Single scattering means that only zero, one or two events of the specified type (e.g. secondary electrons, X-ray, and core-loss electrons) are likely to occur during the passage of a single primary electron. Under high energy electron irradiation, e.g. in TEM and SEM, the main inelastic scattering mechanisms are:
i) Phonon excitation (heat).
ii) Plasmon excitation (valence electrons).
iii) Single electron excitation (inner and outer shell scattering).
iv) Direct radiation losses (Bremsstrahlung radiation due to deceleration of the electron beam in the Coulomb field of an atom).
v) Excitation of conducting electrons leading to secondary electron emissions.
The outer-shell (valence) electrons can undergo single-electron excitations:
i) The valence electrons can transit to the condition band across the band gap (so-called interband transition for insulators and semiconductors).
ii) The conduction electrons transit to a higher energy state (for metal).
iii) Emission of secondary electrons.
In general, the requirements of TEM specimen thickness for EELS and EFTEM measurements are:
i) The specimens should be sufficiently thin to prevent any multiple inelastic scattering, but the degree of single inelastic scattering should be relatively high. For most materials, the optimized specimen thickness is in the range of 25-100 nm, depending on the average atomic number and the beam energy.
ii) To avoid surface effects, the specimen thickness cannot be less than 25 nm if low-loss EELS spectra are measured.
The valence electrons in alkali metals act as free particles so that the collective form of excitation response (plasmon effect) detected by EELS is predominant (compared with single-electron excitation). In contrast, in rare gas solids, plasmon effects in EELS are weak or nonexistent. The other materials fall in between these plasmon excitation and single-electron excitation.
In this above cases, the high energy electrons normally lose several to tens of electron volts and are scattered at small angles (e.g. 1 ~ 5 mrad for incident electrons of 100 ~ 200 keV).