Fine structure in high resolution 4f7–4f65d excitation and emission spectra of X-ray induced Eu2+ centers in LuPO4:Eu sintered ceramics

Abstract

X-ray induced effects in LuPO4:Eu3+ sintered thermoluminescent material were investigated by absorption and photoluminescence measurements between 20 and 300 K. Evidence for Eu3+→Eu2+ conversion upon exposure to X-rays was obtained as narrow band blue Eu2+ photoluminescence was observed. The low temperature luminescence of Eu2+ ions in X-rayed LuPO4:Eu ceramics showed a unique fine structure with a sharp zero-phonon line at 425.8 nm and well-resolved vibronic structure. Excitation spectra of the Eu2+ luminescence revealed a rich structure in which individual 4f7→ 4f6(7FJ)5d1 zero-phonon lines accompanied by vibronic transitions were identified. A detailed analysis allowed an accurate calculation of the Eu3+-like 4 f6(7FJ) core levels in the 4 f65d1 excited configuration. The 4f6 core splitting is different from that of the 7FJ states for Eu3+ in LuPO4, providing evidence for the role of 4f6–5d interaction on the splitting of the 4f6 configuration. The unique luminescence of Eu2+ with a small Stokes shift and well-determined energies of 4f6(7FJ)5d1 excited states make LuPO4:Eu a model system for testing theoretical models which are presently developed to calculate and predict the energy level structure and Stokes shift of 4fn–4fn−15d1 transitions of lanthanides.