Donor and acceptor states in lightly doped polyacetylene,(CH)x

The results of an experimental study of the infrared absorption of the ${\left(\mathrm{CH}\right)}_x$ system, lightly doped (< 0.1%) with acceptors and donors, are presented. Additional absorptions are observed near 1370 ${\mathrm{cm}}^{-1\mathrm{}}$ (width ∼ 50 ${\mathrm{cm}}^{-1\mathrm{}}$) and 900 ${\mathrm{cm}}^{-1\mathrm{}}$ (width ∼ 400 ${\mathrm{cm}}^{-1\mathrm{}}$) upon doping with iodine, As${\mathrm{F}}_5$, and sodium. The two additional absorption maxima appear to be general features of lightly doped ${\left(\mathrm{CH}\right)}_x$ independent of specific dopant or of cis (trans) content. Measurements on stretch-oriented films demonstrate that these absorption maxima are polarized primarily along the polymer chains. The narrow mode at 1370 ${\mathrm{cm}}^{-1\mathrm{}}$ is attributed to a molecular vibration made ir active by the doping. The broader absorption centered at 900 ${\mathrm{cm}}^{-1\mathrm{}}$ is discussed in terms of quasi-one-dimensional donor and acceptor bound states along the ${\left(\mathrm{CH}\right)}_x$ chain. We include an experimental determination of the room-temperature dielectric constant in undoped ${\left(\mathrm{CH}\right)}_x$; ${\epsilon }_{\parallel }\simeq 10-12$, with uncertainty arising from the incomplete orientation of the ${\left(\mathrm{CH}\right)}_x$ films. The bound-state energy is thus consistent with a one-dimensional hydrogenic model in which the Coulomb potential along the chain is reduced by ${\epsilon }_{\parallel }$. Alternatively viewing semiconducting ${\left(\mathrm{CH}\right)}_x$ as a Peierls distorted one-dimensional metal, we discuss localized domain-wall-like charged donor (acceptor) states induced by charge-transfer doping.