Inelastic mean free path, values from

The reasons AES is a surface-specific technique have been given in Sect. 2.1.1, with reference to Fig. 2.2. The normal range of kinetic energies recorded in an AES spectrum would typically be from 20 to 1000 eV, corresponding to inelastic mean free path values of 2 to 6 monolayers. 

Depending on the energy tico of the incident photons, valence band states and even core level electrons can be excited. UPS is a surface-sensitive technique since electrons have a very short inelastic mean free path, Xi, which depends on the kinetic energy Ek, and has a minimum value of 0.5 nm for T k 100 eV. The leading edge of the valence band is taken as the VBM or HOMO maximum and has to be referred to which has to be determined from a clean inorganic metal surface. Those electrons with k > 0 are removed from the sample and transmitted to the detector. The fundamental equation of the photoemission process is (Einstein, 1905) ... 

We see that for EELS analysis, the resolution is degraded only linearly with foil thickness. Comparison with the values in Table 3.1 shows that if the collector aperture is about 10 mrad, and the energy losses are 200 eV or more, nanometer resolution can be maintained for foils up to about 100 nm thick. This thickness coincides approximately with the mean free path for inelastic scattering, so that spectra from thicker foils suffer degradation of energy resolution anyway. 

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