Low-frequency Raman spectra of disordered molecular systems

Low-frequency Raman spectra of liquids and amorphous solids of simple organic compounds were measured to study the molecular motion in disordered systems. Weak and broad bands were observed around 100 cm(-1) in the spectra of amorphous solids made of aromatic molecules. These bands are not the so-called Boson peaks related to acoustic phonons in the crystal, but the bands arising from the optical phonons consisting of librational motions of molecules. For the liquids, similar broad bands due to the same origin were observed. In addition, a very-low-frequency component of scattering attributed to the relaxation motions of molecules was revealed for several liquids. The possibility of observing the related scattering for amorphous molecular solids is discussed.