THEORY OF EVEN-PARITY STATES AND 2-PHOTON SPECTRA OF CONJUGATED POLYMERS

The two-photon absorption (TPA) spectrum of interacting pi-electrons in conjugated polymers is shown to be qualitatively different from any single-particle description, including the Hartree-Fock limit. Alternating transfer integrals t (1 +/- delta) along the backbone lead to a weak TPA below the one-photon gap E(g) for arbitrarily weak correlations at delta = 0, for intermediate correlations at delta = 0.07 in polyenes, and for strong correlations at any delta < 1. More intense TPA is derived from two-electron transfer across E(g); this even-parity state shifts from 2E(g) in single-particle theory to E(g) in the limit of strong correlations in Hubbard models and is around 1.5E(g) for Pariser-Parr-Pople (PPP) parameters. The PPP model, which accounts for one- and two-photon excitations of finite polyenes, is extended to even-parity states in polydiacetylenes (PDA's), polyacetylene (PA), and polysilanes (PS's). Previous experimental data for PDA and PS support both the strong TPA above E(g) and weak TPA slightly below E(g) for delta = 0.15 in PDA and above E(g) for delta approximately 0.3 in PS. The strong TPA expected around 1.5E(g) in isolated PA strands shifts to approximately E(g) due to interchain pi-electron dispersion forces. TPA intensities in correlated states are shown to reflect both ionicity and mean-square charge separation. The even-parity states of conjugated polymers, like those of polyenes, show qualitatively different features associated with electron-electron correlations.