Electron-specimen interactions

Abstract

elastic scattering of electrons by the Coulomb potential of a nucleus is the most important of the interactions that contribute to image contrast. Cross sections and mean-free-path lengths are used to describe the scattering process quantitatively. A knowledge of the screening of the Coulomb potential of the nuclei by the atomic electrons is important when calculating the differential cross sections at small scattering angles. The inelastic scattering is concentrated within smaller scattering angles, and the excitation of energy states results in energy losses. The dominant mechanisms are plasmon and interband excitations, which can be described by the dielectric theory. These inelastic scattering processes are less localized than elastic scattering and cannot contribute to high resolution. Inner-shell ionizations result in edge-shaped structures in the electron energy-loss spectrum (EELS), on which are superposed a near-edge structure (ELNES) and an extended energy-loss fine structure (EXELFS), which can be used for analytical electron microscopy at high spatial resolution. Even quite thin specimen layers, of the order of a few nanometers, do not show the angular or energy-loss distribution corresponding to a single scattering process. Multiple-scattering effects have to be considered as the specimen thickness is increased, and this can also result in electron-probe broadening. © 2008 Springer Science+Business Media, LLC All rights reserved.