Relationships between the Raman Excitation Photon Energies and Its Wavenumbers in Doped trans-Polyacetylene

Abstract

The resonance Raman spectra of trans-polyacetylene films doped heavily with electron donor (Na) and acceptor (HClO4) have been measured with excitation wavelengths between 488- and 1320-nm, and the relationships between the Raman excitation photon energies (2.54-0.94 eV) and its wavenumbers were discussed. We found the linear dependence of the Raman shifts with the exchanges of excitation photon energies. In particular, the Raman wavenumbers in the C=C stretching <TEX>$(V_1$</TEX> band) showed a dramatic decrease with the increase in Raman excitation photon energies. In the case of acceptor doping, its change is larger than that of donor doping. The observed wavenumber (1255-1267 <TEX>$cm^{-1}$</TEX>) of the <TEX>$V_2$</TEX> band (CC stretch) of Na-doped form is lower than that of the corresponding band (1290-1292 <TEX>$cm^{-1}$</TEX>) of its pristine trans-polyacetylene, whereas the contrary is the case for the HClO4 doped form (1295-1300 <TEX>$cm^{-1}$</TEX>). The origin of doping-induced Raman bands is discussed in terms of negative and positive polarons.