DYNAMICS OF PHONON INTERACTIONS IN ORGANIC SOLIDS

Abstract
Investigations carried out on three aspects of phonon interactions are presented in this paper. The phonon dephasing induced by anharmonic phonon-phonon scattering is investigateci by temperature dependence of linewidths, lineshapes and frequencies in the Raman spectra. The dephasing of phonons in organic solids is found to be primarily caused by a T₁-relaxation involving a three-phonon inelastic scattering. Second, a similar investigation on dephasing of vibrations within the molecule suggests that for these intramolecular vibrations with frequencies greater than twice the Debye cut-off frequency, it is the fourth-order anharmonic coupling with phonons leading to an elastic scattering which provide the dominant mechanism for dephasing in organic solids. Finally, the role of phonon interactions in determining solid state reactivity is discussed and a new concept of phonon assisted reaction is introduced. A thermal intermolecular reaction is found to be assisted by a phonon mode softening. A photochemical dimerization reaction is shown to be assisted by a strong-electron phonon coupling in the excited state whereby a polaron is formed.