X-ray-excited charge transfer luminescence in YAG : Yb and YbAG

We report on measurements of X-ray-excited charge transfer luminescence occurring in Yb3+-doped yttrium aluminum garnet (YAG : Yb) and ytterbium aluminum garnet (YbAG). Born cycle model of the localized levels of Yb in the gap of the host lattice using configuration coordinate diagram was used to describe the process involving the charge transfer state (Yb2++hv) which can be formed under UV excitation after the capture of an electron of the valence band or after the capture of an electron of the conduction band produced by high-energy excitation. The temperature dependence of luminescence was investigated in variously Yb3+-doped crystals. Under X-ray excitation, the fluorescence intensity and decay-time drop drastically at temperature below 120 K, where thermoluminescence peaks are detected. The strong afterglow occurring in the temperature range of the glow peaks reflects trapping effects. The afterglow is shown to be important in weakly Yb-doped crystals for a given lattice excitation and under low-energy excitation for a given ytterbium concentration. This is explained by the fact that the mean distance separating filled traps and Yb centers is expected relatively large in both the cases.