Optical Absorption and Luminescence of the I--Center in CdCl2

Abstract

The optical absorption and the luminescence of I - -ions in CdCl 2 single crystals were measured at liquid nitrogen temperature to study optical properties in layer-structure ionic crystals. It was found that addition of low concentration of CdI 2 , 10 -5 to 10 -3 mole fraction, produces two additional absorption bands at 5.03 eV and 5.44 eV on the low energy side of the absorption edge of CdCl 2 and that intensities of these bands increase linearly with increase of I - -ion concentration. The shape of the 5.03 eV band is Gaussian around the band maximum and follows Urbach's rule in the low energy tail region. These results suggest that additional absorption bands can be associated to the localized exciton at isolated I - -ion in CdCl 2 . Illumination in the 5.03 eV band gives rise to an emission band at 2.19 eV. The observed large Stokes' shift suggests that a large lattice relaxation follows the electronic excitation.