Dynamical properties of no impurity molecules in alkali halides. I. Theory

The effect of an impurity molecule on the lattice vibrations is studied. The translational and librational motion of the molecule and lattice vibrations are considered from a unified point of view. Green's functions describing all types of motions as well as their mixing are calculated in the harmonic approximation. Optical response functions (far infrared absorption coefficient and Raman scattering tensor) are found. It is shown that, unlike the case of point impurities, the spectral functions can display interference features, such as asymmetric peaks or antiresonances.