POLARIZATION DICHROISM IN THE OPTICAL SPECTRA OF JAHN-TELLER EXCITONS.

A theory of moments of the optical spectra of degenerate Frenkel excitons is developed for an arbitrary exciton-phonon interaction, including the interaction with the Jahn-Teller vibrations. Expressions are obtained for changes in the moments of the spectra of the polarization dichroism induced by external (deformation, electric, and magnetic) fields. The theory is applied to the cation excitons in alkali halide crystals. A model microcalculation of the temperature dependences of the second and third moments is carried out for the A exciton band of CsCl due to the **1A//1//g(t**6//1//u) yields **1T//1//u (t**5//1//ua//1//g) transition in the Cs** plus ion. The interaction with the Jahn-Teller vibrations make the dominant contribution to these moments and this is responsible for the specific structure of the A band. A comparison is made of the theory and experiment and the results are used to determine the parameters of the exciton band of CsCl.