Anisotropy of the optical constants of pure and metallic polyacetylene

The polarized optical reflectivity at nearly normal incidence has been measured on pure highly oriented trans-polyacetylene and cis-polyacetylene of nonfibrous morphology. The anisotropic optical constants and the dielectric function for polyacetylene (cis and trans) are derived from the polarized reflectivity measurements via a modified Kramers-Kronig calculation. The low scattering of our samples allowed a band-edge analysis leading to a value of 1.5 eV for the one-dimensional band gap (with three-dimensional effects neglected) of trans-polyacetylene. The maximum of the absorption coefficient in chain direction according to the {π}-π* transition is α?=7.4×{}10⁵ cm^-1 and located at 1.9 eV, with an anisotropy of α?/{α}=26. trans-polyacetylene doped with AsF₅ to a room-temperature conductivity in chain direction of σ?=1200 Ω^-1 cm^-1 behaves like a metal (closed gap) in the parallel reflectivity spectrum and from the Kramers-Kronig analysis we derived the position of the plasma edge (ε₁=ε₂) at EP=3.51 eV, whereas the maximum of the loss function is located at 3.25 eV. Thus the strong anisotropic behavior is preserved in the highly doped system.