Ab initio embedded cluster study of the excitation spectrum and Stokes shifts of Bi3+-doped Y2O3.

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi(3+)-doped yttria crystal. Our results indicate that the Bi(3+) impurity can absorb light over a wider energy range in the C(2) site than in the S(6) site. Even if the computed absorption energies seem to be about 0.4 eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6 eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S(6) site than in the C(2) site. Combining all these results confirms the assignment of the violet emission to the S(6) site and that of the green emission to the C(2) site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.