On the dynamics of photostimulated processes in additively coloured CaF2:La crystals

Crystals doped with Y3+, La3+, Ce3+, Gd3+, Tb3+, or Lu3+ ions show after an additive coloration very similar absorption spectra and a characteristic photosensitive behaviour. Light irradiation makes the electrons leave their traps and shift to another ones. Three different mechanisms of such transfer are described in the present paper. Mathematical relations describing such mechanisms are applied to experimentally recorded progresses of the dynamical photosensitive behaviour. The theoretical models are compared with the experimental observations. This practice showed, that irradiation of different wavelengths (410 and 650 nm) induces different types of dynamical photosensitive behaviour. This let us assume different mechanisms of electron transfer, too. The models proposed in [13, 15, 16] assuming a double-model which consists of an impurity ion and an F-centre are supported. On the contrary to the model of the authors of references [17 to 19] this model traps only one electron. Trapping of two electrons should result in the lost of the photosensibility. Following our model the photochromic process is identical with the transfer of one electron between the partners of the above-mentioned double model. The present investigations are interpreted by assuming an 410 nm-induced electron transfer from the Fg-centre to the Latrig3+. with possible participation of the conduction band. 650 nm-irradiation induces the reverse process but without the conduction band.