Secondary electron emission and fundamentals of charging mechanisms in XPS

Fundamentals of charging mechanisms in XPS are investigated with special attention to the X-ray-induced secondary electron emission (SEE) yield from insulators, δX(hν), that is, first, responsible of charging and of differential charging. In the defocused mode, the SEE emission is compensated by three different types of effects: inner trapping effect in the positively charged layer; internal effects occurring at the specimen/substrate interface; external effects resulting from emitted secondary electrons retuning back to the specimen. A detailed investigation of these mechanisms leads to an analytical expression for time evolution of the surface potential as a function of irradiation time, VS = f(t), via incident flux and characteristics of samples. In particular, it is shown that internal effects lead to three different regimes for VS = f(t) depending upon the thickness of the specimen while, in the focused mode, external effects are a function of the lateral dimension of the sample and of its surrounding. Numerical applications permit to obtain order of magnitude in laboratory XPS as well as in spectro-microscopy developed from the use of synchrotron X-ray sources. Various practical details concerning for instance the influence of heterogeneities of the specimen either in thickness or in composition as well as the change of incident angle in angle resolved XPS or the change of incident photon energy have been deduced.