Electron-energy-loss spectroscopy of polydiacetylenes

Electron-energy-loss spectra in the range 0.2-300 eV were measured for two types of polydiacetylene thin films with different sidegroups: polymers of 2,4-hexadiyn-1-ol and Langmuir-Blodgett films of the pentacosa-10,12-diinoic acid—Cd salt. The valence excitations are dominated by a $\pi$-electron plasmon above the 2-eV fundamental absorption edge. The plasmon dispersion is linear with a slope identical to similar plasma excitations in polyacetylene [${\mathrm{}\left(\mathrm{CH}\right)\mathrm{}}_x$]. This implies that the $\pi$-electron bandwidth and interband excitation spectra are similar in these materials as suggested by recent theoretical band-structure calculations. Absorption begins at 2.0 eV for all momenta even at low temperature, consistent with the dominance of localized excitations at the fundamental edge, thus confirming the interpretation of earlier experimental work.