We report on calculations of the third-order susceptibility of pure dimerized trans- polyacetylene, pristine ${\left(\mathrm{CH}\right)}_x$ with neutral solitons, and doped ${\left(\mathrm{CH}\right)}_x$. Line shapes for pure dimerized ${\left(\mathrm{CH}\right)}_x$ are in agreement with recent experiments. Pristine ${\left(\mathrm{CH}\right)}_x$ with neutral solitons and doped ${\left(\mathrm{CH}\right)}_x$ show additional structure below the lowest frequency for which experimental observations have been reported. Our results indicate that third-harmonic generation may be a useful technique for studying the properties of neutral and charged solitons.

• Received 14 November 1988

DOI:https://doi.org/10.1103/PhysRevB.39.3442