Electrical charging of insulating samples during XPS is of direct concern for referencing the binding energy scale. Results are presented on charging of composite insulating samples that consist of an organic overlayer of polystyrene (PS) or polydimethylsiloxane (PDMS) deposited on NaCl particles. The samples were analysed using small-spot monochromatized radiation, the surface charge being stabilized with an electron Ñood gun. The charging shift of the NaCl substrate is inÑuenced by three di †erent factors : the total photoelectron Ñux, the energy of the electrons produced by the Ñood gun and the Ñux of electrons originating from the overlayer. The overlayer is acting as an additional electron source, which makes the substrate more negative and reduces inhomogeneities of the local potential a †ecting the peak full width at half-maximum. Comparison of substrate peak intensities with computed values showed that PS overlayers were less uniform compared to PDMS overlayers. The overlayer thickness and uniformity a †ect both the total photoelectron Ñux and the Ñux of electrons from the overlayer to the substrate. Overlayer charging, like substrate charging, is probably determined by the balance of electron Ñuxes in the phase considered. The kinetic energy of the photoelectron is determined by the electrical potential in the phase where photoemission occurs and not with respect to the sample surface. This was demonstrated by di †erential charging between the NaCl substrate and PS overlayer.

No abstract