Two-Phonon Spectra of Molecular Crystals

Abstract

The object of this chapter is to discuss processes where the absorption ( or scattering ) of a single photon results in the excitation of two vibrational quanta on different molecules. These processes differ from combination and overtone transitions of isolated molecules in several respects and mainly because they are affected by the intermolecular interactions. The very occurrence of double ( or multiple ) transitions of the type we want to discuss is a direct consequence of the existence of interaction forces between the molecules. As such double excitations are not specific of the solid state and, in fact, they have been observed in the infrared absorption spectra of compressed gases and of liquid mixtures¹⁴⁷. However, in ordered solids, because of the translational symmetry, vibrational excitations can be precisely characterized by their frequency ω and wave vector k and the selection rules for multiple excitations, that we call “multiphonon processes”, are of particular significance. For these reasons, the structure of multiphonon bands in crystals can be studied in more detail and, in turn, the information that can be obtained on intermolecular forces and phonon densities of states are particularly valuable.