Oliver S. Wenger, Sophie Bénard and Hans U. Güdel
Crystal Field Effects on the Optical Absorption and Luminescence Properties of Ni²⁺-Doped Chlorides and Bromides: Crossover in the Emitting Higher Excited State
Inorg. Chem. 41, 5968-5977 (2002)                 

Abstract: Single crystals of CsCdCl₃, CsCdBr₃, CsMgBr₃, and MgBr₂ doped with 0.1/5% Ni²⁺ were grown by the Bridgman technique and studied by variable-temperature optical absorption and luminescence spectroscopies. At cryogenic temperatures all these systems are dual emitters; i.e., they emit light from two distinct, thermally nonequilibrated excited states. The emitting higher excited state is ¹T_2g in Ni²⁺:CsCdCl₃ and Ni²⁺:CsCdBr₃ and ¹A_1g in Ni²⁺:CsMgBr₃ and Ni²⁺:MgBr₂. This crossover manifests itself in a change from red broad-band to yellow sharp-line luminescence, and it is rationalized on the basis of crystal field theory. Temperature-dependent luminescence as well as two-color pump and probe experiments reveal that in Ni²⁺:CsMgBr₃ and Ni²⁺:MgBr₂ the ¹T_2g state lies only about 70 and 170 cm^–1, respectively, above ¹A_1g. The effect of crystal field strength on thermally activated nonradiative multiphonon relaxation processes in the bromides is examined for both ¹A_1g/¹T_2g higher excited state and ³T_2g first excited-state emission. Two-color excited-state excitation experiments are used to monitor Ni²⁺ excited-state absorption transitions originating from ³T_2g.