The change in the
internal energy of a particle may be due to radiative processes, i.e. the emission or absorption
of a photon. A particle in an
excited state may undergo a transition to a lower energy level by emission of a photon. This is
known as radiative de-excitation. If such a transition occurs spontaneously its
probability per second for a given excited particle is termed the transition
probability for
spontaneous emission.
Source:
PAC, 1985, 57, 1453
(Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation
sources (Recommendations 1985))
on page 1463