Excited states in polyene radical cations. An ab initio theoretical study

The lowest π- and σ-doublet excited states of butadiene, bauntriene and octatetraene radical cation are studied using multiconfigurational second-order perturbation theory. The calculated excitation energies are found to be within ⩽ 0.2 eV of the experimental energies. Our results allow us to interpret the different features observed in photoelectron spectra which are measured at the ground state geometry of the neutral molecules as compared to the electronic absorption spectra which are taken at the ground state geometry of the cations.