Binding-energy reference in X-ray photoelectron spectroscopy of insulators

The surface potential of a sample in XPS depends on the flux and energy of incident electrons and on the resistance, R, between the surface and the spectrometer. When R is very low, the kinetic energy Em of the ejected photoelectron, relative to the spectrometer vacuum level, is given by the well known relation Em=hν φBφS, where hv is the photon energy, φB the binding energy relative to the sample Fermi level, and φS the work-function of the spectrometer. However, when R is very high and the residual charge left by the ejected photoelectron is solely compensated by a sufficient flux of low-energy monoenergetic electrons of energy φe, the sample charges to φe. The measured kinetic energy is now given by Em = hνφBφR+φe, where φR, is the work-function of the sample. Consequently, binding energies on insulated samples are measured relative to the vacuum level, not the Fermi level, since Em now depends on gfR. A good conductor can be examined both shorted and insulated. The difference in measured kinetic energy is Em (shorted)Em (insulated) = φRφSφe. This may provide a method for measuring changes in the work-function while monitoring surface composition.