The influence of elastic scattering on the concentration dependence of the photoelectron line intensity

Elastic scattering of signal photoelectrons in targets of complex chemical composition has been studied by means of the Monte Carlo technique and the transport approximation. The angular distribution of photoelectrons leaving a sample is shown to be a function of the ratio of the inelastic and the transport mean free paths. The analytical expression for the concentration dependent transport mean free path is derived. Elastic scattering of photoelectrons on their way out of the target is shown to lead to some unusual concentration dependent effects, especially pronounced for light elements in heavy matrices and vice versa. These effects cannot be explained by the widely adopted formalism of XPS when elastic collisions are ignored. In particular, the photoelectron line intensity at a certain emission direction may be no longer linearly dependent on the element concentration and even decrease with increasing concentration for a fixed value of the inelastic mean free. The consequences for the quantitative XPS analysis of multiple component samples are discussed.