Abstract
Optical absorption spectra of $Co^{2+}$ ions in $CdGa_{2}S_{4}:Co^{2+}$ single crystals were investigated in the region of 2500 to 25000 {AA}. The absorption band originating from the charge-transfer transition was observed beyond the absorption edge. We also observed two groups of several absorption bands in the visible and infrared regions corresponding to the $^{4}A_{2}(^{4}F){\rightarrow}^{4}T_{1}(^{4}P)$ and the $^{4}A_{2}(^{4}F){\rightarrow}^{4}T_{1}(^{4}F)$ transitions. The fine structures of these absorption bands are assigned to electronic transitions between the split energy levels of the $Co^{2+}$ with $S_4$ symmetry in the $CdGa_{2}S_{4}$ host lattice. The spin-orbit splittings of the energy levels of $Co^{2+}$ ions in $CdGa_{2}S_{4}$ are discussed and compared to those previously reported for $CdGa_{2}Se_{4}$.