Photomodulation Spectroscopy of Iodine-Doped Polyacetylene; Phase Transition from Soliton Lattice to Metal

Using the photomodulation technique for iodine (${\mathrm{I}}_3^{-}$)-doped polyacetylene, we have found spectroscopic evidence for an abrupt phase transition at 5% ${\mathrm{I}}_3^{-}$. The transition is from the lightly doped phase best described as soliton lattice to the heavily doped metallic phase. The phase transition is characterized by gradual disappearance of the single photoinduced soliton band in the semiconducting phase and a sudden appearance of two correlated photoinduced absorption bands associated with photogenerated charged defects in the metallic phase.