Infrared-active vibrational modes of heavily doped ‘‘metallic’’ polyacetylene

Abstract

A direct comparison of the infrared-active vibrational (IRAV) modes in heavily doped, lightly doped, and photoexcited trans-(CH${)}_{\mathrm{x}}$ is presented. The infrared absorption spectrum of heavily doped trans-(CH${)}_{\mathrm{x}}$ exhibits doping-induced IRAV modes and an onset of electronic absorption which are substantially red shifted with respect to the corresponding absorptions observed at light doping levels and which have an unusually strong temperature dependence. Analysis of the data leads to the following conclusions: For heavily doped trans-(CH${)}_{\mathrm{x}}$, all the doping-induced charges are involved in the generation of the IRAV modes in the ``metallic'' state; the pinning of the \ensuremath{\pi}-electron charges which cause the IRAV modes has virtually disappeared; and there is an energy gap (or pseudogap) in the excitation spectrum with magnitude \ensuremath{\sim}1500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, or \ensuremath{\sim}0.2 eV. Thus, although many of the most fundamental issues associated with metallic polyacetylene at high doping levels remain unresolved, the existence of a pseudogap and IRAV modes demonstrate that it is not a simple metal.