Dynamics of molecular impurities in crystals

Publisher Website
Google Scholar
Abstract
Group-theoretical methods are used to formulate the incoherent-neutron-scattering law for rotational molecular motion in solids in terms of symmetry-adapted functions. The relevant correlation functions are calculated in the framework of a microscopic model by using a self-consistent method for the calculation of frequency-dependent transport coefficients. As a concrete example, we consider a dumbbell molecule in an octahedral potential. The problem is treated in terms of classical physics. At temperatures low in comparison with the potential barriers, the resonances of the correlation functions correspond to librations and reorientations. These modes can be classified according to the irreducible representations of the symmetry-adapted functions and the sign of the potential. At high temperatures, above the barrier heights, the spectra characterizing both types of motion become similar due to the increasing importance of free rotations. Our results are in full qualitative agreement with Raman-scattering data by Callender and Pershan.