Direct hole and delayed electron capture on a picosecond timescale by Eu2+ centers in CaGa2S4 monitored by synchroscan with horizontal blanking

A Eu2+ concentration and temperature dependent energy transfer study from the host lattice to Eu2+ luminescence centers in Ca(1–x)EuxGa2S4 (x = 0.001 to 0.05) was performed with a special streak camera that combines the high timing resolution of a conventional synchroscan operation (<2 ps) with the ability to study long lived states (10?ns???1 ms) typical for rare earth and transition metal ions. Two transfer mechanisms from the CaGa2S4 host lattice to the Eu2+ ions were identified. A fast transfer process (<4 ps) is interpreted as sequential hole-electron capture by the Eu2+ ions, and slower process (>1 ns) is interpreted as the sequential capture of a hole and an electron by Eu2+ but with the electron first entering an intermediate state trapped near Eu3+. Energy transfer via a self-trapped excitonic (STE) state is unlikely because of the absence of an anti-correlation between the STE decay-time and the Eu2+ rise-time as well as between the STE emission intensity and the Eu2+ intensity. All rise-time spectra were successfully fitted with a model representing the above transfer mechanisms.RRR/Radiation, Radionuclides and ReactorsApplied Science