Momentum-dependent dielectric functions of orientedtrans-polyacetylene

The momentum-dependent dielectric functions parallel and perpendicular to the chain axis of highly oriented nonfibrous crystalline trans-polyacetylene were determined by electron-energy-loss spectroscopy in the energy and momentum range 0.2--30 eV and 0.05--1.2 A^-1, respectively. Information on the electronic structure of {π} and {σ} bands as well as on collective excitations was obtained. In particular, a total width of the {π} bands of 4t₀=12.8±{}0.5 eV is derived. In addition, from the sum rule for {π}-electron excitations the ratio of the correlation energy U to the total width of the {π} bands U/4t₀{~}0.7 is estimated. Most of the experimental results can be explained in a single-particle picture. The momentum-independent absorption edge is most likely explained by excitonic transitions.