Valence band photoemission cross-section

Fig. 2 Valence band photoemission profiles of empty C82 and Gd C82, recorded at room temperature with He Ia (21.22 eV) radiation. The inset shows the region close to the Fermi level on an expanded scale. For this photon energy, the photoionisation cross sections of the C 2s and C 2p levels dominate that of the Gd 4f levels...

In 1978, the same authors (Platau and Karlsson, 1978) performed a thorough XPS and UPS study of the valence band emission of Ce. Using the different energy dependence of photoemission cross sections of the 4f level and the sd... 

The cross section for photoemission of an electron in a core or molecular level is dependent on the photon energy. An important consequence of this statement relates to the valence-band electrons which have large cross sections for photoemission by UV light but very small cross sections for photoemission by x-rays. A good choice of anode for XPS valence-band photoemission is Zr, whose line has a photon energy of 151.4 eV and a line width of ca. 0.77 eV. With Zr radiation the valence-band cross sections are large, the spectral resolution is not significantly limited by the photon line width, and electrons in core levels with BEs between 20 and 145 eV can also be excited. The latter are not accessible with most UV photon sources and are useful because they are quite sensitive to chemical. state, even if a compositional analysis cannot be made. 

The possible contribution of 5 f states to the BIS intensity at Ep suggests that some 5 f character may exist (even in Th, with a 5 f configuration) in the composition of the occupied valence band. In fact a small tail of occupied 5f states (0.5 states per atom. Ref. 57) is supposed to contribute to the intensity of the XPS experimental spectrum the rather high intensity of which may be due to the high cross-section for 5 f excitation. Energy dependent photoemission should then be able to identify this contribution. 

In conclusion, the partial localization effects in the valence band spectra of the light actinides, although extremely important if convincingly verified, still need much experimental and theoretical investigation. It is expected that the situation will improve considerably when resonant photoemission studies become available for actinide compounds in which the 5 f and 6 d emissions do not overlap. In addition, theoretical calculations of 5 f and 6 d cross sections near the 5 d threshold will be very helpful. 

Experimental information on the valence levels comes essentially from photoemission XPS and UPS measure densities of states (DOSs) convoluted with absorption cross sections, and these DOS values can be compared with those computed from VEH valence-band structures [195]. This has now been done for several CPs and the agreement is good. It would be more instructive to compare the actual band structure to angle-resolved (ARUPS) measurements, but this has never been done. What comes nearest is an ARUPS study of a series of long alkanes taken as models for polyethylene, a nonconjugated polymer [196]. 

To a first approximation angle integrated photoemission measures the density of occupied electronic states, but with the caveat that the contribution of a given state to the spectrum must be weighted by appropriate ionisation cross sections. Comprehensive tabulations of ionisation cross sections calculated within an independent electron framework are available [8]. At X-ray energies cross sections for ionisation of second and third row transition metal d states are often very much greater than for ionisation of O 2p states, so that valence band X-ray photoemission spectra represent not so much the total density of states as the metal d partial density of states [9],... 

The interpretation of photoemission spectra of the valence band is straightforward in the itinerant limit. In this case the band states are spread across the N unit cells of the crystal and the change in electronic density in a single unit cell is proportional to 1 /N and should be negligible. Then the band energies represent well the excitation spectrum if the appropriate scattering cross-sections are taken into account. This is the case of compounds with a broad 5f band, where the photoemission spectra correlate well with the results of band-structure calculations (Naegele et al. 1988). 

The electronic structure of the TMC(IOO) surface has been studied most extensively among the low-index TMC surfaces. It is well known that the use of angle-resolved photoemission spectroscopy (ARPES) can give direct information about the valence band structure around the surface (18,19), and extensive ARPES studies have been performed on the valence band structure of TMC(IOO) surfaces such as TiC(lOO) (20,21), ZrC(lOO) (22,23), VC(IOO) (24-27), NbC(lOO) (28-31), and TaC(lOO) (32,33). These studies have shown that most of the features in ARPE spectra can be understood as emissions from the bulk bands, and thus the electronic structures of TMC(IOO) are well regarded as the cross sections of the bulk electronic states. However, in some systems, surface induced electronic states have been identified as described in the following. 

In the intrinsic part of the photoemission spectrum, that is, the elastic lines, there are basically three observables associated with each core-level or valence band peak line positions, line intensities, and line widths or line shapes. From these, different pieces of information can be gained. In core-level emission, the rough line positions reveal the elemental composition of the sample surface, whereas the exact positions are characteristic of the specific chemical environment of the atoms [6j. The intensities are determined not only by the atomic concentrations but also by the photoelectric cross sections and instrumental effects such as the photon flux and the transmission of the spectrometer. Finally, information on the many-body dynamics of the solid after the sudden creation of a photohole (the missing electron that has been ejected) is contained in the shape and width of the peak. In the simplest case, the line shape is Lorentzian and its width is a measure of... 

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