Cathodoluminescence (CL) is the emission of photons of characteristic wavelengths from a luminescent material that is under high-energy electron bombardment.
This is achieved by exciting an electron out of the valence band into the conduction band, which subsequently leaves an electron hole in the valence band.
The excited electron in the conduction band will subsequently drop down into the valence band to fill the e- hole.
This decrease in energy will cause a photon (γ) to be released as visible light, ultraviolet or infrared, which can be detected by a CL detector.
