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The elastic and inelastic scattering at single atoms influences the contrast in all types of electron microscopes, though the dense packing of atoms in a solid changes the energy states of valence electrons and is known as band structure (Fig. 1.1b). Elastic scattering results from the Coulomb potential, due to the attractive force of the nucleus of charge +Ze (Fig. 1.1a). The band structure influences the elastic scattering, due to the screening of the nuclear charge by the modified atomic electron cloud. Inelastic scattering with an energy loss ∆E results from the excitation of electrons from the initial state to a free state beyond the Fermi level EF (Fig. 1.1b). The most frequent interactions come from the valence electrons, due to intra- and interband transitions and collective plasmon oscillations, whereas the influence of the dense packing on inner-shell ionizations decreases with increasing ionization energy. The sequence of elastic and inelastic scattering processes results in a diffusion cloud and a slowing down of electrons within a range R (Fig. 1.2).
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