Energy levels of rare-earth ions in Gd2O2S

The energies of the ground 4fn levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd2O2S crystal has been determined. It is shown that the Pr3+, Tb3+, and Eu3+ ions can be luminescence centers in Gd2O2S. The levels of the Nd3+, Dy3+, Er3+, Tm3+, Sm3+, and Ho3+ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce3+ ion is near the midgap, due to which Ce3+ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd2O2S:Pr and Gd2O2S:Tb phosphors.