Optical Absorption Spectra of Chromium-Doped Zinc Sulfide Crystals

We have measured the optical absorption spectra of twinned cubic crystals of chromiumdoped zinc sulfide between 5000 and 29 000 ${\mathrm{cm}}^{-1\mathrm{}}$ (2-0.34 μm) at 4.2-500°K. Two broad absorption bands are specific to chromium; at 77°K, one has a maximum at 24 400 ${\mathrm{cm}}^{-1\mathrm{}}$, the other, at 5700 ${\mathrm{cm}}^{-1\mathrm{}}$. Weak photoconductivity is observed in the former; but none in the latter. Twelve narrow bands between 5100 and 5250 ${\mathrm{cm}}^{-1\mathrm{}}$ are identified as zero-phonon transitions in ${\mathrm{Cr}}^{2+}$ (3${\mathrm{d}}^4$) substitutional at cation sites, two for the cubic structure, ten for the twin boundaries. The cubic crystal field is determined. Thirteen phonon-assisted transitions are identified and the phonon wave numbers are evaluated. We propose that the 24 400-${\mathrm{cm}}^{-1\mathrm{}}$ band involves transitions to the conduction band edge.