absorption

The term absorption has a particular meaning in the study of radiative transfer and spectroscopy to indicate the removal of photons from a beam of electromagnetic radiation (e.g., light) through interactions of photons with particles such as molecules, atoms, ions, or electrons. In this usage, a photon which is scattered out of the beam is considered absorbed. It is the inverse of emission, the addition of photons to the beam. The two concepts are used widely in astrophysics, for explaining and modeling stars, for dealing with the effects of the Earth's atmosphere, for explaining and modeling gas clouds, etc.

Absorption is complicated by the variety of processes that cause it, and varies by the density, the constituents, and the temperature of the material the beam is passing through. Absorption lines are a result of a type of absorption that is very sensitive to wavelength. A classification of types of absorption is based upon the status of an electron with which the photon is interacting:

• bound-bound - with an electron that is bound to an atom both before and after the interaction: this is the type of absorption that results in spectral lines.
• bound-free - an electron being freed from an atom by the absorption process, such as a neutral atom being ionized (photoionization).
• free-free - a free electron absorbing a photon, which can only happen when near a charged particle such as an ion.
• electron scattering - Thomson scattering or Compton scattering.

The first three have an inverse which constitutes emission. Electron scattering is both absorption and emission, the latter for the beam along the photon's new trajectory.

The term absorption is also used within astronomy in a more general sense. It may refer to that of EMR but without the attention to detail listed above. Also, it may be used for other types of particles.