The change in the @I03103@ of a particle may be due to radiative processes, i.e. the emission or absorption of a photon. A particle in an @E02257@ 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 @P04855@ per second for a given excited particle is termed the transition @P04855@ for @S05886@.