the vacuum level of the appropriate metal and are therefore absolute energies suitable for gas-phase comparison. For a given noble-metal host, the magnitude of the binding-energy shift decreases monotonically from Ne to Xe, while for a given rare-gas core electron the shift is largest in Ag and smallest in Cu. Investigation of both these trends allows for the study of the two contributions responsible for these shifts. First, the self-consistent potential experienced by the core level is changed upon implantation (the initial state). Second, the polarization of metal-host electrons upon photoionization (the final state) provides relaxation energy not present in the free atom. Both these effects have been calculated using a model which represents the metal-host valence $s$ electrons as free, the host outer $d$ electrons as a dielectric, and the neutral and photoionized rare gas by a pseudopotential. The model is treated using a density-functional method that allows for both self-consistency and nonlinear screening by the host electrons. All the parameters in the model are empirically determined independently of the present experiment. The results show that the potential shift and polarization energy are of comparable magnitude and opposite sign, so that significant cancellation occurs. The calculated trends in total binding-energy shift for the series of gases in each metal are in excellent agreement with the experimental results, and the trends for each gas from metal to metal are reasonably well reproduced. An essentially uniform discrepancy of 1.4 eV between the absolute calculated and measured values is ascribed to limitations of the model and detailed knowledge of the implantation-site geometry.

Mixed valence complexes of ruthenium ammines with 4,4'-bipyridine as bridging ligand

Abstract: Die Reaktion von Aquopentaamminruthenium(II)-hexafluorophosphat mit 4,4′-Bipyridyl im Verhaltnis 2 : 1 unter Argon liefert den Komplex (Ia), der durch Oxidation mit Brom in (Ic) ubergeht, das als Tosylat gefallt wird.

X-ray photoelectron studies of thorium and uranium

Abstract: The X-ray induced photoemission from both clean and deliberately oxidized thorium and uranium surfaces is described. The results are compared with previously reported XPS and X-ray emission measurements of electron binding energies for the heavy elements. While agreement between photoelectron and X-ray spectroscopic results is fair for most of the electron energy levels studied large discrepancies were observed in the energies of the P1, P2 and P3 levels. The authors discuss also the effect of oxidation on the spectra and on the valence bands for the metals.

Theoretical foundations of purely semiempirical quantum chemistry

Abstract: All the purely semiempirical quantum chemical theories of molecular electronic structure, such as the Pariser‐Parr‐Pople theory and its all valence electron generalizations like MINDO, assume the existence of an abstract true effective Hamiltonian which acts only within the space spanned by a minimum basis set of valence shell orbitals. Instead of following the customary procedure of attempting to determine the properties of this true effective Hamiltonian by fitting its matrix elements to experiment, this effective Hamiltonian is derived from the full N‐electron molecular Schrodinger equation under the condition that a complete configuration interaction calculation within the valence shell using this true effective Hamiltonian reproduces the exact ground and valence state energies. It is shown that this true effective Hamiltonian also exactly reproduces the projection of the exact ground and valence state wavefunctions on the space spanned by the minimum valence shell basis in the fixed core approximatio...

Ab initio calculations of transition metal complexes

Abstract: The He(I) photoelectron spectra of the isoelectronic series Fe(CO)2(NO)2, Co(CO)3NO and Ni(CO)4 are reported and interpreted by means of ab initio SCF-MO calculations. For the nitrosyl complexes it is found that ionization potentials calculated assuming Koopmans' theorem are seriously in error due to the considerably greater orbital relaxation accompanying ionization from metal than from NO valence orbitals. When such allowance is made for orbital relaxation by performing restricted Hartree-Fock (RHF) calculations on the ionic states, the experimental spectra are accurately reproduced and the observed similarity of the spectra of all three molecules is explained.

Ab initio evaluation of correlation contributions to the true π‐electron Hamiltonian: Ethylene

Abstract: The first ab initio calculations of the correlation contributions to the matrix elements of the true effective π electron Hamiltonian are presented for ethylene. It is shown that there is an additional core ``correlation'' energy in the valence singlet excited (V) state that has no counterpart in the triplet (T) and ground (N) states. This correlation energy contributes to the reduction of the V–T separation and the contraction of the π*g orbital of the V state. Its neglect in ordinary Pariser‐Parr‐Pople theory, etc., results in a serious cancellation of errors in traditional semiempirical valence theories. When the correlation contributions are evaluated by second order perturbation theory, the final true parameters are much less dependent on the basis set (inclusion of diffuse orbitals) than are the original ``theoretical'' integrals.

Study of the ground state potential curve and dipole moment of OH by the method of optimized valence configurations

Abstract: Accurate theoretical potential and dipole moment curves are presented for the X2Πi state of the hydroxyl radical. The theoretically determined dissociation energy is 4.53 eV as compared to the experimental value of 4.63 eV. The computed dipole moment at the experimental equilibrium internuclear separation is 1.674 D, which is in excellent agreement with the most reliable experimental value of 1.66±0.01 D. A detailed, general prescription for constructing optimized valence configuration wavefunctions for diatomic hydrides is presented with OH as a specific example.