Exciton spectra of KCN andK(CN)xCl1−xcrystals. Exciton-vibron coupling in ordered and disordered systems

The reflectance spectra of KCN and $\mathrm{K}{\left(\mathrm{CN}\right)}_x{\mathrm{Cl}}_{1-x}$ crystals have been measured in the energy range from 5 to 11.5 eV at temperatures between 5 and 300 K. In KCN seven vibrational bands of the $A{}_{}{}^1{}_{}{}^{}\Pi \leftarrow X{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ transition of the C${\mathrm{N}}^{-}$ ion appear above 6.8 eV. Below 80 K a resolved splitting of the exciton bands is observed. The effects of exciton-phonon coupling and of the reorientational motion of the C${\mathrm{N}}^{-}$ ions on the reflectance bands are discussed. Other optical transitions occurring at higher energies are interpreted on the basis of a simple electronic energy-level scheme. In $\mathrm{K}{\left(\mathrm{CN}\right)}_x{\mathrm{Cl}}_{1-x}$ mixed crystals the Wannier excitons of chlorine and the vibronic excitons of C${\mathrm{N}}^{-}$ are observed throughout the whole composition range. The concentration dependence of the $A{}_{}{}^1{}_{}{}^{}\Pi$ exciton progression is discussed in terms of a point-dipole lattice model. The observed inhomogeneous line broadening of the $A{}_{}{}^1{}_{}{}^{}\Pi$ transition originating either from orientational disorder above 80 K or from nonequivalent molecular environments in mixed crystals is compared with the one derived from simulation calculations.