Relation between Absorption and Emission Probabilities in Luminescent Centers in Ionic Solids

It is pointed out that the Einstein relationship between spontaneous emission probability and induced absorption probability for radiative transitions is not generally valid for impurity centers in condensed media. An alternative approach to the problem is available by which estimates of the possible discrepancies can be made. Several mechanisms are discussed, particularly the Jahn-Teller effect, which depend on the existence of the phonon field and the lattice relaxation following absorption of a photon. In some simple cases, such as the $F$ center in KCl and LiF, the decay time predicted from the integrated absorption cross section may differ by an order of magnitude from that observed. In other simple and cubic systems, where the atomic symmetries are appropriate, the discrepancy may be even larger. In noncubic crystals similar discrepancies may be expected.