Optical absorption from polarons in a model of polyacetylene

Abstract

We present a theoretical calculation, within a continuum electron-phonon-coupled model, of the optical absorption due to polarons in polyacetylene [${(\mathrm{CH})}_{x}$]. Our results can be applied to both trans- and cis-${(\mathrm{CH})}_{x}$, as well as potentially to other polymers (polypyroles, polyparaphenylenes) in which polarons are present. We establish that the essential signature of polaron absorption is the existence of three separate absorption peaks in the gap with qualitatively different features. For trans-${(\mathrm{CH})}_{x}$, we compare and contrast this structure with that from the kink solitons, which are expected to dominate the optical absorption at all but the lowest doping levels. For cis-${(\mathrm{CH})}_{x}$ and related polymers, we discuss polaron (and multipolaron) absorption and the relation of polarons to possible excitons in these materials. Finally, we evaluate briefly the existing experimental situation regarding optical absorption in polyacetylene and indicate possible future experiments that could confirm the existence of polarons in ${(\mathrm{CH})}_{x}$ and similar polymers.