Raman dispersion in polyaniline base forms

Raman spectra of polyaniline in its emeraldine (EB) and pernigraniline (PB) base forms were obtained using several excitation laser lines and Raman dispersion of the bands was measured. Only the νCN band value depends on excitation wavelength for both polymeric forms. These changes of vibrational frequencies with excitation wavelength were analyzed using amplitude mode model (AMM). ∏n(ωnR/ωn0)2 versus excitation photon energies plot was linear in the electronic absorption transition region, for EB at ca. 640 nm and for PB at ca. 620 nm. The dispersion values, D   (slope of ∏n(ωnR/ωn0)2 versus excitation photon energies plot) were 0.058 and 0.038 eV−1 for EB and PB, respectively, which indicate that EB has the first excited state of even parity presenting lower energy when comparing to that of PB.