Colour Centres and Energy Transfer in BaF2-xClx:Eu2+ Phosphors

The optical absorption spectra of BaF2-xClx:Eu2+ after ultraviolet (UV) light excitation were investigated. The differences between the absorption spectra after and before excitation (DAS) were observed. The DAS increase at both the high and the low energy side of F band in BaF2-xClx: Eu2+ after 245 nm UV light excitation. The bleach effect of UV light and the absorption of electrons in the valence band may account for the former and the formation of Fa centres (association of F(Cl−) centres), whose absorption band matches the HeNe laser better, may explain the latter. In the write-in process, the transfer of electrons is via tunneling. In the readout process, the transfer of electrons captured in F(F−) and Fa centres is more likely via tunneling, and that of F(Cl−) centres is more likely via conduction band.