COULOMB INTERACTIONS AND LINEAR, NONLINEAR, AND TRIPLET ABSORPTION INPOLY(PARA-PHENYLENEVINYLENE)

Within a model Hamiltonian with variable on-site and long-range Coulom b interactions between the pi electrons for poly(para-phenylenevinylen e), we conduct a thorough search in the parameter space to determine t he magnitudes of the effective Coulomb interaction parameters necessar y to fit all four absorption bands that are seen in the experimental a bsorption spectra of this material. We find best agreement between the calculated and experimental absorption spectra with Coulomb interacti ons that are slightly smaller than the standard Pariser-Parr-Pople par ameters. For these values of the Coulomb parameters, the primary photo excitation in poly(para-phenylenevinylene) is to an exciton with bindi ng energy close to 0.9+/-0.2 eV. This result, obtained from fitting th e linear absorption, is in agreement with nonlinear absorption studies , viz. electroabsorption, two-photon absorption, and picosecond photoi nduced absorption, within our model. We have also calculated the energ ies of the lowest triplet state, and the final state to which triplet absorption occurs. The excited triplet state is an exciton. We show th at the latter result, taken together with the known experimental tripl et absorption energy, indicates that estimates of 0.2 eV or less for t he binding energy are incorrect. We briefly discuss the possibility th at the binding energy has an intermediate magnitude.