2AgExciton State below 1BuExciton State in Trans-Polyacetylene Observed by Second-Harmonic-Generation Spectroscopy

Abstract
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 2Agexciton state at about 0.96 eV, which is located below the 1Buexciton 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.