ON THE MEAN INNER POTENTIAL IN HIGH-ENERGY AND LOW-ENERGY ELECTRON DIFFRACTION

New electron microscopies spanning the energy range from millivolts to kilovolts image different beam-energy-dependent potentials. We review the theories of the average value of the mean inner potential of a solid in order to understand its role in image interpretation and its relationship to charge density and atom positions in crystals. Three common definitions of this quantity are compared, that from high-voltage transmission electron interferometry, the theory of surface dipole layers and the work function, and that from low-energy electron diffraction (LEED). Energy-dependent corrections due to exchange and virtual inelastic scattering are proposed for LEED, photoelectron holography (PEH), the low-energy electron microscope (LEEM) and the point-projection microscope (PPM) operating in the sub-kilovolt range. A calculation of the mean free path of low-energy electrons as a function of their energy is also reproduced.