Excitation spectrum for ultrafast photogeneration of charged solitons in polyacetylene

Ultrafast photoinduced absorption by infrared-active vibrational modes is used to detect charged solitons in oriented trans-polyacetylene. Soliton pairs are photogenerated within \ensuremath{\sim}250 fs with quantum efficiencies $({\ensuremath{\varphi}}_{\mathrm{ch}})$ approaching unity. The excitation spectrum of ${\ensuremath{\varphi}}_{\mathrm{ch}}$ shows an onset at \ensuremath{\sim}1.0 eV with a weak photon energy dependence up to 4.7 eV. The results are consistent with the ultrafast soliton formation predicted by Su and Schrieffer and with the Su-Scrieffer-Heeger threshold of ${2E}_{g}/\ensuremath{\pi}$ for soliton pair production. The recombination dynamics of charged solitons is very fast (initial $\mathrm{decay}<1\mathrm{ps})$ with a modest dependence on the pump photon energy.

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