The scattering of incident electrons by electrons outside the nucleus belongs to inelastic scattering process. In the process of scattering, part of the energy lost by incident electrons is converted into heat, and some atoms in the substance are ionized or form free carriers (in the case of semiconductors), resulting in various useful information, such as secondary electrons, Auger electrons, characteristic X-rays, characteristic energy-losing electrons, cathodoluminescence, electron-induced conductance and so on.

1. secondary electrons: secondary electrons refer to extranuclear electrons bombarded by back-incident electrons. Because the binding energy between the nucleus and the valence electrons in the outer layer is very small, when the extranuclear electrons of the atom gain energy greater than the corresponding binding energy from the incident electrons, they can leave the atom and become free electrons. If this scattering process occurs near the sample surface, those free electrons whose energy is greater than the work function of the material can escape from the sample surface and become free electrons in vacuum, that is, secondary electrons. The secondary electrons come from the surface area of 5- 10nm, and the energy is 50eV. It is very sensitive to the surface state of the sample and can effectively display the micro-morphology of the sample surface. Secondary electrons are generated because the incident electrons originate from the surface of the sample and have not been reflected many times.