ONE-DIMENSIONAL EXCITON SYSTEM IN TRANSPOLYACETYLENE

Recently, we clarified that a sharp absorption/reflection peak at 1.5 eV of directly synthesized trans-polyacerylene films comes not from sq uare-root singularity of joint density of states between one-dimension al bands, but from one-dimensional 1B(u) exciton state. In this letter , we found a peak at about 0.96 eV in second-harmonic-generation spect ra, and attributed this peak to resonance enhancement at the energy co rresponding to the fundamental wavelength. The resonance state is not electric-dipole-forbidden but an electric-quadrupole- or magnetic-dipo le-allowed state, most probably a 2A(g) exciton state.