INFRARED ACTIVITY OF PHONONS AROUND A SOLITON IN TRANS-POLYACETYLENE.

The dynamical conductivity due to phonons around a charged soliton in trans-polyacetylene is calculated, using three kinds of models: the Takayama-Lin-Liu-Maki (TLM) model, the Su-Schrieffer-Heeger (SSH) model, and the multi-phonon-band model. In the TLM model, two of the three localized modes, i. e. the Goldstone mode and the third localized mode, are found to be infrared (ir) active. These modes are found also in the SSH model to contribute to the optical absorption, while the staggered localized mode yields no contribution to the conductivity. In the last model, three such ir modes, which have their origins in the third localized mode, are found in addition to the already known three intense ones. The vibrational frequencies and relative optical intensities agree well with a recent photo-induced absorption experiment. The isotope effect on the absorption spectrum is also discussed.