Rotational dynamics of molecular impurities in alkali halide crystals

We have carefully investigated the orientational dynamics of a single substitutional diatomic molecular impurity in alkali halide crystals. The orientational relaxation functions of $E_g$ and $T_{2g}$ symmetries are calculated by using Mori’s projection-operator technique with truncation first applied to the rigid-lattice case by de Raedt and Michel [Phys. Rev. B 19, 767 (1979)]. For the ${\mathrm{CN}}^{\mathrm{-}}$ impurity in a rigid KBr lattice we find that the librational and central-peak behaviors of the spectral weight function for the two symmetries are strongly influenced by the hexadecapole moment of the molecular ion. We find that the rotation-translation coupling modifies the single-site cubic potential by self-energy corrections, suppresses librational motion, enhances diffusive motion, and narrows the width of the central peak.