Single crystal with a gradient in the lattice spacing

The triply-degenerate local mode of theU-center is coupled to non-degenerate, doubly-degenerate, and triply-degenerate lattice vibrations. Because of the coupling to degenerate lattice vibrations an optical Jahn-Teller-Effect is to be expected which influences the structural form of the side-band. To investigate this influence the absorption band belonging to the triply-degenerate lattice vibrations has been calculated. A response formalism employing double-time thermodynamic Green's functions has been used. The numerical calculation is done for KBr. Our result exhibits a considerable deviation of the structural form from the one in which the dynamical mixing due to the Jahn-Teller-effect has been neglected.