Molekulare Schwingungsexzitonen in Fliissigkeiten

Abstract

The intermolecular coupling of molecular vibrations in molecular crystals is usually treated by the exciton concept. This is identical with the assumption that a potential, which describes the interaction of a vibrating molecule with its neighbours, perturbs the isolated molecule energies and thereby couples the motions of molecules. In most cases this potential will be the transition-dipoletransition- dipole interaction. If one assumes validity of the pair interaction approximation this concept should be able to describe the effects in the liquid state, too. The aim of these considerations is to show, how collective excitation can influence the shape of vibrational bands. Because dipoledipole interaction is strongly orientation dependent, molecular reorientations perturb the coupling of the vibrations and cause motional narrowing. The behaviour of the shapes of the vibration bands of CH3J is discussed on the basis of these considerations. One finds that the band shape of ν2 is mainly broadened by vibration coupling. As expected a distinct motional narrowing is observed, if one increases the temperature from the melting point to the boiling point.