Valence band photoelectron spectra

Fig. 5.5 Valence band photoelectron spectra from a 7 ML thick Fe(llO) films on tungsten taken with left LCP) and right circularly polarized RCP) radiation (hv = 28 eV) for opposite magnetization directions. Upper panel experimental data, lower panel calculations for Fe(llO) with magnetization direction in-plane along the [110] axis. The corresponding asymmetry is shown in each diagram. Reprinted from [26], Copyright (1998), with permission from Elsevier...

A study of the valence band photoelectron spectrum and the X-ray emission spectrum of poly(ethylene oxide) was carried out by Brena and co-workers [102] in order to understand the effect of conformation on the observed spectra. Up to 12 monomers were used in the calculations for the valence band photoelectron... 

The most direcdy measurable feature of the electronic band structure, the valence band photoelectron spectrum, involves the density-of-valence-states (DOVS), or p(e), which is obtained from the electronic band structure using the standard definition,... 

Finally, Fig. 58 e may illustrate intra-3 d-band fluctuation and decay of a 3 d-valence hole in metallic Ni. This process is probably one of the essential elements in the description of the valence band photoelectron spectrum which seems to show 3 d-band narrowing, decreased splitting of the majority and minority spin bands and a pronounced shake-up structure about 6 eV below the Fermi level (see e.g.143 175) and references therein, and also Sect. 8.3.4). 

Like for the core levels, a valence band photoelectron spectrum is composed of peaks or bands having the following characteristics ... 

When the polymer backbone contains a large number of substituents, or long side chains, it is clear that its valence band photoelectron spectrum will contain a lot of peaks each side group will bring its own fingerprint in the XPS spectrum, and the interpretation of the data will be very difficult. An example of this limitation has been found in the study of polydiacetylenes, which until now were not synthesized with sufficiently short side chains to reduce the number of bands (molecular orbitals) appearing in the valence spectrum (, 25). 

Theoretical descriptions may be to some extent verified experimentally. For instance, the valence-band photoelectron spectrum of the compound [Fe(CO)7r-Cp]4 shows that, in agreement with the calculations discussed above, the highest occupied valence orbitals are actually metallic orbitals of the Fe4 core. Mossbauer experiments of the same compound and of its oxidation product [Fe(CO)7T-Cp]4 indicate that the unpaired electron interacts with the iron moiety. This feature is consistent with the theoretical model which establishes the unpaired electron would occupy a delocalized MO with marked metal character. 

The results of a preliminary study of a sample of berkelium oxide (Bk02, Bk203, or a mixture of the two) via X-ray photoelectron spectroscopy (XPS) included measured core- and valence-electron binding energies (162). The valence-band XPS spectrum, which was limited in resolution by photon broadening, was dominated by 5f-electron emission. 

The photoelectron spectra of pyridazine have been interpreted on the basis of many-body Green s function calculations both for the outer and the inner valence region. The calculations confirm that ionization of the first n-electron occurs at lower energy than of the first TT-electron (79MI21201). A large number of bands in the photoelectron spectrum of 3,6-diphenylpyridazine in stretched polymer sheets have been assigned to transitions predicted... 

However, UPS and XPS do not both image the density of states in entirely the same way. In XPS, the photoelectrons originating from the valence band leave the sample with kinetic energies over 1 keV. In UPS, the exciting energy is on the order of 21 eV, and the kinetic energy of the electrons is low, say between 5 and 16 eV. This means that the final state of the photoelectron is within the unoccupied part of the density of states of the metal. As a result, the UPS spectrum represents a convolution of the densities of occupied and unoccupied states, which is sometimes called the "Joint Density of States."... 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

Experimental data can be obtained by ultra-violet absorption spectroscopy, electron energy loss spectroscopy and photoelectron spectroscopy. UV absorption and EELs have been described briefly in Chapter 3. The former provides information only about the band-gap, while EELs gives more general information about the conduction bands. Both X-rays and UV photons can be used to generate photoelectrons these two methods are given the acronyms XPS and UPS. The energy spectrum of the emitted electrons provides information about the density of electron states in the valence bands. In principle the size of the band gap can be obtained, but care must be taken as the absolute energy... 

Intrinsically, a UV lamp has a much better resolution (a few meV), that in fact turns out to be very useful to study gases. But in a solid, the molecular orbitals are grouped into bands, broadened by vibrational excitations, and become unresolved so that it has been proven that a photoelectron valence band spectrum recorded with a UV laiip do not offer substantial advantages compared to a XPS experiment. 

When excited with a UV lan, the photoelectrons have a kinetic energy range (0-40eV) where the escape depth is small (—sS) the contamination problem is then crucial in this case. On the contrary, when excited with a X-ray source, the electrons emitted from the valence levels have a larger escape depth, so that the surfaces features do not contribute as crucially to the valence band spectrum than to the core level peaks. 

Fig. 4.28. The experimental valence-band x-ray photoelectron ( photoemission ) spectrum of a-FeO, compared with a calculated spectrum based on a configuration-interaction calculation on an FeO, cluster. The dotted and dashed curves represent O 2p emission and integral background, respectively. The bottom panel shows a decomposition into configuration components for each final-state line (after Fujimori et al., 1986 reproduced with the publisher s permission).

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