Far-Infrared Optical Absorption ofFe2+in ZnS

The optical absorption of substitutional ${\mathrm{Fe}}^{2+}$ impurities in natural single crystals of cubic ZnS has been measured in the far infrared (10 to 100 ${\mathrm{cm}}^{-1\mathrm{}}$) at temperatures from 4 to 25°K. Several absorption peaks are identified with electric- and magnetic-dipole transitions between the five spin-orbit levels of the ${}_{}{}^5{}_{}{}^{}E$ ground term of tetrahedral ${\mathrm{Fe}}^{2+}$. The positions and absolute intensities of these peaks agree reasonably well with crystal field theory and with values obtained for the various parameters from previous measurements of the optical absorption in the near infrared. The separation $K$ of the spin-orbit levels of the ${}_{}{}^5{}_{}{}^{}E$ term is found to be 15.2±0.4 ${\mathrm{cm}}^{-1\mathrm{}}$. Oscillator strengths for the transitions are in the range 5×${10}^{-9\mathrm{}}$ to 5×${10}^{-8\mathrm{}}$. Lifetimes for spontaneous radiative decay of the excited levels are calculated to be of the order of ½ to 30 h, and actual lifetimes are therefore determined by nonradiative processes. These observations support the conclusion that no pronounced Jahn-Teller effect occurs in the ${}_{}{}^5{}_{}{}^{}E$ state of ${\mathrm{Fe}}^{2+}$ in ZnS.