One-dimensional exciton system in trans-polyacetylene

Recently, we clarified that a sharp absorption/reflection peak at 1.5 eV of directly synthesized trans-polyacetylene films comes not from square-root singularity of joint density of states between one-dimensional bands, but from one-dimensional 1Bu exciion state. In this letter, we found a peak at about 0.96 eV in second-harmonic-generation spectra, and attributed this peak to resonance enhancement at the energy corresponding to the fundamental wavelength. The resonance state is not electric-dipole-forbidden but an electric-quadrupole- or magnetic-dipole-allowed state, most probably a 2Ag exciton state.