EXPERIMENTAL AND THEORETICAL BASIS FOR POLSON MODEL OF DOPED POLYACETYLENE

New-type polyacetylene, stretched and doped with perchlorate or iodine, shows a nearly metallic conductivity up to 2.5 x 10(4) S cm-1. The molecular structure of the heavily doped chain is discussed to understand the metallic state. Ab initio molecular orbital calculations of model complexes, reflection spectra and ultraviolet photoemission spectra (UPS) are studied at several dopant levels. The transitions from insulator to metal are discussed based on optical conductivity spectra (sigma(omega)), which are obtained from the reflection spectra measured from the far-infrared to ultraviolet regions. The UPS of alkali metal-doped or perchlorate-doped polyacetylene thin films are measured; finite density of states is found at the Fermi level in both heavily n-type and p-type doped films. The changes of UPS are correlated to the calculated energy levels of the charged soliton-antisoliton and polson-antipolson chains. These results are analyzed based on transitions in stages from pristine to charged soliton, then to polson chain in the isolated state, and finally to polson chains interacting three dimensionally. The importance of three-dimensional interaction is found in sigma(omega), UPS and electrical conductivity analyzed by the variable range hopping model.