Soliton-antisoliton configurations and the linear and nonlinear optical response of degenerate-ground-state conjugated polymers

The contributions of soliton-antisoliton (SS¯) configurations to the linear and third-order nonlinear optical response are investigated for conjugated polymers having a degenerate ground state. We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(1\right)\mathrm{}}$(ω). With the help of the oscillator strength sum rule, we decompose the π-${\mathrm{\pi }}^{\mathrm{*}}$ oscillator strength into two parts; a contribution where the final states are charged SS¯ pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid lattice). The linear optical coefficients calculated from ${\mathrm{\chi }}^{\mathrm{}\left(1\right)\mathrm{}}$(ω) are in general agreement with optical data obtained from trans-polyacetylene. The results imply that approxiamtely 25% of the integrated oscillator strength of trans-polyacetylene arises from the direct photoproduction of solitons.