Ultraviolet Photoelectron Spectroscopy of Alkaline-Metal Doped Polyacetylene

Abstract

Abstract Sodium- and potassium-doped highly conducting polyacetylenes were studied by ultraviolet photoelectron spectroscopy (UPS) using synchrotron radiation. Upon doping, the UPS spectra immediately show a large shift toward the higher binding energy side relative to EF. At intermediate doping level, a new state is created in the empty energy gap region. The gap state in the UPS spectra is assigned to the charged soliton band. This is the first direct observation of emission from the charged soliton state in doped polyacetylene. At heavily doped level, the quasi-metallic density of states is found just below the Fermi level and the broadening of the UPS spectra occurs at the regions of C 2p and C 2s levels. However, no finite density of states was observed at the Fermi level in the case of unoriented polyacetylene. This result is in contrast to the case of oriented polyacetylene doped with perchlorate, where the existence of a finite density of states at EF is reported. The spectral shape of UPS near the Fermi level is in accordance with Tomonaga–Luttinger liquid model of strong electron correlated system. Ab initio molecular orbital calculations taking account of nearest-neighbor chain effect gives a good description of the UPS for all the C 2s, C 2pσ, and C 2pπ bands.