Optical Properties of U Centers in Alkali Halides and Alkaline Earth Fluorides

The two-electron orbitals for the U center have been computed numerically from the Hartree-Fock-Slater (HFS) equations in the point-ion lattice potential. The lattice relaxation of the nearest-neighbor ions is included in the model. The five lowest-lying U-center states for NaCl, KCl, CdF₂, CaF₂, SrF₂, and BaF₂ are given. The low-lying singlet states have the following order for increasing values of the energy: ¹S(1s, 1s), ¹P(1s, 2p), and ¹S(1s, 2p). The energy levels for the triplet states ³S(ls, 2s), and ³P(1s, 2p) lie between the energy levels for the ¹S(1s, 1s) and ¹P(1s, 2p) states. The ordering of the triplet states depends upon the host crystal and the lattice relaxation. The predictions based upon the numerical HFS wave functions are compared with the predictions based upon past variational wave functions and with experiment.