Optical constants and excitons of alkali silver halide crystals

The anisotropic reflection spectra of K₂CuCl₃ and K₂CuBr₃ have been measured in the range from 4 to 26 eV using synchrotron radiation. The optical constants have been obtained through the Kramers-Kronig analysis and compared with the calculated interband transition probability. The first fundamental absorption bands at about 4.5 eV are interpreted as being due to the excitons of the Frenkel type tightly bound on a cuprous ion. The absorption bands in the range from 6 to 12 eV are probably caused by the virtual excitons with holes in the p-bands of halogen ions. The anisotropic absorption bands due to the core excitons of potassium ions have been found around 20 eV. There is no clear structure due to plasmons of the valence electrons in the energy loss spectra.