Multiphonon absorption in molecular crystals

Abstract The classification of molecular crystal multiphonon absorption is suggested taking into account quantitative difference of the energy spectrum of internal and external phonon modes. The experimental observations are given of phonon side bands following the bands of high frequency internal modes of the naphthalene crystal. The absorption observed is considered as a two‐phonon one caused by internal and external mode interaction. The dependence of the phonon side band shapes on the different geometry of the atom displacement for different internal modes is discussed.