Many features of the behavior of luminescent sulphides can be correlated by means of a mechanism based on the energy-band theory of the solid insulator. It is imagined that after excitation, which is essentially an ionization of an activator atom, the dissociated electron is highly likely to be trapped and held localized in a discrete energy level in the forbidden region, recombining with an ion and emitting the characteristic luminescence only after it has been freed from this trapped state by thermal excitation or other agencies. This picture, with appropriate modifications, is also useful in explaining the properties of silicate phosphors.
R. P. JOHNSON, "Luminescence of Sulphide and Silicate Phosphors," J. Opt. Soc. Am. 29, 387-391 (1939)