OFF-CENTER DISPLACEMENT OF GE2+ IONS IN KCL CRYSTALS (I) OPTICAL-ABSORPTION SPECTRA

Optical absorption spectra of KCl:Ge2+ crystals have been investigated for the first time in the temperature range 15-300 K, and compared with the spectra of the Sn2+ centers in alkali halide crystals. Six absorption bands are observed at 276, 255, 240, 231.1, 219.0 and 211.7 nm at 16 K. The first two, the middle and the last three bands are attributed to the spin-orbit-allowed, vibration-induced and dipole-allowed bands due to an s2-->sp transition in the Ge2+ ion, respectively. It is suggested from the asymmetric triplet structure of the dipole-allowed band that the Ge2+ ion is at the off-center position at low temperatures.