Temperature dependence of phonon-frequencies and — Linewidths for weakly anharmonic molecular crystals

Abstract

Results of model calculations of the shift of phonon frequencies and of phonon widths with temperature and pressure are compared with experimental results for α-N2 , NH3 , benzene, naphthalene and anthracene. The molecular crystals are assumed to be perfect, neglecting the influence of electronic excitations, impurities and lattice defects. A short description of harmonic models, often used for the lattice dynamics of molecular crystals preceeds the treatment of the models of anharmonic crystal vibrations. Within the framework of the theory shifts and widths are in qualitative, often in quantitative agreement with experiment. At low temperatures the width of external and low energy internal phonons can be nearly always explained by a population relaxation effect.