Ultraviolet Photoemission Spectra of Perchlorate-Doped cis- and trans-Polyacetylene

The UPS spectra of cis- and trans-polyacetylene were measured by careful treatment of fresh samples. This is the first UPS measurement of cis-polyacetylene (cis-transoid). Assignments of the observed spectra are given by comparing with energy levels of cis- and trans-C20H22 obtained by ab initio MO calculation. The change of UPS spectra upon stepwise perchlorate doping was measured; at first the intensity of the C2pπ band decreased and the whole band shifted to higher binding energy. By further doping, a new C2pπ band appeared close to the Fermi level E F when the conductivity reached 11 000 S/cm. These changes show insulator to metal transition accompanied with structural transformation. The UPS spectrum near E F showed a strong correlational effect characteristic of a low-dimensional conductor. The spectra of heavily perchlorate-doped and potassium-doped polyacetylene near E F are analyzed by a theory of Kopietz, Meden, and Schönhammer (Phys. Rev. Lett. 1995, 74, 2997).