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 $\ensuremath{\pi}$ 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 $\ensuremath{\pi}$ electrons, and specifically to the large $\ensuremath{\pi}$-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 $\ensuremath{\sim}14$ sites generates many local modes, but only special ones are coupled strongly to $\ensuremath{\pi}$ electrons. At the same time, local vibrations are sensitive to chain lengths much longer than the defect due to the long electronic coherence length.