Far-Infrared Absorption of ZnS:Fe2+in Strong Magnetic Fields

We have investigated several optical transitions between the $^{5}E$ energy levels of the orbital ground state of substitutional ${\mathrm{Fe}}^{2+}$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$ wave numbers caused by trace impurities other than Fe.