Core-level

There are complicating issues in defmmg pseudopotentials, e.g. the pseudopotential in equation Al.3.78 is state dependent, orbitally dependent and the energy and spatial separations between valence and core electrons are sometimes not transparent. These are not insunnoimtable issues. The state dependence is usually weak and can be ignored. The orbital dependence requires different potentials for different angular momentum components. This can be incorporated via non-local operators. The distinction between valence and core states can be addressed by incorporating the core level in question as part of the valence shell. For... 

Note that in core-level photoelectron spectroscopy, it is often found that the surface atoms have a different binding energy than the bulk atoms. These are called surface core-level shifts (SCLS), and should not be confiised with intrinsic surface states. Au SCLS is observed because the atom is in a chemically different enviromuent than the bulk atoms, but the core-level state that is being monitored is one that is present in all of the atoms in the material. A surface state, on the other hand, exists only at the particular surface. 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

Flimpsel F J, McFeely F R, Morar J F, Taleb-lbrahimi A and Yarmoff J A 1990 Core level spectroscopy at silicon surfaces and interfaces Proc. Enrico Fermi School on Photoemission and Adsorption Spectroscopy and Interfaces with Synchrotron Radiation vo course CVIII, eds M Campagna and R Rose (Amsterdam Elsevier) p 203... 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

Xps requires a source that can provide a single x-ray line reasonably narrow in energy. The absolute energy requirement for this x-ray line is that it must be energetic enough to generate photoelectrons from core levels of a majority of the elements with reasonable resolution. Of the many possible x-ray sources, those which best meet this requirement are the A1 x-ray line at 1486.6 eV and the Mg x-ray line at 1253.6 eV. These lines have... 

Not usually, sometimes from available core levels No, some beam damage to radiation-sensitive material... 

Yes, but complicated using valence levels for core levels as for XPS... 

The ability of XPD and AED to measure the short-range order of materials on a very short time scale opens the door for surface order—disorder transition studies, such as the surface solid-to- liquid transition temperature, as has already been done for Pb and Ge. In the caseofbulkGe, a melting temperature of 1210 K was found. While monitoring core-level XPD photoelectron azimuthal scans as a function of increasing temperature, the surface was found to show an order—disorder temperature 160° below that of the bulk. 

BEs of the electrons in all the elements in the period table up to Z= 70 are plotted in Figure 2, as a function of their atomic number Z, up to the usual l486.6-eV accessibility limit. Chance overlaps of BE values from core levels of different elements can usually be resolved by looking for other core levels of the element in doubt. 

Typical chemical shift values for XPS core levels. 

The spectra of Figure 3 illustrate two further points. All the C Is peaks in Figure 3a are of equal intensity because there are an equal number of each type of C atom present. So, when comparing relative intensities of the same atomic core level to get composition data, we do not need to consider the photoionization cross section. Therefore, Figure 3c immediately reveals that there is four times as much elemental Si present as Si02 in the Si 2p spectrum. The second point is that the chemical shift range is poor compared to the widths of the peaks, especially for the solids in Figures 3b and 3c. Thus, not all chemically inequivalent atoms can be distin-... 

Figure 4 Schematic electron energy level diagram (a) of a core-level photoelectron ejection process (one electron process) (b) core-level photoelectron ejection process with shake-up (two- electron process) (c) schematic XPS spectrum from (a) plus (b) (d) Cu 2pa/2 XPS spectrum for Cu in CU2O and Cu in CuO. The latter shows strong shake-up features.

As stated earlier, the major use of UPS is not for materials analysis purposes but for electronic structure studies. There are analysis capabilities, however. We will consider these in two parts those involving the electron valence energy levels and those involving low-lying core levels accessible to UPS photon energies (including synchrotron sources). Then we will answer the question why use UPS if XPS is available ... 

Table 1 lists core levels and their BEs for elements commonly used in technology, which are sufficiendy sharp and intense, and which are accessible to laboratory He I or He II sources (21.2-eV or 40.8-eV photon energy) or to synchrotron sources (up to 200 eV or higher). The analytical approaches are the same as described in the XPS article. For example, in that article examples were given of Si 2p spectra obtained using a laboratory A1 Ka X-ray source at l486-eV photon energy. The... 

Element Core level Approximate binding energy (eV) Usable r radiation... 

Table 1 Narrow, Intense core levels of some elements commonly ...

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