Mechanism for photogeneration of metastable charged solitons in polyacetylene

We present a detailed investigation of the optical excitations, both electronic and vibrational, from photogenerated charged solitons in cis-rich and all-trans-polyacetylene prepared by the Shirakawa method. We find that the saturation concentration of photogenerated solitons on the trans sequences in cis-rich material (3040 % trans) is substantially higher than in all-trans material, and that the lifetimes in this cis-rich material are considerably longer than in all-trans, in excess of 10 msec at low temperatures. For both the cis-rich and the all-trans materials, we find that photoinduced bleaching occurs only above the band edge, as expected if the photogenerated soliton states are formed from band states. These experimental results provide strong support for the stabilization of the photogenerated charges as like-charged pairs of solitons on the same chain rather than as charged solitons formed by trapping of free carriers (polarons) on neutral soliton defects.