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-${\pi }^{\mathrm{*}}$ transition is ${\alpha }_{?}$=7.4×${10}^5$ ${\mathrm{cm}}^{\mathrm{-}1}$ and located at 1.9 eV, with an anisotropy of ${\alpha }_{?}$/α=26. trans-polyacetylene doped with ${\mathrm{AsF}}_5$ to a room-temperature conductivity in chain direction of ${\sigma }_{?}$=1200 ${\Omega }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$ 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 (${\epsilon }_1$=${\epsilon }_2$) at $E_P$=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.