Energy transfer between Eu3+ ions in calcium diborate glasses

The evolution of the D-5(0) --> F-7(0) emission of Eu3+ ions in calcium diborate glasses has been analysed using time resolved fluorescence line narrowing measurements in order to give a complete view of the energy transfer processes between these ions. At low concentration (2.5 mol% of Eu2O3) and exciting within the high energy side of the inhomogeneous F-7(0) --> D-5(0) absorption band, the luminescence spectrum mainly consists of a narrow resonant peak that repeats the exciting profile, indicating that the migration processes between Eu3+ ions within the D-5(0) level is not important. However, at higher concentrations (5 to 11.5 mol% of Eu2O3) the luminescence spectrum contains not only a narrow emission but also a broad band due to ions excited by energy transfer (background fluorescence), which for long times well reproduces the inhomogeneous profile. The temporal evolution of the narrow band fluorescence and the shape of the background fluorescence have been analysed using a previously proposed model. The purpose is to understand the dynamics involved in the energy transfer processes caused by the interaction between Eu3+ ions and the implications in their luminescence. A very good agreement with the experimental results is found taking into account an energy dependent quadrupole-quadrupole (S = 10) non-radiative energy transfer process assisted by a phonon from Eu3+ ions at high crystal held sites to ions at low crystal field sites. The temperature dependence of the energy transfer processes is analysed in the range from 13 to 60 K.