A measure of the likelihood of an electron making a jump (transition) between a particular pair of energy levels in an atom or ion. The atom can either absorb energy as the electron jumps to a higher level (the likelihood of which is the absorption probability), or emit energy as it drops to a lower level (the likelihood of which is the emission probability). There is also the probability of stimulated emission taking place, in which an atom is in a higher level but the probability of it making a transition to the ground state is low. However, if the atom is struck by a photon of just the right frequency (i.e. having the same energy as the difference in energy between the higher level and the ground state), the electron is stimulated into making the transition.
Subjects: Astronomy and Astrophysics.