Interchain coupling and localized vibration modes in conjugated polymers of finite length

The effects of interchain coupling on vibration modes localized around a polaron in polyacetylene of finite length are investigated. It is shown that the eight localized modes found in an isolated finite chain are delocalized between chains due to the interchain coupling. New modes of lattice vibration emerge because of the delocalization of the charged polaron. The well-known Goldstone zero-frequency mode becomes pinned by the chain ends for sufficiently short chains. Our results can qualitatively account for the frequency shifts observed recently in Raman scattering and infrared absorption spectra.