The different processes of radiation-matter interaction
Theory of atomic spectra

The interaction of electromagnetic radiation with matter can take many different forms, and we will distinguish between emission, absorption, fluorescence and scattering.

The emission of radiation, accompanied by the transfer of a certain flow of energy from the source to the external environment, requires that the material constituting this source has been previously excited, i.e. that it has itself received a corresponding quantity of energy. This excitation can take many forms (thermal, electrical, electron bombardment, etc.). In all cases, it is said to be a spontaneous emission of radiation, characterized by the fact that the vibrations emitted by the various points of the source are incoherent.

In the case of absorption, on the other hand, it is the incident radiation that provides the absorbing material with a certain amount of energy....