Dispersion of resonant Raman scattering in pi-conjugated polymers: Role of the even parity excitons

Resonant Raman scattering dispersion of the most strongly coupled phonon frequencies with the excitation laser photon energy is measured in terms of a dispersion rate parameter D and quantified in a variety of pi conjugated polymer films. D was found to be large in nonluminescent polymers and small in luminescent polymers. We show that D is determined by the dependence of the even parity excitons (A(g)) on the polymer conjugation length, and this may serve as a useful spectroscopy for the 2A(g) exciton in nonluminescent polymers where it is otherwise optically inactive.