Quantification of XPS Spectra

The intensity of an XPS peak (Ia) is a strong function of (i) the incoming photon flux, (ii) the concentration of the given element, (hi) its photoionization cross-section (which is excitation-energy dependent), (iv) the mean free path of the emitted photoelectron, and (v) further instrumental parameters (such as photoelectron collection and detection efficiency). By defining atomic sensitivity factors (S, as an overall factor summing up the effects of iii-v), the atom fraction of any element in a sample can be calculated as  

Based on the Lambert-Beer-type law of exponenhal attenuation of peak intensity (I) with excitation depth (z), 

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Quantification is in many cases the most important feature of XPS [52]. The X-ray photoelectron current produced by an incident X-ray photon of energy hv, which ionises core level Z in an atom of type A in a solid matrix (M), is a function of both energy- and matrix-dependent terms. Quantification of XPS spectra is thus far from simple. In fact, already the exact definition of quantification is complicated. Even for an ideal sample the actual escape depth may vary substantially. The depth of photoelectron detection, for a given XPS system, may vary dramatically from measurement to measurement, and even within the same measurement (in relation to the energy scale). After some experimental... 

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