Crystal field effects on the optical absorption and luminescence properties of Ni2+-doped chlorides and bromides: crossover in the emitting higher excited state

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 1T2g in Ni2+:CsCdCl3 and Ni2+:CsCdBr3 and 1A1g 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 1T2g state lies only about 70 and 170 cm-1, respectively, above 1A1g. The effect of crystal field strength on thermally activated nonradiative multiphonon relaxation processes in the bromides is examined for both 1A1g/1T2g higher excited state and 3T2g first excited-state emission. Two-color excited-state excitation experiments are used to monitor Ni2+ excited-state absorption transitions originating from 3T2g.