Optical Properties of a Polydiacetylene Single Crystal Evidence for van Hove Singularities

Abstract

Abstract Reflection spectra of polydiacetylene‐bis(toluene‐sulphonate) single crystals have been measured in the temperature range between 9 K and 300 K. By means of Kramers‐Kronig analysis the dielectric constants ε 1 and ε 2 and the absorption constant a have been derived. As they display van Hove singularities it is concluded that the optical absorption results from transitions between delocalized one‐dimensional states of the polymer chain forming broad bands. All absorption bands are split into doublets, the splitting energy decreases from 32 meV at 9 K to less than 10 meV at 300 K. Splitting is interpreted in terms of a threedimensional band model and finite intermolecular interaction.