Spectra of charged solitons and temperature dependence of the mobility of neutral solitons in trans-polyacetylene

On the basis of the dynamics of charged solitons within the Su-Schrieffer-Heeger SSH model geometries of the chain as functions of time are obtained. The SSH model was chosen for the simulation because this one-particle model is based on renormalized parameters that essentially already contain the effects of electron-electron interactions. Furthermore, at least for charged solitons, the theory gives quite correct soliton widths as compared to Pariser-Parr-Pople calculations. Thus the present study is also aimed as a first step to investigate whether the SSH model is really able to yield reliable geometries in time simulations. With the help of the Pariser-Parr-Pople model, Moller-Plesset perturbation theory of second order and the random phase approximation, the spectra as a function of time are calculated at two different doping levels and for an excited chain. Then these spectra calculated at different times are superimposed. All features in the spectra are consistently roughly 0.5 eV too high in energy than expected. Further, the spectra show much more local minima and maxima than the experimental ones. It is suggested that this could be due to a chain length distribution present in the real material. The results show clearly that photogenerated charged solitons appear lower in energy than doping-generated ones and that this effect is not necessarily due to the presence of counterions in the doped material as suggested previously. Further, with the help of explicitly calculated lattice and electron dynamics and again within the Su-Schrieffer-Heeger Hamiltonian, it can be shown that neutral solitons in trans-polyacetylene start to become slightly mobile from 10 K and completely free above 100 K. This behavior agrees with experimental findings found in the literature. The derivation of equations of motion is given. Further it is shown, that without explicit consideration of electron dynamics the experimental results cannot be reproduced. (C) 2000 John Wiley & Sons, Inc.