Electron Traps and Infrared Stimulation of Phosphors

Abstract

Extensive studies have been made of the relation between electron trap distributions, as shown by thermoluminescence experiments, and the infrared stimulation characteristics of phosphors. A large number of conventional zinc and alkaline earth sulfide phosphors, as well as the special types of infrared sensitive phosphors, have been investigated. Experiments were made over a wide range of temperature from 77 to 600 K. The principal results of these experiments are summarized as follows: 1. There appears to be no direct relation between the stimulation spectrum of a phosphor and a particular group of electron traps. In general, during stimulation, electrons are ejected most readily from the shallowest electron traps present. The ability of the stimulating radiation to empty deeper traps increases as the wavelength of the radiation decreases. There are, however, two types of association between the stimulation spectrum bands and groups of electron traps. In certain phosphors, which usually have a complex stimulation spectrum, monochromatic irradiation in one part of this spectrum reduces preferentially this part of the spectrum, although it may affect the whole spectrum. Such irradiation appears, however, to have no selective effect in ejection of trapped electrons. The second kind of association is the following. In some phosphors certain groups of electron traps must be filled for a particular band in the stimulation spectrum to appear. However, when these traps are filled, they can be emptied by stimulation at shorter wavelengths. 2. Large differences are found between the light sums due to the release of trapped electrons by thermal and by optical activation. The light sum for stimulation is often much less than that for thermoluminescence in conventional phosphors, but very much the reverse is true of some infrared sensitive phosphors, as shown previously by other workers. 3. When phosphors are excited at a given temperature and then cooled in the dark, the stimulation spectrum obtained at the low temperature is similar to that observed at the excitation temperature, though reduced in intensity. The intensity decreases with temperature and, below a certain critical temperature, it is impossible to obtain stimulation in particular spectral bands. From these results, it is inferred that a thermal activation process is essential to stimulation; this is similar to the photoconductivity effects in alkali halides due to the optical ejection of electrons from F centers. 4. In previous studies at this laboratory, it was shown that, in the phosphorescence and thermoluminescence of sulfide and silicate phosphors, no retrapping of the released electrons takes place. However, when traps are emptied by infrared stimulation, a considerable degree of retrapping occurs. The theoretical implications of this effect are discussed.The above studies have furnished significant evidence on the stimulation process in phosphors, but they indicate no simple relation between the thermal and optical release of trapped electrons. It is hoped to clarify the position in this respect by making more precise measurements of stimulation, absorption, and thermoluminescence on single crystals of less complex luminescent solids.