Electron transfer processes in rare earth doped insulators

The optical properties of insulators activated with rare earth ions are det ermined by the energy levels of the impurity; hence, the study of intra-ion transitions has been the principal focus of numerous investigations in the past. This has occurred to the neglect of other properties, particularly t hose involving the most energetic states of the impurity. To understand the optical properties in regions of higher energy, i.e. UV, the intrinsic ban ds of the host materials and the charge transfer dynamics between impurity and host must be taken into account. In such cases, the exact positions of the impurity states relative to the host conduction and valence bands becom e important and need to be determined in order to establish the physical be havior of the activated material. A complete treatment of the impurity-host system leads to a donor-acceptor model, similar to the common approach in semiconductor physics. Here we apply this model to discuss recent experimen ts on rare earth doped materials, including the luminescence efficiency of rare earth doped scintillator and phosphor materials, and laser cooling of solids. (C) 2000 Elsevier Science S.A. All rights reserved.