Abstract

Phosphors formed from alkali halides with the CsCl structure that contain activator ions with the ground-state configuration ns², differ from their analogues with the NaCl structure in several respects. In addition to the usual A(¹A_1g → ³T_1u), B(¹A_1g → ³E_u, ³T_2u) and C(¹A_1g → ¹T_1u) absorption bands, a variable number (up to three) charge-transfer bands may be distinguished and in CsI:Tl⁺ these CT states actually lie between the ³T_1u and ¹T_1u unrelaxed excited states instead of above ¹T_1u as is usually the case. In CsI:Tl⁺ the CT bands lie in their usual location between the C band and the absorption edge while in CsBr:In⁺ no CT bands are seen, presumably because the CT states overlap the intrinsic exciton states. The emission spectrum of CsBr:In⁺ contains three bands, none of which are CT bands. Two of these are from coexistent minima of tetragonal and trigonal (or possibly orthobombic) symmetry and the third is from the relaxed B state. The CsI:In⁺ emission spectrum also contains these three bands and, in addition, a fourth band that is excited only in the CT absorption band. The emission spectrum of quenched CsI:Tl⁺ crystals consist of three bands all of which can be excited in the A band. The coexistence of three types of minima in the adiabatic potential energy surface that represents the relaxed excited ³T_1u state is a most unusual occurrence - indeed unique so far in these phosphors - and two possible models for the RES are formulated

© 1984 Optical Society of America