Nature of one-dimensional excitons in polysilanes

One-dimensional (1D) exciton states have been studied by nonlinear optical spectroscopy on organosilicon polymer polysilanes (PS’s). From systematic variation of the linear and nonlinear optical spectra upon changing the backbone conformation, the following two characteristic exciton parameters have been evaluated; the ratio |${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$|/α of the modulus of third-order nonlinear optical susceptibility |${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$| to the absorption coefficient α; and the energy difference Δ${\mathit{E}}_{12}$ between the lowest and second lowest exciton energies ${\mathit{E}}_1$ and ${\mathit{E}}_2$, which are theoretically related, respectively, to the Bohr radius and to the binding energy of excitons. It was found that |${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$|/α increases, whereas Δ${\mathit{E}}_{12}$ remains almost unchanged, with increasing valence and conduction-band widths. These features cannot be accounted for either by the Wannier- or Frenkel-type exciton model. From analysis by a unified 1D exciton model, it has been concluded that the 1D excitons in PS’s represent a unique system having an intermediate character in between the Frenkel- and Wannier-exciton regimes, where the on-site Coulomb (or exchange) interactions play a crucial role in determining the characteristic exciton structures as observed. © 1996 The American Physical Society.