Showing papers on "Valence (chemistry) published in 1977"

Valence Ionization Energies of Hydrocarbons

Abstract: Experimental valence ionisation energies of 143 hydrocarbons, CnHm, have been determined from their He (IIα) (40.80 eV) and He (Iα) (21.22 eV) excited photoelectron spectra. Ionization energies, usually up to 26 eV are given in tabular form for 108 hydrocarbons with n≤6, together with their (tentative) assignment. The ionization energies up to approximately 25 eV, of 35 selected hydrocarbons with 7≤n≤10, are presented by means of their He (IIα) photoelectron spectra.

A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds

Abstract: A correlation has been established between the stabilities of metal hydrides (measured in terms of the free energies of formation of the hydride phase per mole H 2 ) and the radii of tetrahedral holes in both hexagonal AB 5 (D2 d ) and cubic AB (B 2 ) intermetallic compounds. The correlation demonstrates that as the tetrahedral hole size increases, the stability increases. The results of this correlation show conclusively that hole size can be employed effectively in determining the stabilities of intermetallic compound-hydrogen phases. A model, which depends solely upon lattice parameters, was developed to compute the radii of tetrahedral holes in hexagonal AB 5 intermetallic compounds. A similar model, which requires a ratio of metallic radii in addition to the lattice parameter, has been employed to compute the radii of tetrahedral holes in the cubic AB intermetallic compounds. The thermodynamic and structural properties of hexagonal AB 5 -hydrogen systems and cubic AB-hydrogen systems have been compiled and are presented. The quantitative correlations are excellent. A change in valence of the A atom in the hexagonal AB 5 intermetallic compounds may have a significant effect on the stabilities of the hydride phases formed. In those cases where the B atoms were partially substituted in the hexagonal AB 6 and the cubic AB intermetallic compounds, a good correlation of stability with hole size was found. Gross deviations of the Ce-base AB 5 hydride data from the correlation line established by all the other AB 5 hydrides were observed. These were rationalized on the basis of well-documented stress-induced lattice contractions of Ce intermetallic compounds. The resulting contracted hole sizes of the CeNi 5 and CeCo 5 compounds modified the stability of their hydrides. These compound hydrides demonstrate a displaced but similar correlation to the other AB 5 hydrides, i.e . the smaller the hole size, the less stable is the system.

Bond ionicity and structural stability of some average-valence-five materials studied by x-ray photoemission

Abstract: Core-level and valence-band spectra have been obtained by means of x-ray photoemission spectroscopy for the group IV, V, and VI elements Ge, Sn, Pb, As, Sb, Bi, S, Se, and Te and the group IV-VI compounds GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. These results, taken under ultrahigh-vacuum conditions with unmonochromatized x rays, are presented and discussed in terms of the bonding in materials with an average valence of 5. The effects of relaxation on the chemical shifts are found to be relatively small. The chemical shifts are found to vary in a manner similar to that expected from the magnitude of the elemental electronegativities except for the ordering of the shifts of GeS and GeSe. Relative charge transfers are calculated from the chemical shifts and are found to be in general agreement with ionicities calculated using the Phillips-Van Vechten theory although there is some disagreement as to their magnitudes. A consideration of the structures of the compounds relative to the charge transfers demonstrates the importance of metallic as well as covalent and ionic bonding in determining the most stable structure. The value of critical ionicity carried over from the average-valence-4 materials does not apply to the average-valence-5 materials and this concept does not appear useful in understanding their bonding because of the increased importance of nondirectional metallic bonding.

Abinitio configuration interaction study of the valence states of O2

Abstract: Configuration interaction calculations have been performed for the 62 electronic states of O2 arising from O atoms in the lowest 3P, 1D, and 1S states. The calculations used an extended one‐particle basis set, and included internal and semi‐internal electron correlation effects. Numerical values for potential curves are reported for all states. Detailed comparison with experiment and other calculations is made for the seven lowest bound states: X 3Σg−, a 1Δg, b 2Σg+, c 1Σu−, C 3Δu, A 3Σu+, and B 3Σu−. For these seven states the maximum error in the calculated spectroscopic constants Re, De, Te, and ωe are 0.04 A, 0.4 eV, 0.2 eV, and 120 cm−1, respectively.

Electrons in Glass

Abstract: An outline is given of some of the steps which in the last ten years have led to an understanding of many of the properties of conduction and valence bands in non-crystalline materials. The concept of a mobility edge is described and some of the experimental evidence for its existence is presented. A discussion is given of the nature of ‘states in the gap’, thought of as deep donors or acceptors. These pin the Fermi energy and act as recombination centres. A distinction is made between materials such as silicon or germanium with fourfold co-ordination and those like As2Te3 or SiO2, where for the chalcogen (Te or O) co-ordination is twofold; for the former an electron in a deep state produces only moderate distortion of the lattice, for the latter the distortion is large. Photoluminescence and other properties of chalcogenides are discussed in terms of a recent model of ‘charged dangling bonds’. It is suggested that free holes in As2Te3 and As2Se3 are not self-trapped but that in SiO2 they are. Fi...

