Fluorescence of Trivalent-Europium-Doped Yttrium Oxysulfide

Abstract

The fluorescence spectrum of trivalent europium in the site of C3υ symmetry of polycrystalline Y2O2S:Eu3+ is investigated in the region 4000–9000 Å. Of the total of 363 transitions between the crystal components of the J = 0 to J = 3 states of D5 and those of the J = 0 to J = 6 states of 7F, almost two thirds are observed. Transitions originating in the D35 state of Eu3+ are particularly intense at 77°K. Some lines originating in unidentified levels, higher than D35 in energy, are also present. Thirty-nine of the 44 crystal components of D0–35, 7F0–6 are identified. Y2O2S:Eu3+ is perhaps an optimal case of identification of degeneracies of the crystal components without benefit of the additional information afforded by polarization studies or by the Zeeman effect. From a consideration of all possible assignments of symmetry species for the crystal levels of F1–47 in a systematic fashion, a small number of self-consistent sets of assignments and crystal-field parameters is obtained. Intensity data reduce the number of possibilities to four. For the most likely one of these, the deviation between observed and calculated crystal levels is 9 cm−1. The crystal-field parameters are B02 = 124, B04 = 1344, B34 = 823, B06 = 286, B36 = − 362, and B66 = 554 cm−1. The effect of J mixing is considerable. A fit to the barycenters of F0–47 and D0–25 yields values of the spin–orbit and electrostatic parameters η = 1326 cm−1 and F2 = 395.8 cm−1. A mechanism involving lattice vibrations is proposed which may explain the dramatic increase in intensities of transitions originating in D35 when the temperature is lowered from 300° to 77°K.