Valence band spectra

Figure 5-5. Valence band spectra of r/wu-polyacetyienc, recorded using synchrotron radiation at 27 eV and 50 eV photon energy, and the corresponding DOVS derived from Vl-H calculations. The VEH band structure is shown in the lower part of the figure (from Ref. 1281).

Valence band spectra provide information about the electronic and chemical structure of the system, since many of the valence electrons participate directly in chemical bonding. One way to evaluate experimental UPS spectra is by using a fingerprint method, i.e., a comparison with known standards. Another important approach is to utilize comparison with the results of appropriate model quantum-chemical calculations 4. The combination with quantum-chcmica) calculations allow for an assignment of the different features in the electronic structure in terms of atomic or molecular orbitals or in terms of band structure. The experimental valence band spectra in some of the examples included in this chapter arc inteqneted with the help of quantum-chemical calculations. A brief outline and some basic considerations on theoretical approaches are outlined in the next section. 

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]. 

In principle, it should be possible to obtain experimental valence band spectra of highly dispersed metals by photoemission. In practice, such spectra is difficult to obtain because very highly dispersed metals are usually obtained only on nonconductive supports and the resulting charging of the sample causes large chemical shifts and severe broadening of the photoelectron spectra. The purpose of this section is to discuss valence band and core level spectra of highly dispersed metal particles. 

Figure 3. Valence band spectra of Co/Si(100). Upper curve UPS spectra for 100 nm thick Co/Si(l 1 1) film middle curve thinned 4-5 nm Co/Si(l 1 1) film after ion etching (Co nanoparticles) lower curve clean silicon substrate after removing the Co layer by in situ sputtering. The photoemission data were obtained by He(I) excitation. (Reprinted from Ref [78], 1994, with permission from Springer.)...

Figure 11. Size dependence of the UPS valence band spectra from gold nanoparticles. The sputtering time on curves (a), (b), and (c) are the same as in Figure 9. Curves (bi) and (b2) were recorded after 20 and 25min. (Reprinted from Ref [171], 2002, with permission from Elsevier.)...

Fig. 25. XPS valence band spectra for reactively sputtered Ru Ir, x02 electrodes on a Ti substrate after preparation for different compositions x. Note the shift in t2g band position. After [83].

Fig. 28. Valence band spectra (UPS) of an AIROF electrode in the coloured (1.25 V) and in the bleached (0.0 V) state. The electrode was rinsed after emersion with ultra pure H20. After [67],...

Fig. 32. Valence band spectra (UPS) of a polycrystalline Pt electrode emersed at a different potential where underpotential deposition of Cu (0.3V, 0.1 V) and bulk deposition (—0.2 V) of Cu occurs. Clean Pt and Cu surfaces are shown for comparison.

UPS valence band spectra were also obtained for the UPD system Ag on Pt. These spectra exhibit again a shift of the Ag4d level of the adatom to lower binding energies when compared to the bulk value. For both Cu and Ag adatoms on Pt the UPS spectra clearly show that bulk properties of the adsorbate layer are achieved for coverages of about 3 monolayers. 

To determine the BEs (Eq. 1) of different electrons in the atom by XPS, one measures the KE of the ejected electrons, knowing the excitation energy, hv, and the work function, electronic structure of the solid, consisting of both localized core states (core line spectra) and delocalized valence states (valence band spectra) can be mapped. The information is element-specific, quantitative, and chemically sensitive. Core line spectra consist of discrete peaks representing orbital BE values, which depend on the chemical environment of a particular element, and whose intensity depends on the concentration of the element. Valence band spectra consist of electronic states associated with bonding interactions between the... 

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. 

Fig. 9 a Variation in photoionization cross-sections [42]. b PES valence band spectra for Hf(Sio.5Aso.5)As at three different excitation energies, normalized to the As2 4p peak. Reprinted with permission from [35]. Copyright Elsevier... 

Table 3 Binding energies (eV) of component peaks in valence band spectra of MP (M = Cr, Mn,...

Fig. 29 a Fitted valence band spectra for LaFe4Pi2 and CeFe4Pi2, with peak assignments listed in Table 4. b Overlaid spectra highlighting the presence of a Ce 4f component in CeFe4Pi2. Reprinted with permission from [110], Copyright the American Chemical Society... 

Fig. 30 Fitted valence band spectra for a LaFe4Sbi2 and b CeFe4Sbi2. The peak assignments are analogous to those in Table 4, except that Sb 5s and 5p states account for peaks 1 1. Reprinted with permission from [59], Copyright the American Physical Society...

In principle, valence band XPS spectra reveal all the electronic states involved in bonding, and are one of the few ways of extracting an experimental band structure. In practice, however, their analysis has been limited to a qualitative comparison with the calculated density of states. When appropriate correction factors are applied, it is possible to fit these valence band spectra to component peaks that represent the atomic orbital contributions, in analogy to the projected density of states. This type of fitting procedure requires an appreciation of the restraints that must be applied to limit the number of component peaks, their breadth and splitting, and their line-shapes. 

In addition to studying core levels, XPS can also be used to image the valence band. Figure 3.6 shows valence band spectra of Rh and Ag. The step at Eb=0 corresponds to the Fermi level, the highest occupied electron level. Figure 3.6 illustrates that the Fermi level of rhodium lies in the d-band where the density of states is high, whereas the Fermi level of silver, with its completely filled d-band, falls in the s-band, where the density of states is low (see also the Appendix). 

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