Valence band region

Photoemission has been proved to be a tool for measurement of the electronic structure of metal nanoparticles. The information is gained for DOS in the valence-band region, ionization threshold, core-level positions, and adsorbate structure. In a very simplified picture photoemission transforms the energy distribution of the bounded electrons into the kinetic energy distribution of free electrons leaving the sample, which can easily be measured ... 

The integrated intensities of the fitted component peaks should then be related to the electron population of different valence states, subject to correction factors, according to the same equation used earlier for quantitative analysis of survey XPS spectra (Eq. 3) [10]. Because photoelectron KEs are similar throughout the valence band region, spectrometer-dependent factors and IMFP values can be assumed to be the same for all states, so that the equation simplifies to ... 

In the photoemission spectroscopic method, it is customary to distinguish between a core level region, which is probed in XPS (see Fig. 2), and which contains the response coming from the bound or core levels, and a valence band region, which is explored by both XPS and UPS, and which contains the response coming from the outer electrons in a solid, those of the ground state energy bands. 

Here, the main features of the valence band results for Th02 and UO2 will be illustrated. Since a large number of publications exists in this field (especially for uranium oxides), reference will be made only to a few selected investigations, chosen for the purpose of highlighting those aspects of the oxide bond discussed previously. A very comprehensive review of these results can be found (and references therein electronic and spectroscopic properties in Refs. 109-111). Figure 21 shows the photoemission spectrum of Th02 and UO2 up to Et = 45 eV The valence band region extends to about 10 eV. The marked difference is the appearance in UO2 of a sharp and intense peak at Et =... 

An attempt of identification of 5 f character has been recently reported for UO2 by resonant photoemission of the valence band region with excitation energies of 92 eV (off-resonance) and 98 eV (on-resonance). The core absorption edge used was the 5d ionization threshold, corresponding to a process ... 

It is worthwhile to mention the ample use of screening final states models in understanding core levels as well as valence band spectra of the oxides. The two-hole models, for instance, which have been described here, are certainly of relevance. Interpretational difference exists, for instance, on the attribution of the 10 eV valence band peak (encountered in other actinide dioxides as well), whether due to the non-screened 5f final state, or to a 2p-type characteristics of the ligand, or simply to surface stoichiometry effects. Although resonance experiments seem to exclude the first interpretation, it remains a question as to what extent a resonance behaviour other than expected within an atomic picture is exhibited by a 5 f contribution in the valence band region, and to what extent a possible d contribution may modify it. In fact, it has been shown that, for less localized states (as, e.g., the 3d states in transition metals) the resonant enhancement of the response is less pronounced than expected. 

Zolotarev, V. M., 1970. The optical constants of amorphous Si02 and Ge02 in the valence band region, Opt. Spectrosc., 29, 34-37. 

Figure 2.21 UVPS (Hell spectra) in the valence-band region showing final-state effects (a) Sm (b) Tb. The valence band of Sm shows the coexistence of both divalent and trivalent species due to valence instability (mixed valence). (After Rao Sarma, 1982.)...

Analysis of the valence-band spectrum of NiO helped to understand the electronic structure of transition-metal compounds. It is to be noted that th.e crystal-field theory cannot explain the features over the entire valence-band region of NiO. It therefore becomes necessary to explicitly take into account the ligand(02p)-metal (Ni3d) hybridization and the intra-atomic Coulomb interaction, 11, in order to satisfactorily explain the spectral features. This has been done by approximating bulk NiO by a cluster (NiOg) ". The ground-state wave function Tg of this cluster is given by,... 

Figure 21. Variable photon energy photoelectron spectr of the valence band region of D2(j-Cu(II)C1 and T -Cu(I)Cl ...

Fig. S a Valence band spectra of Gd C82 (grey) and C82 (black) measured with Al Ka x-rays, b Symbols Gd 4f photoemission after subtraction of the empty C82 C 2s/2p spectrum. The vertical lines are individual components of atomic calculations for a 4f> multiplet, and the solid curve is their broadened sum. c Gd-N4>5 x-ray absorption spectrum (Gd 4d-4f excitations) of Gd C82. The complex lineshape comes from the widely spaced multiplet components resulting from the strong Coulomb interaction between the single hole in the 4d shell and the eight electrons present in the 4f shell in the x-ray absorption final state [see Fig. lc]. The arrows represent the two photon energies used for the data shown in panel d. d Resonant photoemission data of the valence band region of Gd C82 recorded off (hv=137 eV) and on (hv=149 eV) the Gd 4d-4f giant resonance...

The bottom spectra are taken from a freshly evaporated CdS film. The only emissions observed are from Cd and S core levels and Auger levels and from the valence band region. With increasing deposition time of ZnO, the Cd and S levels are attenuated, while the emissions from the growing ZnO film... 

Figure 9 Density of electronic states plotted in the valence band region of the poIy(Gly-Ala-Ser) polymer (in relative units)...

Additional evidence for the effect of polymerization appears in the x-ray photoelectron spectral intensities of sihcates. DVM-Aa calculations on the energies and intensities of spectra by Sasaki and Adachi (1980a,b) satisfactorily reproduce relative intensities in the upper-valence-band region for SO/ [Fig. 5.8(a)] but seriously underestimate the intensity of the 5 i orbital feature of Si02 using a SiO/ cluster model [Fig. 5.8(b)]. This error may be a result of the influence of polymerization in SiOj, although the calculated spectrum is also somewhat different from that observed for olivine in Fig. 5.7. 

Fig. 14. Total DOS and atom projected PDOS curves of the valence band region for the V10O31H12 based vacancy cluster modeling an 0(2) surface vacancy, see text.

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