Anharmonic oscillator representation of nonlinear optical susceptibilities of a charged soliton, a neutral soliton, and a polaron in conjugated polymers

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Abstract
The signatures of a chargedsoliton, a neutral soliton, and a polaron in the resonant and off‐resonant optical susceptibilities of conjugated polymers are calculated using the Pariser–Parr–Pople (PPP) model which includes Coulomb interactions. The optical response is obtained by solving equations of motion for the reduced single‐electron density matrix, derived using the time dependent Hartree–Fock (TDHF) approximation. The density matrix clearly shows the electronic structures induced by the external field. The roles of charge density, spin density, bond order, and spin bond order waves and how they contribute to the optical response are analyzed. Both charged and neutral solitons show one absorption peak inside the gap of the half‐filled case; the frequency of the neutral soliton peak is about 0.7 eV higher than that of a chargedsoliton. A polaron shows two absorption peaks inside the gap.