Femtosecond dynamics of charged solitons in trans-polyacetylene

Abstract

Recently, directly photogenerated charged solitons as predicted by Su and Schrieffer have been observed.1 Here we report studies of their formation and decay dynamics using transient absorption with 0.35-ps pulses. Above band gap (2.1 eV) radiation creates intrachain electron–hole pairs which degenerate into charged soliton pairs. These are probed using synchronized pulses tunable from 2.5 to 5.5 μm which are generated by mixing amplified femtosecond pulses with white light continuum generated in water.2 The charged soliton pairs are shown to recombine geminately in ~0.5 ps with power-law decay dynamics characteristic of 1-D diffusion. At high pump fluences, saturation of the charged soliton absorption can be attributed to a volume filling mechanism. Varying the probe wavelength allows us to observe femtosecond spectral relaxation of the charged solitons and to interpret this in terms of conformational relaxation. Work on oriented Durham trans-polyacetylene has enabled us to probe the dynamics following interchain electron-hole pair creation as well.