recombination line emission
recombination line emission(ree-kom-bă-nay -shŏn) Electromagnetic radiation resulting from the recombination of a free electron (one not bound to a nucleus) and an ionized atom. The spectrum of the radiation consists of a continuum from the transition of free electrons to bound states and a number of distinct recombination lines, each one due to a transition between two energy levels in the atom. The emission lines occur at radio, infrared, and optical frequencies.
The energy levels are numbered in sequence starting at one for the ground state. Recombination lines are then classified according to the number of the energy level to which the electron moves and the number of energy levels that it passes. Recombination line emission occurs, for example, in emission nebulae.