Intensities of two-photon absorptions to low-lying even-parity states in linear-chain conjugated polymers

Interest in the lowest even-parity two-photon states in linear-chain π-conjugated polymers has been largely limited to their energies relative to the lowest odd-parity one-photon state. We show that the intensities of the two-photon absorptions to even-parity states that occur below the lowest one-photon states should be tiny in the infinite chain, because of cancellations. This has important implications for the interpretation of third-harmonic generation spectra of polydiacetylenes. A resonance seen in third-harmonic generation of polydiacetylenes that has commonly been assigned as a two-photon resonance is interpreted here as a three-photon resonance.