Cathodoluminescence is the excitation of light by means of an electron beam (cathode ray). The emitted light is reflected at two mirror surfaces above and below the TEM specimen, respectively, and guided via optical fibres into the spectrometer.
But there are various reasons for the emission of light within an electron microscope:
Cathodoluminescence describes the de-excitation of an electron initially excited into the conduction band, when spontaneously falling back into the valance band of a semiconductor or insulator.
Plasmonics describes the emission of light when an electronic surface resonance collapses. The energy of the surface oscillation is emitted as photons.
Transition radiation is the radiation emitted as soon as an electron passes across an interface of two media with different dielectric properties, such as the vacuum/specimen interface. Due to the annihilation of the mirror charge inside the specimen, being initially created by the approaching electron, the collapsing electric dipole field causes the emission of radiation.
Vavilov-Cerenkov radiation is created as soon as the speed of the probe electrons is faster than the speed of light inside the specimen. The speed of light inside the specimen is reduced by a factor 1/n, with n being the refractive index of the probed sample.
Diffraction radiation is generated by the Smith-Purcell effect, as soon as a swift charge passes by a rough interface.
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