Direct observation of Coulomb correlation effects in polyacetylene

We report the direct observation of electron-electron correlation effects in polyacetylene. Pristine 99% ${}_{}{}^{13}{}_{}{}^{}\mathrm{enriched}$ cis-rich polyacetylene has been investigated by electron nuclear-nuclear triple (double-ENDOR) resonance spectroscopy. We observe an induced negative spin polarization of ${\rho }_{\mathrm{-}}$/${\rho }_{+}$≃-0.33. Negative spin densities arise from spin exchange mechanisms which can only result from Coulomb effects. Theoretical analysis of experimental results within the soliton framework yields 3 eV for the effective on-site Coulomb repulsion.