Elastic scattering and quantification in AES and XPS

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Abstract
In the mathematical formalism of quantitative AES and XPS, the elastic electron collisions are not taken into account. However, recent calculations have shown that the neglect of the elastic collisions may result in considerable errors. Theoretical analysis of the actual electron transport in a solid requires two major problems to be considered. (i) calculation of the differential elastic scattering cross‐sections for a given potential and electron energy and (ii) description of the multiple electron scattering. Both problems are extensively reviewed. The Monte Carlo method is usually used to describe the electron trajectories in a solid at energies of surface‐sensitive electron spectroscopies. Such simulations have indicated that the elastic collisions of photoelectrons affect considerably the angular distribution of the measured intensity and the photoelectron escape depth. The latter parameter may be diminished by > 30%. Elastic collisions of Auger electrons decrease the current recorded by the typical analysers and also considerably decrease the escape depth. Thus, the actual electron transport in a solid should be considered in calculations associated with determining the inelastic mean free path using the overlayer method or overlayer thickness measurements. The usual formalism of quantitative AES and XPS can be extended easily to account for the elastic electron collisions. The values of the corresponding correcting factors are extensively compiled. To check the validity of the Monte Carlo calculations, the results of simulations of forward electron scattering and electron back‐scattering are compared with the available experimental data. Excellent agreement between experimental and theoretical angular distributions of photoelectrons has been found. A very good agreement was also observed in the case of elastic electron back‐scattering from surfaces.

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