Electroabsorption of polyacetylene

We present the results of electroabsorption measurements for nonoriented cis- and trans-polyacetylene and for oriented trans-polyacetylene. The electroabsorption spectrum of the as-grown (nonoriented) material is dominated by a series of field-induced absorptions and bleachings near the band edge. These spectral features are interpreted as vibronic sidebands (arising from electronic coupling to the excited-state phonon modes) which are broadened by the applied electric field. The existence of such vibronic sidebands in the as-grown material indicates that the band-edge states are localized. In oriented trans-(CH${)}_{\mathrm{x}}$ the vibronic features (the strongest of which is observed as a ‘‘knee’’ near 1.5 eV in the absorption spectrum) are highly suppressed, while the induced absorption at 1.28 eV is enhanced relative to the unoriented case. No electroabsorption signal is present when the external field is applied perpendicular to the chain direction. The dependences of the electroabsorption spectrum on temperature, electric field strength, incident light polarization, and intensity are also presented. Since stretch orientation suppresses the vibronic sidebands, we conclude that such postsynthesis processing leads to improved structural order and to delocalization of the band-edge electronic states.