Renormalized atoms: Cohesion in transition metals

Abstract: The renormalized-atom method is used to calculate the cohesive energy of the 3d and 4d transition-metal elements and the equilibrium lattice constant and bulk modulus of two representative elements, Ti and Cu The results agree with experiment to within 20% for most elements The method of calculation allows the cohesive energy to be decomposed into a number of contributions whose relative importance can be investigated both as a function of valence and as a function of density No evidence of d--d repulsion is found for the transition or noble metals Instead, the ''spring'' which holds the atoms apart is the result of the increasing kinetic energy of the conduction electrons as the density is increased The d--d interaction is uniformly attractive and produces the minimum in the Wigner--Seitz radius near the center of a transition-metal period Band structures calculated from renormalized-atom and Xcap alpha potentials are compared, and the relationships among them are discussed in some detail

Strong Correlation Effects in inner Valence Ionization of N2 AND CO

Abstract: The results of many-body calculations of the valence and inner-valence ionization potentials and their intensities are reported for N2 and CO. For N2 we find that the 2σg line is smashed to several pieces of roughly equal intensity. It is not possible to identify any of these lines as the ǒmainǒ line representing the ionization of a 2Ug electron and the remaining ones as satellite lines. For CO there survives a line which carries about half of the 3e intensity and which can be interpreted to represent the ionization of an electron out of the 3σ orbital. The results explain the peculiar shape of the broad innervalence peaks of N2 and CO. For both N2 and CO rich satellite structure is found in qualitative agreement with experimental X-ray photoelectron spectra.

Self-consistent numerical-basis-set linear-combination-of-atomic-orbitals investigation of the electronic structure and properties of TiS2

Abstract: A fully-self-consistent numerical-basis-set linear-combination-of-atomic-orbitals calculation of the electronic structure of Ti${\mathrm{S}}_{2}$ is reported using the method described previously. The calculated band structure differs considerably from those previously obtained by non-self-consistent muffin-tin models. Comparison with experiment shows that the calculated optical properties for energies below 16 eV and the various characteristics of the valence and conduction bands agree very well with optical-absorption and electron-energy-loss data as well as with photoemission, x-ray absorption, and appearance-potential spectra. A small indirect gap (0.2-0.3 eV) occurs at the points $M$ and $L$ in the Brillouin zone with a larger direct gap (0.8 eV) at $\ensuremath{\Gamma}$. We suggest that the characteristic semi-metallic large $g$ value observed experimentally originates from a near coincidence of the band gap with the enhanced spin-orbit splitting which is consistent with the soft-x-ray data and our band model. The bonding mechanism in Ti${\mathrm{S}}_{2}$ is discussed in detail; it is shown by a direct calculation of the self-consistent charge density and the transverse effective charge that the system is predominantly covalent with small static ionic character and large dynamic ionicity. In contrast with muffin-tin $X\ensuremath{\alpha}$ models, the bonding is found to be largely due to Ti $4s4p$ to S $3p$ bonds and a much weaker Ti $3d$ to S $3p$ bond. The effects of muffin-tin approximation and self-consistency are discussed in detail. Extrapolation of these results to the case of Ti${\mathrm{Se}}_{2}$ is made and the possible origin of its charge-density wave is discussed.

Metal-non-metal transition in silver chalcogenides

Abstract: The paper reviews the crystallographic, optical, magnetic and transport properties of silver chalcogenides Ag2S and Ag2Se. In the low temperature β-phases, the influence of stoichiometry on the respective magnitude of ionic and electronic conductivities is important. In the disordered high-temperature α-phases, the characteristic feature is the statistic distribution of lattice silver ions. This situation is analysed according to the theory established by Mott (e.g. 1974, and Mott and Davis 1971) for amorphous semiconductors. Owing to the disorder, tails appear both in valence and conduction band, and in α-Ag2Se, the bands overlap.

