Polyacetylene oligomers:: π-electron fluctuations, vibrational intensities, and soliton confinement

We present a simple, internally consistent model for calculating ground-state properties of polyacetylene (PA) oligomers. The model describes pi electrons according to Huckel theory and accounts for electron-phonon coupling with an exponential dependence of hopping integrals on the bond length. Careful tuning of model parameters against the geometry and Raman frequencies of pristine PA allows the calculation of vibrational frequencies and nonresonant Raman intensities of oligomers, as well as infrared active vibrational spectra of solitons on short chains. The available experimental data are well reproduced by the proposed model. Both infrared and nonresonant Raman intensities are found to scale superlinearly with the chain length. By relating this behavior to the scaling of the optical gap, we gain insight on the coherence length of pi-electron motion in PA. [S0163-1829(99)04635-4].