Origin of the 0.89 eV peak inχ(3)(−3ω;ω,ω,ω)of polyacetylene: Electron correlation effects

The third-order optical susceptibility ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ of polyacetylene is calculated by the Lanczos-based density-matrix renormalization group. The theoretical results now remarkably agree with two different sets of experimental data: both the positions of two major peaks and their relative intensities. The 0.89 eV peak, originally assigned to a two-photon resonance, is actually a three-photon resonance due to the ${\mathrm{nB}}_u$ state. This state lies above the band-edge state ${\mathrm{jB}}_u$ which itself accounts for a hidden hump on the right shoulder of the 0.6 eV peak. The electron correlation effect is the main origin of these rich features.