Pb valence in iron garnets

Abstract: A model for the incorporation of Pb in garnets is presented in which self‐compensation occurs via the formation of Pb2+ and Pb4+ and allows an explanation of the following observations: high levels of Pb substitution achievable, the temperature dependence of Pb incorporation, lattice mismatch behavior of epitaxial films of Pb‐doped iron garnets, the Pb‐induced uniaxial anisotropy behavior in EuIG : Pb, the electrical conductivity type, and the high Pb‐induced optical absorption. Spectroscopic evidence for the simultaneous existence of Pb2+ and Pb4+ in Y3Ga5O12 (YGG) is given and analyzed in terms of Pb2+, Pb4+ impurity levels lying respectively 1.1 eV above the valence band and 2.36 eV below the conduction band. Intervalence transition between Pb2+ and Pb4+ occurs at 18 000 cm−1 (2.23 eV) in YGG : Pb. Using the separation of Pb2+ and Pb4+ levels determined in YGG and a simple energy‐band model for YIG the Pb‐induced absorption processes in YIG : Pb are enumerated and shown to give a qualitative explanatio...

The electronic structure of lithium‐based liquid semiconducting alloys

Abstract: The resistivity, ρ, and the thermoelectric power, S, for liquid lithium alloys of the type Li x M1-x where 0

Quantum yields for the production of O(1D) from photodissociation of O2 at 1160–1770 Å

Abstract: The quantum yield for the production of O(1D) by photodissociation of O2 was measured in the 1160–1770 A wavelength region For wavelengths longer than 1390 A, the quantum yields are unity and constant, with a sharp cutoff at about 1750 A For wavelengths shorter than 1390 A, the O(1D) quantum yields depend strongly on wavelength The positions of many of the structures correspond to Rydberg states identified by various authors, and the data show by which of the two principal dissociative channels, O(3P)+O(3P) or O(1D)+O(3P), the excited molecules predissociate The total oxygen atom yields were also measured and clearly show that all photon absorption leads to dissociation in the spectral region studied Possible identification of absorption to the 3Πu valence state has been made, with a peak at 1356 A (914 eV)

The homogeneous chemistry of atmospheric sulfur

Abstract: Sulfur is present in the atmosphere as a constituent of more than 15 compounds and in valence states ranging from −2 to +6. Nearly all of the compounds have both natural and anthropogenic sources. This paper reviews the present information on the reactions of these compounds and their products with typical atmospheric species. It is shown that atmospheric sulfur chemistry proceeds monotonically in the direction of increasing sulfur oxidation and that the gas phase transformation of reduced sulfur compounds to sulfate aerosol in all ambient atmospheres can be described by a common chain consisting of five chemical steps followed by a heterogeneous loss process.

Electronic Structure of CeN Studied by X-Ray—Photoemission Spectroscopy

Abstract: The core levels and the valence-band region of CeN have been studied by x-ray photoemission. Clear evidence is found that Ce has a mixed valence between 3 and 4 in this compound. A comparison of the $4d$ spectra of the Ce pnictides and of pure Ce at different temperatures leads to the conclusion that the $\ensuremath{\alpha}$ phase of this metal corresponds very likely to an intermediate valence.

Polymer Primary Structures Studied by ESCA and EHCO Methods

Abstract: The electronic structure (core and valence levels) of various simple polymers has been investigated by ESCA (Electron Spectroscopy for Chemical Analysis), with the aid of original EHCO (Extended H?ckel Crystalline Orbitals) calculations giving corresponding density of states and band structure schemes. Simple monosubstituted homopolymers derived from polyethylene ([CH2-CHX]n) have been chosen in order to study the influence of X substituents (X = -F, -Cl, -OH, -CH3, -CH2CH3, -C6H5, -C6H11 corresponding respectively to poly(vinyl fluoride), poly(vinyl chloride), poly(vinyl alcohol), poly(propylene), poly(1-butene), poly(styrene), and poly(vinyl cyclohexane)) through the chemical shifts measured for the core levels, and the alteration of the valence band structures. Assignment of the valence molecular orbitals is made with the aid of the theoretical calculations and with comparison of spectra recorded for molecules similar to the monomeric units. Emphasis is put on the very significant information provided by the valence band spectra for polymers containing only carbon and hydrogen atoms. Valence band data are used to calculate physical parameters (first ionization potential, energy gap, work function) of importance to deduce electrical properties for the polymers.

Surface and interface states on GaAs(110): Effects of atomic and electronic rearrangements

Abstract: Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surfaces of III–V semiconductors. In this paper we will briefly review these new developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band‐gap materials (e.g., GaSb, InAs, and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can produce first‐order effects in the valence‐band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage, the Ga 3d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first‐order changes in the valence‐band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3d chemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating practical surfaces. In particular, the effect of mixed As and Ga oxides, the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain‐induced interface states are discussed.

Complete breakdown of the quasiparticle picture for inner valence electrons

Abstract: It is pointed out that the quasiparticle picture, which has been found to be valid for both outer valence and core electrons and is commonly used in atomic, molecular and solid-state physics, may completely break down in the inner valence region. Many-body calculations of inner valence ionisation potentials and their relative intensities are reported for several atomic and molecular systems. The calculations demonstrate that the absence of quasiparticles is a common phenomenon.