Mechanisms of persistent luminescence in Eu2+, RE3+ doped alkaline earth aluminates

Persistent luminescence of Eu2+ doped calcium aluminate, CaAl2O4 : Eu2+, was studied under UV- and visible illumination. Broad band luminescence was observed in the blue region (λmax=440 nm) due to the 4f65d1→4f7 transition of Eu2+. The threshold for the visible excitation was found at about 530 nm, which is not high enough in energy to create a monovalent Eu+ ion required by the mechanisms presented previously. The new mechanism proposed involves the excited state absorption of two 530 nm photons via deep traps followed by trapping of electrons in shallow traps. The persistent luminescence is observed after a temperature controlled electron–hole recombination and subsequent energy transfer to the Eu2+ luminescent center.