Calculation of Electronic Spectra ofTrans-Polyacetylene

Abstract

We present in this work theoretically calculated electronic spectraof charged trans-polyacetylene chains containing solitons. Thesecalculations are based on the random phase and rotating waveapproximation. Further we use correlation corrected energy levelscomputed in the framework of the Pariser–Parr–Pople (PPP)Hamiltonian. The geometries of the chains as functions of time havebeen taken from simulations within theSu–Schrieffer–Heeger (SSH) model. The spectra obtained formany time steps within a simulation are subsequently superimposed andaveraged. The Su–Schrieffer–Heeger model was chosen forthe simulation because it is often stated, that this one-particlemodel is based on renormalized parameters which essentially containalready the effects of electron–electron interactions. Furtherat least for charged solitons the theory gives quite correct solitonwidths as compared to Pariser–Parr–Pople calculations.Thus the present study is also aimed as a first step to investigatewhether the SSH model is really able to yield reliable geometries intime simulations. We found that our spectra reproduce theexperimentally known peaks of the solitons and band to bandexcitations quite well within an uncertainty of 0.3–0.5eV.Further, the ratio of intensities of the two peaks as obtained in ourcalculation agrees quite well with the corresponding experimentalones. The remaining small error in intensities should be due to ouruse of transition dipole moments calculated from HF orbitals. Chainlength might play a role in the appearance of several minima andmaxima of the absorption which are not present in the experimentalspectra. These different effects are currently under furtherstudy.