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

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(⁷F_J)5d¹ zero-phonon lines accompanied by vibronic transitions were identified. A detailed analysis allowed an accurate calculation of the Eu³⁺-like 4 f⁶(⁷F_J) core levels in the 4 f⁶5d¹ excited configuration. The 4f⁶ core splitting is different from that of the ⁷F_J 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(⁷F_J)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.