The Mechanism of the Luminescence of Solids

In this paper, effort is directed toward explaining various diverse luminescent properties of solids in terms of a simple model of potential energy versus configuration coordinate. Three states of different multiplicities—a normal, metastable and an emitting state—are involved. The luminescent process consists of the excitation of an electron to the metastable level, the activated release of the electron to the emitting level, and a forbidden transition between the emitting and the ground state. With high excitation energy, the metastable state is bypassed. At high temperatures, the electron in the metastable state surmounts a larger potential barrier to undergo radiationless recombination with the activator atom. Among those significant phenomena that have been measured experimentally and treated quantitatively by calculations based on this model are the temperature dependence of luminescent efficiency and the effect of type and wave-length of excitation on this temperature dependence; the three types of phosphorescence—spontaneous, metastable, and recombination phosphorescence; the phenomena of two-stage afterglow and the effect of type of excitation and temperature on the two stages; the relationship between buildup and afterglow kinetics; and the release of electrons from metastable states by thermal energy. The concept of the Absolute Rate theory are used to clear up the essential criteria for the different types of afterglow. A detailed theoretical analysis of ``glow curves'' is presented and quantitatively applied to improved glow curves obtained at linear rates of heating 100 times slower than those previously reported. The slower rates of heating allow one metastable level to be operative at a time. Both monomolecular and bimolecular mechanisms are treated, and it is concluded that glow curves result from discrete metastable states that are emptied thermally by predominantly monomolecular kinetics. The explicit expressions for the specific rate constants involved in the release of electrons from metastable states are calculated.