VIBRATIONAL RELAXATION AND DEPHASING IN ORGANIC SOLIDS.

A review of experimental studies conducted in the author's laboratory in the area of vibrational relaxation and dephasing in organic solids is presented. The investigation utilizes temperature dependence of linewidths, lineshapes and frequencies in the Raman spectra. Theoretical background is presented on various anharmonic interactions for lattice phonons and internal vibrations in relation to their relaxations and dephasing. The effects of experimental conditions and disorders on linewidth are discussed. Some selected results of experimental investigations are discussed. It is found that for a lattice phonon transition the dephasing occurs by T//1-relaxations due to cubic anharmonic coupling. For an internal vibration the process responsible for dephasing depends on the frequency and the delocalization of the vibration. For a localized and low frequency internal mode, the dephasing occurs by an elastic phonon-scattering (T//2** prime -process) which does not destroy the excitation. For a delocalized internal vibration or for a high frequency internal vibration T//1-relaxations (but of different types) contribute to dephasing. The dominant contribution to the temperature dependence of vibrational frequencies for both phonons and internal vibrations are derived primarily from the thermal expansion of the lattice.