emissionabsorption

Emission-absorption is the interplay of emission and absorption processes by matter interacting with electromagnetic radiation. Atoms or molecules in a medium can absorb photons at specific energies, removing those photons from a beam. The same species can also emit photons, spontaneously or in response to incident radiation, at the same or related energies. The observed radiation reflects the balance of these processes and the properties of the intervening material.

In radiative transfer, the change of specific intensity Iν along a path is described by dIν/ds = -κν Iν + jν, where κν is the absorption coefficient and jν is the emission coefficient. The ratio jν/κν defines the source function Sν. In local thermodynamic equilibrium, Sν equals the Planck function Bν(T); in non-LTE conditions, Sν can deviate from Bν, leading to complex line formation and brightness variations.

Spectral signatures of emission-absorption include absorption lines formed when cooler material lies in front of a brighter source, as well as emission lines from excited gas. Some environments produce both effects in the same spectral feature, yielding profiles such as the P Cygni shape, where blue-shifted absorption accompanies red-shifted emission, indicating expanding material.

Applications span stellar and planetary atmosphere spectroscopy, nebular diagnostics, laboratory plasmas, and laser physics. Understanding emission-absorption allows inference of temperature, composition, density, and velocity fields in astronomical and terrestrial plasmas.