Far-infrared absorption of ZnS : Fe2+ in strong magnetic fields

We have investigated several optical transitions between the E-5 energy levels of the orbital ground state of substitutional Fe2+ in natural crystals of cubic ZnS for magnetic inductions up to 14 T (140 kG) at liquid-He temperatures. In these experiments we have used both gas lasers and a Fourier-transform Michelson spectrometer covering the range 10-100 cm(-1) wave numbers. The results show that simple crystal-field theory is adequate to account for the observed energy levels and their oscillator strengths as a function of the applied magnetic field. There is also evidence for local modes at 43 and 63 cm(-1) wave numbers caused by trace impurities other than Fe.