In EELS, a beam of high-energy electrons is directed at a specimen. As these electrons interact with the specimen, they lose energy through various mechanisms such as inelastic scattering. The amount of energy lost is characteristic of specific interactions within the material, such as excitations of core electrons, collective oscillations of electrons (plasmons), or interband transitions. The energy loss is measured using a spectrometer, which disperses the electrons based on their energies, allowing the energy loss spectrum to be recorded.
