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 (φch) approaching unity. The excitation spectrum of φch 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/π 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.