Migrations of excitons and holes in luminescent crystals of CsBr

Abstract

For CsBr, CsBr:Tl, CsBr:In and CsBr:Na luminescence has been studied for the uv excitation within the CsBr fundamental absorption. Optically created excitons in CsBr at 80°K give unpolarized emission of 3.49 eV with the decay time of emission τ=10.5 μs. It was concluded that emission arises after the excitons have migrated distances of several lattice constants and then have been self-trapped. Optically creation of excitons in CsBr:Tl and CsBr:In induces the luminescence not only of self-trapped excitons but also of In + and Tl + centres. The intensity of activator emission does not depend on temperature in the interval from 80 to 140°K and increases with temperature if T 140°K. At 80°K the decay time of exciton emission in CsBr equals that of CsBr:In. The emission of In + and Tl + centres at 80°K is excited by nonrelaxed excitons. At high temperatures, T 150°K, axially relaxed excitons diffuse to impurity centres and excite them. The temperature dependences of the ionization of In + centres by holes have been investigated. At T<130°K nonrelaxed holes ionize In + centres and the ionization efficiency does not depend on temperature. At T 130°K the V k centres are mobile and also ionize In + centres. Various properties of relaxed and nonrelaxed excitons and holes are discussed.