Infrared intensity and local vibrations of charged solitons

Abstract: We present a simple microscopic model for infrared active vibrations (IRAV's) of charged solitons in trans-polyacetylene (PA), by combining the Su-Schrieffer-Heeger model for pi electrons and a force field based on pristine PA. The model rationalizes the one-to-one correspondence between Raman modes of pristine PA samples and IRAV's of doped or photoexcited samples and provides a microscopic basis for the phenomenological amplitude mode formalism. The softening of the IRAV modes relative to Raman frequencies and their huge intensities are clearly due to their coupling to pi electrons, and specifically to the large pi-electron fluctuations induced by small nuclear displacements along a single local coordinate. The model accounts for effective mass and infrared spectra of photoexcited samples, and qualitatively, for dopant-induced spectra. Our microscopic picture also clarifies two long-standing puzzles. A soliton defect extending over similar to 14 sites generates many local modes, but only special ones are coupled strongly to pi electrons. At the same time, local vibrations are sensitive to chain lengths much longer than the defect due to the long electronic coherence length.