Vibrational, torsional, and librational excitons in molecular crystals: Raman spectra of neat and isotopic mixed durene

A theoretical comparison of the conventional phonon and exciton approaches is made and the validity of the concept of a librational exciton is discussed. An experimental investigation of the Raman active librations, torsions, and other vibrations, in the neat and isotopic mixed crystals of durene at 100 and 25°K, is presented. The results obtained are explained in a uniform and consistent way by using the exciton concept (librational exciton, torsional exciton, and vibrational exciton). The librational excitons obey the amalgamation limit while the torsional and vibrational excitons fit the separated band limit of the mixed crystal. Davydov splitting has been observed on one torsional, one vibrational but not on any librational excitons (phonons). Intermolecular contributions to the methyl torsions are discussed.