Interpretation of the Properties of Alkali Halide-Thallium Phosphors

An attempt is made to interpret the properties of alkali halide-thallium phosphors. The experimental basis of this interpretation is the work of Pohl and his numerous collaborators on these substances. It is concluded that the long wave-length absorption peaks of the phosphors which are not contained in the pure alkali halides are to be attributed to the excitation of the thallium ions. These are assumed to be distributed throughout the crystal, replacing alkali metal ions. The structure of the observed peaks is interpreted in terms of the details of the spectrum of Tl+ and the probable effects of crystalline perturbations. The nature of the fluorescent and phosphorescent spectra is then correlated with the behavior of the normal and excited levels during the time after absorption and before optical radiation in which the crystal again comes to mechanical equilibrium. Distinction is drawn between two cases: That in which the excited Tl ion has no thallium ions among its twelve positive ion neighbors and that in which it has one. It is concluded tentatively that excitation to the lowest excited levels, associated with a triplet in the free ion, will give rise to fluorescence in either case, and that excitation to the higher, singlet level will lead to the alternative possibility of phosphorescence in the second case, but not in the first. The theoretical justification of these tentative conclusions involves assuming that the homopolar forces between the excited ion and its neighbors depress singlet excited levels more than triplet excited levels, and that singlets are depressed more when one of the neighbors of a Tl ion is another Tl ion than when none are.