It was recalled that the 3T1(F)-3T1(P) transition of isolated Ni2+ produced structured near-infrared absorption and emission bands. Crystals which were doped at the sub-ppm level exhibited exceptionally sharp lines which were suitable for the study of the dependence upon external fields. Here, the fine structure of the perfectly cubic and of one axial Ni2+ center was studied. A definite attribution of the observed optical transitions was obtained on the basis of high-resolution absorption and emission investigations at various temperatures and in magnetic fields of up to 15T. The experimental results furnished evidence of intermediate dynamic Jahn-Teller effects in both the 3T1(F) ground state and the 3T1(P) excited state. These were attributed to a coupling to local T2-type modes. A detailed comparison showed that there was good agreement between experimental data for Ni-doped ZnS and for the better understood case of Ni-doped CdS. A Fano-type structure in the 3T1(F)-3T1(P) absorption was explained by the coupling of a distinct local mode of the Ni center to a continuum of phonon states. In addition, the interaction of low-frequency acoustic phonons with the Ni center was investigated. No thermal equilibrium was established in the excited 3T1(P) state during its lifetime, thus giving rise to hot emission lines.

R.Heitz, A.Hoffmann, I.Broser: Physical Review B, 1994, 50[23], 17028-36