An emission line in a spectrum arising from the recombination of a free electron with an ion. As the electron drops down through the energy levels of the atom, it emits recombination lines at wavelengths that depend on the difference in energy between the levels. For hydrogen, these range from radio lines (caused by jumps between the outer, lowest-energy levels), via infrared, to optical lines produced by jumps down to level 2. Recombination to level 1 (the ground state) produces an ultraviolet photon which itself causes further ionization. From recombination lines, the gas temperature and density in ionized nebulae can be estimated.
Subjects: Astronomy and Astrophysics.