Photo-induced charge state conversion of Eu2+ in Ca2ZnSi2O7

Eu2+ doped sorosilicate Ca2ZnSi2O7 (melilite) exhibits a broad band emission peaking at similar to 600 nm (similar to 2 eV) due to the electric dipole allowed transition of 4f(6)5d(1) to 4f(7) of Eu2+ by an excitation with blue light (460 nm). Strong O2- ligand field with low symmetry due to the layered tetragonal crystallographic structure of the melilite may play a dominant role in lowering the emission band energy to similar to 2 eV. In addition, line emissions attributable to the transitions from D-5(0) to F-7(J) of Eu3+ ions are detected by an excitation with deep UV light with sub-360 nm wavelengths. This is due to the formation of transient Eu3+ ions via charge transfer from Eu2+ to the matrix. The lifetime of the transient Eu3+ ion is found to be 58 ms by a pump-probe measurement, in which UV pulse laser and green continuous wave laser are employed as pump and probe lights, respectively. Based on these results, the energy diagram of Eu2+ in Ca2ZnSi2O7 is proposed.