Localized Excitons and Band Excitons in Alkali Chloride-Alkali Iodide Solid Solutions

The optical absorption spectra of KCl-KI and NaCl-NaI solid solutions in the whole concentration range are presented. Measurements are made in the range from 5.5 eV to 7.5 eV at 80°K. The changes in the profile, the positions of the resolved structures and their intensities in the absorption spectra in changing the concentrations of iodide are observed. With increases of the concentration of I - ions, the localized exciton bands due to cluster of I - ions appear on the low energy tail of those due to an isolated I - ion. Above certain concentration of I - ions, the exciton can be regarded as the band exciton rather than localized one. The percolation theory seems useful for explaining this. Taking account of the band structures in KI and NaI, it is proposed that the absorption bands at 6.69 eV and 7.49 eV in KCl : I and at 6.90 eV and 7.60 eV in NaCl : I are assigned as the spin-orbit pair of the localized exciton band.