We measured second-harmonic-generation spectra of centrosymmetric trans-polyacetylene films. A resonance peak was observed and assigned to the electric-quadrupole transition to the 2A g exciton state at about 0.96 eV, which is located below the 1B u exciton state at about 1.5 eV. We have also discussed the relative magnitude of the transition dipole between the excited states by comparing the various nonlinear optical spectra.
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Nishioka et al. (1997) studied this question.
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