1,721,023 research outputs found
Coulomb correlation energy versus covalence in transition-metal-compounds
No full textWe have performed optical and photoemission studies on Mott-Hubbard and charge transfer insulators with formal ionic configurations 3d(3) (CrCl3, CrBr3) and 3d(8)(NiCl2, NiBr2, NiI2). The photoemission spectra have been analyzed in terms of a cluster model leading to estimates of the on-site Coulomb repulsion energy, charge transfer energy and hybridization energy parameters. The ionicity parameter f(i) in Cr and Ni compounds has been calculated by the means of the Phillips-Van Vechten theory: its value is about 0.80 in Cr halides, and it varies from 0.70 in NiI2 to 0.80 in NiCl2 in Ni halides. We have also considered the ionicity scale f(i)(DT) of the dielectric theory. The two scales allow a good understanding of the chemical bond character and electron correlation in these materials
Multiplet splitting of core-electron binding energies in chromium compounds
No full textThe multiplet structure of core-electron binding energies in Cr3s and Cr3p levels of CrCl3 and CrBr3 compounds is investigated by means of x-ray photoelectron spectroscopy. In Cr3s core levels a doublet splitting of about 4.3 eV is observed for the main emission in both halides. The presence of satellite structures is observed both in Cr3s and Cr3p energy levels. In the Cr3s spectrum the main emission is assigned to intra-atomic multiplet splittings accompanied by correlation-induced satellites. Final state screening (charge-transfer) effects do not affect the satellite structures in a relevant way. This explains why the Cr3s splitting in chromium halides is proportional to the local magnetic moment
Splitting of core-level lines in photoemission spectra of transition metal compounds
No full textThe multiplet structure of core-electron binding energies in Cr 3s, Cr 3p and Cr 2p levels of CrCl3 and CrBr3 compounds has been investigated by X-ray photoelectron spectroscopy. The Cr 3s levels show a doublet splitting (about 4.3 eV) for the main emission in both halides. Satellites features are observed in Cr 3s, 3p and 2p levels at higher binding energies. In the Cr 3s spectrum, the main emission is assigned to unscreened intra-atomic multiplet splittings with correlation-induced satellites. The Cr 3p and 2p spectra can be better explained by the multiplet splitting arising from the interaction between valence band 3d electrons and core p holes in the crystal field. Final state screening (charge-transfer) effects do not play a major role in Cr 3s main emission nor do they affect the satellite structures in a relevant way. This explains why the Cr 3s exchange splitting in chromium halides is proportional to the local magnetic moment
Trends in science: heterogeneous catalysis
No full textCatalysis is of great academic interest and is extremely important in industry and for society in general. Catalysts have the remarkable property of facilitating a chemical reaction repeatedly without being consumed. The classical heterogeneous catalyst contains nanoscale metal particles dispersed on a solid support the function of which is that of selectively accelerating the reaction performance. This article deals with gold, one of the most attractive catalysts, and describes how ordinarily inert gold becomes an exceptional catalyst when shrung to nanoparticle size. Whatever its industrial future, gold' s uncanny knack for catalysis already provides a fascinating scientific puzzle
Electron correlation and hybridization in late-transition-metal compounds
No full textIn this article we discuss the transition-metal d-d Coulomb interaction U and the anion-to-cation charge transfer energy Delta for nickel halides and relate these to the observed bandgaps. In addition, we describe the degree of hybridization for NiCl2 and NiBr2 and transition-metal chlorides by considering the ionicity of the chemical bond of these compounds and the results obtained with the Phillips-Van Vechten dielectric theory. The degree of hybridization in transition-metal chlorides has been obtained by considering a simple model which utilizes the photon energy dependence of the photoionization cross-sections for the 3p band of the chlorine ions and the 3d band of the transition-metal ions. Good agreement between the two approaches has been found and a degree of hybridization between 0.15 and 0.25 for transition-metal chlorides is confirmed
AiRabbi, per studiare i pollini della Val di Rabbi
No full textLe temperature più elevate stanno influenzando la fenologia delle piante, che si manifesta ad esempio con l’anticipo della fase di fioritura, i pollini aerodispersi sono sentinelle di biodiversità e cambiamento climatic
Time- and angle-resolved photoelectron spectrometer based on a picosecond Nd:glass laser system
No full textA time- and angle-resolved photoelectron spectrometer based on an amplified picosecond Nd:glass laser system is described. The kinetic energy of photoemitted electrons, excited by 0.4 ps laser pulses at 264 nm, is measured with a time-of-flight electron energy analyzer. A theoretical model that takes into account the electron drift inside the mu -metal shielded tube as far as the electron analyzer has been introduced to calculate the energy distribution curves of photoelectrons. As a test of the experimental apparatus we present photoemission spectra of cesiated silver excited with 4.7 eV photons. Good agreement between the experimental and the calculated distribution curves of photoemitted electrons has been found. Finally, we have briefly discussed the electronic properties of silver in terms of free electron and d-band transitions, together with Auger processes for photon energies near 4 eV
Optical properties and electronic structure of InSe, GaSe, GaS semiconductors in the 2-9 eV range
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