Depth profiling inelastic mean free paths

XPS may be used to non-destructively depth profile a surface. This is an important ability since the most common depth profiling method used in surface techniques is acheived by sputtering surface atoms from the material with the use of a high flux ion gun, such an approach w ould be inappropriate for analysing polymers due to the extreme levels of damage w hich would occmr The surface sensitivity of XPS derives from the low inelastic mean free path of electrons in solids. This means that photoelectrons emitted from deep in the sample have a very small probability of... 

Figure 3. Variation with primary energy Ep of the N(E ELS spectrum from an annealed tin oxide surface. Because of the relationship between KE and inelastic mean free path, the set of spectra corresponds to a depth profile of the annealed material. The growth of the 27-eV feature is due to increasing oxygen deficiency as the spectra become more surface specific. (From Ref. 2.)...

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