Possible mechanism of energy storage in optically stimulable materials: Doped alkali halides

Radiation-induced effects in doped alkali halides, mainly in KBr:In, are studied by the luminescence technique. The activator luminescence during a 10 s under UV-light or electron irradiation and, after it, the pulsed photostimulated luminescence on a phosphorescence background were investigated. The obtained results allow us to conclude that the main host lattice excitation relevant to both the luminescence processes mentioned above is a very mobile excitonic excitation including a photon phase and the self- trapped exciton in its composition. The photon phase, as we suppose, represents a free exciton luminescence at room temperature. In this phase, via multiple reabsorption in the low-energy exciton absorption band, the excitonic excitation can overcome large distances. In the phase of the self- trapped exciton, the induced decay within a perturbed region around the activator hole center takes place. As a result, the pairs of F-type electron and activator hole centers, contributing to the photostimulated luminescence under stimulation with light in the F absorption band, are formed. The above-mentioned defect formation process is dominant over the excitation of activator ion at low activator concentration. At high activator concentration, when strong competition between reabsorption in the exciton absorption band and absorption in the local center band connected with the activator takes place, the activator luminescence prevails. The activator luminescence spectrum is found to be sensitive to the F or F-aggregate centers formation in the region near the activator hole center.