Electroabsorption in poly(paraphenylene vinylene) and PtI: Exciton vs band descriptions

We contrast the electroabsorption (EA) spectra calculated within an exciton model (Pariser-Parr-Pople Hamiltonian) with that in a band description. The former model combined with small lattice relaxation of excited states is found to give a very good EA description for poly(paraphenylene vinylene) (PPV). A band picture gives qualitatively different EA behavior. As an illustration, we consider a two-band charge-density-wave model, for which we find three kinds of linewidth broadening behavior: (1) For weak fields, the broadening decreases with electric field; (2) for intermediate field strength, the broadening is dominated by damping effects and is independent of the field; (3) for strong fields the traditional Franz-Keldysh law holds. The EA spectrum calculated using the two-band model is in very good agreement with experiments on PtI, a halogen-bridged mixed-valence metal linear-chain complex.