Localized phonons associated with solitons in polyacetylene: Coupling to the nonuniform mode

We report the observation of photoinduced infrared absorption due to the recently predicted localized phonons in trans-(CH${)}_{\mathrm{x}}$ and its deuterated analog (CD${)}_{\mathrm{x}}$. The infrared activity results from coupling of the three lattice degrees of freedom to a soliton shape-modulation mode involving nonuniform translation of the soliton. Our results agree with the energies and weak oscillator strengths predicted for these new modes and for the related absorptions which result from coupling to the uniform translation of the soliton. This agreement provides confirmation of the accuracy and applicability of the electron-phonon model in describing the nonlinear soliton excitation in polyacetylene.