Spectroscopic relaxation processes of rare earth ions in crystals

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Relaxation processes between Stark groups in the (4f)n energy levels of rare-earth salts are discussed. The low temperature spontaneous relaxation rates are exponential in the energy gap to the next lowest Stark group. The rates are determined primarily by the order of the process and the strength of the orbit-lattice interaction. With increasing temperature, stimulated emission of phonons by thermal phonons increases the rate. The presence of molecular groups and 4fn-1 5d bands can cause quite different rates and temperature dependences. Excitation migration to special sites is suggested as a primary mechanism in ion-pair relaxation.