Far-Infrared absorption in polyacetylene below the nonmetal-metal transition

Far infrared (20-700 cm−1) transmission measurements have been made on in situ iodine-doped polyacetylene (CH[I3]y)x at doping levels from zero to the metallic state, u<y<0.02. A broadband absorption, too weak to be the pinning mode of a soliton, was observed; it increased with doping level. The data yield estimates for the ac conductivity and the dielectric constant of the material. At low doping levels the ac conductivity far exceeds the dc conductivity, while at higher levels the two conductivities are the same. The dielectric constant diverges as the nonmetal-metal transition (yc ≃ 0.0215) approaches.