Photoionization cross section energy-level dependence

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 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 iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

Fig. 20. A plot showing the dependence of the peak of the photoionization cross section (P) on the level energy ( ,) for the Lucovsky (5-function) model and for a more accurate ( full calculation") evaluation of impurity wave functions. [After Blow and Inkson (1980a, Fig. 5).]...

Each core level of a component of a specimen appears as a characteristic peak in the energy spectrum (Fig. 1-23 a). These core level peaks provide a qualitative measure of the composition of the surface. The size of the signal is proportional to the surface concentration of the element under study. For a quantitative evaluation, the relative sensitivities of the elements, which depend to the Erst order on their photoionization cross section (PICS), have to be taken into account. The PICS are listed in appropriate tables (Wagner et al., 1978). The exact energy position of an XPS signal depends on the charge or valency of the atom. Therefore, the chemical shift of an element... 

The intensity of a given photoelectron line is proportional to the X-ray flux, cross-section for exciting the particular level, density of the particular atom in the lattice x, the escape depth for electrons of the resulting kinetic energy, asymmetry factor in the angular distribution of the photoionization event, transmission of the analyzer, which includes the acceptance angle and area, and the efficiency of the electron detector. In addition, there are several secondary factors, some of which depend on the matrix. The intensity of line is given by... 

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