Excitation spectrum for ultrafast photogeneration of charged solitons in polyacetylene

Abstract

Ultrafast photoinduced absorption by infrared-active vibrational modes is used to detect charged solitons in oriented trans-polyacetylene. Soliton pairs are photogenerated within ;250 fs with quantum efficiencies (fch) approaching unity. The excitation spectrum of fch shows an onset at ;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 2Eg /p for soliton pair production. The recombination dynamics of charged solitons is very fast ~initial decay,1 ps) with a modest dependence on the pump photon energy.