Raman studies on the self-localized excitations in lightly and heavily doped trans-polyacetylene with sodium

The difference of Raman spectra between lightly and heavily Na-doped trans-polyacetylene (trans-PA) was observed with the 1320 nm laser line, and these were discussed on the basis of the Raman spectra of the negatively charged anions Ph(CH)_nPh^- [DP_n^-, n = 3, 5, 7, 9, and 13] and the radical-anion species Ph(CH)_nPh^.- [DP_n^.-, n = 4, 6, 8, and 10] and 19,19′,20,20′-tetranor-β,β′-carotene [TNBC^.-, n = 22] of the model compounds, which correspond respectively to negatively charged soliton and negative polarons in trans-PA. The observed wavenumbers of the v₁ and v₄ bands (C=C and C-C stretches) of lightly doped trans-PA are lower than those of the corresponding bands of pristine trans-PA, whereas the contrary is the case for heavily doped trans-PA. Raman spectra of the negatively charged soliton models (DP_n^-) and the negative polaron models (DP_n^.- and TNBC^.-) were very similar to their spectrum patterns as a whole, but their wavenumbers and relative intensifies are mutually different to some extent. In particular, the Raman wavenumbers of heavily doped trans-PA are very similar with those of the polaron models. The dispersion of the C=C stretching band (ν₁) in doped PA with the various wavelengths of excitation laser line were explained as the presence of charged domains having various lengths of electron localization.