EVOLUTION OF PHOTOEXCITATIONS IN POLYACETYLENE AND RELATED POLYMERS FROM FEMTOSECONDS TO MILLISECONDS

We have measured photoexcitation dynamics in trans (t) and cis (c) isomers of polyacetylene ((CH)_x) and in polydiethynylsilanes, in a broad time interval from femtoseconds to miliseconds and spectral range from 0.1 eV to 2.4 eV, using a variety of experimental techniques. These include picosecond to milisecond transient and cw photomodulation, femtosecond degenerate four-wave mixing, picosecond transient photoinduced resonant Raman scattering and the newly developed technique of absorption detected magnetic resonance. In the degenerate ground state polymer t-(CH)_x we followed the evolution of charged and neutral photogenerated solitons, their geminate recombination in the form of t^-1/2 which dramatically slows down into t^-β (β < 0.5) due to disorder and inhomogeneity in the films, and their diffusion along the chains. In partially isomerized (CH)_x we identified tightly bound soliton pairs in short trans segments and a fast excitons intersystem crossing from cis chains to the more energetically favorable trans chains. In the almost degenerate ground state polydiethynylsilane we observed a subpicosecond geminate recombination with a very small long tail. We identified charged and neutral photogenerated solitons in long chains and bipolarons in short chains where the degeneracy can be easily lifted.