Core-hole effects in the x-ray-absorption spectra of transition-metal silicides

The anisotropic emission of the K-X-radiation of boron and nit'rogen in hexagonal boron nitride BN permits the determination of the x -and a-subbands of the two X-ray spectra. Together with the X-ray photoelectron spectrum a consistent picture of the valence band of BN is obtained. Position and width of the s-, o-, and x-bands are determined. None of the band structure and density-of-states calculations known so far is in satisfactory agreement with the experimental observations. Die anisotrope Emission der K-Rontgenstrahlung von Bor und Stickstoff in hexagonalem Bornitrid BPU' erlaubt die Bestimmung der x -und a-Teilbanden der beiden Ront,genspektren. Zusammen mit dem Rontgen-Photoelektronenspektrum ergibt sich daraus ein konsistentes Bild des Valenzbandes von BN. Breite und Lage der s-, a-und x-Bande werden bestimmt. Keine der vorliegenden Bandstruktur-und Zustandsdichteberechnungen zeigt eine zufriedenstellende Ubereinstimmung mit den experimenbellen Beobachtnngen.

The x-ray emission spectra (SKβ, SL, CuKβ, CuL, FeKβ, FeL) and x-ray photoelectron spectra of CuFeS2 chalcopyrite are reported and are interpreted using SCF-Xα MO calculations on the polyhedral anions CuS−74 and FeS−54 and discrete variational Xα band structure calculations. The highest energy occupied orbitals are found to be of Cu3d-S3p antibonding character. Substantial Fe3d character is observed across a broad range of binding energies indicating strong mixing of the Fe3d orbitals with both the Cu3d and S3p. Both the Xα cluster calculation on FeS−54 and the Xα band structure calculations overestimate the Fe3d orbital binding energies, although the discrepancy is smaller in the band calculation. Both computational methods predict the lowest energy empty orbitals to be of Fe3d-S3p antibonding character with a small (<2 eV) separation from the highest occupied orbitals. This small energy difference provides a possible explanation for the metallic conductivity observed in CuFeS2 at high pressure, particularly if the CuT–S bond is more compressible than the Feiii–S as is expected from compressibility-formal charge systematics.

We present x-ray Si K, Si L, and 0 I( emission bands of hydrogenated amorphous SiO"alloy films covering the concentration range 0 & x~2.2. All spectral features in the emission bands were identified and attributed to Si 3s, Si 3p, and 0 2s, 2p derived states. With increasing x, the shape and the energy position of the main features of the Si E and Si L emission bands change significantly, particularly in the concentration interval 0.5 &x &1.5. A comparison with available ultraviolet (UPS) and x-ray (XPS) photoemission spectra demonstrates that x-ray emission bands for a-SiO are more sensitive to sample composition than UPS and/or XPS spectra. The random-bonding model and the random-mixture model of the structure of SiO"alloys are discussed in view of the x-ray emission spectra. For x =2, the Si X, Si L, and 0 K spectra are very similar to those observed for cx-quartz (c-Si02). This justifies interpretation of all features of a-Si02:H on the basis of c-SiO&. We therefore performed self-consistent pseudopotential calculations of the total density of states and the local partial density of states of Si 3s, Si 3p, Si 3d, 0 2s, and 0 2p of cr-quartz. The calculations confirm that due to the high electronegativity of oxygen nonbonding, Si 3d-like states are created which, for high values of x, clearly show up in the Si L emission bands. Finally, we discuss the long-standing so-called d-orbital controversy about Si02 and give an explanation of it.

We present Si L2 3 emission-band spectra of a series of 3d and 4d transition-metal (TM) silicides, together wtih Si E emission-band spectra of four 3d TM disilicides. The data are compared with augmented-spherical-wave density-of-states (DOS) calculations, and good agreement is found. The trends we find are explained with a general scheme for chemical bonding in TM silicides. The differences between the experimental data and the calculated DOS curves are tentatively attributed to self-energy effects.

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The two conformers of a cyclohexa‐2, 4‐dienone with different substituents at C(6) on irradiation are believed to undergo ring opening stereospecifically affording a mixture of two configurationally isomeric diene‐ketenes (and descendents thereof)‐ Exceptions are generally found for those dienones with one C and one O substituent or even with two C substituents, if one of them carries a polar group at a site able to interact through space with the ring CO group. In these cases, only one of the two anticipated diene‐ketenes (and descendents thereof) is produced. A thorough investigation of the photochemistry of a series of structurally different cyclohexa‐2, 4‐dienones on analytical as well as on preparative scale extends our mechanistic knowledge of the various routes from diene‐ketenes into a variety of compound classes. Novel compound classes accessible to diene‐ketenes are seven‐membered carbocycles (by intramolecular aldolization of the zwitterion of appropriately substituted, transiently formed diene‐(N, O)‐ketene acetals) and β‐lactams (by Staudinger reaction).

Aktml. An experimental and theoretical study of the electronic structure and the x-ray Si K and Si L emission bands of 42aoordinated @ q u a m and 63-coordinated slishovite is presented. The nature of atomic states participating in the x-ray valence band spectra is idcntified. In particular the longstanding controversy a b u t the mle of Si M-like states is analysed; evidently, Si M-like electrons essentially contribute 10 the intensity of Si L emission bands We have calculated the total density of states, the local partial densities of states. the ratios of the s, p and d charge componenls inside silicon and mygen Sited spheres, and the Si K and Si L minion bands. mere is good agreement behveen our measured and alculated spectra. Reasons are given why in contrast with the Si K emission bands the Si L emission bands are inxwitive to the local c y t a l stmcture of aquam and stishovite.