ELECTROABSORPTION IN POLY(PARA-PHENYLENE VINYLENE) AND PTI - EXCITON VS BAND DESCRIPTIONS

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