Photoluminescence and excitation spectroscopy of Ni2+ and Ni+ centres in ZnS crystals

Abstract Excitation spectra have been studied at T = 4.2 K for the luminescence of Ni 2+ and Ni + centres in ZnS crystals. The excitation spectrum of the Ni 2+ (3d 8 ) transitions 3 T 2 (F) → 3 T 1 (F) exhibits all the main bands known from the Ni 2+ crystal-field absorption spectra as well as the threshold for the process Ni 2+ + hv → Ni + + e + vb at v ≅ 20 000 cm -1 . In the excitation spectrum of the Ni + (3d 9 ) emission 2 E(D) → 2 T 2 (D) another threshold is observed for the reaction Ni + + h v → Ni 2+ + e - cb at v ≅ 11500 cm -1 . The thresholds coincide with the excitation and quenching limits of the Ni + EPR signals, respectively. Fine structure is resolved near 11500 cm -1 on the high-energy tail of the recombination transition Ni 2+ + e - cb → Ni + + hv . An interpretation in terms of an excitonic radiative recombination is proposed.