Inelastic-electron-scattering experiments indicate that electronic excitations in polyacetylene, ${\left(\mathrm{CH}\right)}_x$, are similar to those of a semiconductor with wide energy bands and an excitonic or indirect absorption edge. However, the measured bulk-plasmon energy increases linearly with momentum unlike that of known inorganic solids. Heavy doping with As${\mathrm{F}}_5$ or iodine is found to result in an inhomogeneous mixture of metallic and insulating regions.

• Received 26 October 1979

DOI:https://doi.org/10.1103/PhysRevLett.44.1351