Core-level photoionization

XPS. As indicated in Figure 1, core-level B.E. s are characteristic of individual atoms and so one obtains an atomic identification directly from the determined B.E. value. In addition, the core-level photoionization cross-sections are reasonably well-established, both theoretically (13) and experimentally (14), and since the core-levels are atomic in nature, there are no significant variations with chemical environment of the atoms, which means that the atom composition analysis can be made quantitative. All elements which have core-levels, i.e., everything but hydrogen and helium can be detected, though the magnitude of the cross-sections and hence the relative sensitivities to the different elements varies by 102. 

Core level photoionization cross-sections in molecules are similar to the cross-section for single atoms, which indicates that relative XPS line intensities can be used directly for elemental analysis of the sample. A comparison of UPS and XPS methods is presented in Table 1. 

To understand the origin of low-energy satellites in the 3d spectra of light lanthanides, their implication for a dynamical picture of core level photoionization in the presence of semilocalized screening orbitals to establish their relation to the ground state electronic structure, see fig. 1 (Wertheim and Campagna 1978). 

State appears as a peak along with the main core-level photoionization. 

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-... 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

Figure 7. Depiction of origin of EXAFS. An X-ray photon is absorbed by A, resulting in the photoionization of a core-level electron represented as an outgoing ( + ) photoelectron wave which is backscattered (<- ) by a near neighbor, B.

Fig. 1. Relationship between direct photoionization and shake up and shake off phenomena for core levels...

The first levels of information available derive from the measurement of absolute and relative binding energies and relative peak intensities. The distinctive nature of core levels means that identification of elements is straightforward as is the distinction between peaks arising from direct photoionization and Auger processes. 

The lines of primary interest in an xps spectrum are those reflecting photoelectrons from core electron energy levels of the surface atoms. These are labeled in Figure 8 for the Ag 3s, 3p, and 3d electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative intensities of these core level peaks is the photoionization cross-section, q. This parameter describes the relative efficiency of the photoionization process for each core electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same core level of all elements. This difference results in variable surface sensitivity for elements even though the same core level electrons may be monitored. 

In the case of a deep core level, the photoelectron will leave the core region in a time short compared with the time for the localized Is hole to hop between the nuclei, and the deep core hole will in practice be localized on one of the nuclei. Diagrammatically this is shown in Fig. 41, where Figs. 41a-d describe photoionization from the symmetry adapted 1 og and 1 ou MO s and where Fig. 41 e describes the interference between the... 

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