Optical Absorption of Localized Excitons in Cadmium Halide Crystals

Optical absorption spectra of CdBr 2 :I, CdCl 2 :I and CdCl 2 :Br have been measured at temperatures from 10 to 500 K. In every system, well-defined absorption bands due to doped halogen ions were observed on the low energy sides of intrinsic absorption edges of host crystals. Shapes of these bands are Gaussian around the band maxima and follow Urbach's rule in the low energy tail regions. These absorption can be attributed to the creation of excitons localized at doped halogen ions. Effective energies of lattice vibrations related to the central Gaussian part and the exponential tail part are estimated from the measured temperature dependence of half-widths and of Urbach's tails, respectively. Temperature dependence of these of these bands will be discussed using the Franck-Condon model and modified Urbach's rule.