Number of Traps and Behavior of Excited Electrons in Luminescent Materials

The number of traps per unit volume in different types of luminescent materials, and the prolonged lifetimes of excited electrons due to trapping, were studied by means of curves of the rise of fluorescence with time under constant high-energy electron and alpha-particle excitation. These rise curves were taken after various dark decay periods and after different infrared irradiation conditions. They were found to be different in shape for different types of phosphors. Lifetimes longer than one week were found to occur in most of the zinc cadmium sulfides. The number of traps per unit volume responsible for lifetimes longer than about a minute was found to be of the order of ${10}^{15}$ (${10}^{-7\mathrm{}}$ mole fraction) in the zinc cadmium sulfides, and the number responsible for lifetimes longer than a week was of the same order of magnitude. There is no direct correlation between infrared stimulability and the number of traps in which electrons remain for some time. Substances with extremely high stimulability (Fonda type) and those with extremely low stimulability both contained about the same number of these traps per unit volume. Materials with large concentrations of Ni exhibited no more such traps than those with smaller concentrations. Lack of phosphorescence is caused either by the preponderence of excited electrons of very long lifetime or by the radiationless recombination of electrons of short lifetime. Examples of each are given. It was found further that most of the electrons which remain in traps for long periods could be ejected by infrared radiation of wavelength greater than 1.25 μ, indicating that retrapping of both electrons and positive holes is of major importance. The different effects of alpha particle excitation and electron excitation on the filling of the traps is discussed.