Valence band interpretation

The systems discussed in this chapter give some examples using different theoretical models for the interpretation of, primarily, UPS valence band data, both for pristine and doped systems as well as for the initial stages of interface formation between metals and conjugated systems. Among the various methods used in the examples are the following semiempirical Hartree-Fock methods such as the Modified Neglect of Diatomic Overlap (MNDO) [31, 32) and Austin Model 1 (AMI) [33] the non-empirical Valence Effective Hamiltonian (VEH) pseudopotential method [3, 34J and ab initio Hartree-Fock techniques. 

During the last decade, the progress in theoretical methods and the access to quantum-chemical calculations has become more available. Nowadays, the use of quantum-chemical calculations in the interpretation of experimental UPS valence band spectra is a common approach [26-29]. 

With the absorption of a quantum with an energy of more than 3.05 eV resp. 3.29 eV, an electron is lifted out of the valence band and into the conduction band, thereby forming an exciton (Fig. 5). This interpretation is also supported by the molecular orbital theory and the crystal field theory regarding the bonding conditions in the TiC lattice. 

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

Surprisingly little has been done to take advantage of these valence band spectra, perhaps because of some of the challenges in interpretation. In principle, it should be possible to fit these spectra with component peaks that correspond to contributions from individual atomic valence orbitals. However, a proper comparison of the experimental and calculated band structures must take into account various correction factors, the most important being the different photoelectron cross-sections for the orbital components. 

One has to be very cautious in interpreting, e.g., valence band spectra of actinides generally a direct comparison of valence band spectra with the density of states as derived from band ground state calculations is not appropriate. 

A satellite is found in the valence band spectrum of Ni metal (and compounds) at about 6 eV from Ep . This satellite does not correspond to any feature in the calculated density of states. Therefore a brief discussion of this satellite is worthwhile, since its interpretation has inspired that of some structures found in light actinide metals photoemission spectra. 

From the comparison of the spectra with calculated one-electron density of states curves (lines I and II), therefore, the valence band spectra of U, from photoemission and BIS, are interpreted as due to a hybridized (d, f) continuous band, with a much large itinerant 5 f contribution than in Th for the occupied part . As for the broad structure at about 2.5 eV, qualitative agreements point to a 6d character. 

A further argument was advanced by Veal et al.. A linear relationship was found to exist between the XPS intensity of the main oxygen valence band and the oxygen-to-uranium ratio of the different uranium oxides investigated (Fig. 23). This was interpreted as indicating that this band consists entirely of 2p states (perhaps 6 d hybridized) with no 5f contribution, as expected in a localized 5f picture. (The 5f contribution, if present, would have caused deviations from this hnear relationship, especially because of the very large photoionization f cross-section.)... 

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. 

Photoconductive response (the rate of creation, or the rise, of the photocurrent, and the rate of decay of the photocurrent) appears to be divided into fast and slow responses. The fast responses, with time constants for rise and decay of the order of a second or less, have been adequately interpreted by Mollwo, et al. (53-55), Weiss (56), and Heiland (47,57) as bulk processes. These authors have concluded that the fast response processes are associated with the double ionization of interstitial zinc, and have proposed that the photon excites electrons from the valence band, and that the hole immediately recombines with the electron from an interstitial Zn+, producing double-ionized zinc ions. 

Another interpretation would be to suppose that the adsorbed sulfide ion forms a surface state that can be directly oxidized by a hole in the valence band. In this case the shift in current onset to lower voltages would be due to an increase in the charge transfer rate rather than the decrease in the recombination rate discussed in the preceeding paragraph. The corrosion suppression associated with the sulfide could then be partially attributed to the rapid kinetics of hole capture by these surface sulfide ions and partially due to reduction of oxidized corrosion sites by sulfide ions in solution. 

The metallic lustre of the elemental substances formed by the heavier Group 14 elements in the diamond structure can be interpreted in terms of the valence band/conduction band picture. The spectrum of excited states which can arise from promotion of an electron from the valence band to the conduction band covers the whole of the visible region, leading to opaqueness and specular reflectance. In the case of diamond itself, the lowest electronic excited state lies well into the ultraviolet. 

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