The lowest pi- and sigma-doublet excited states of butadiene, hexatrie
ne and octatetraene radical cation are studied using multiconfiguratio
nal second-order perturbation theory. The calculated excitation energi
es are found to be within less-than-or-equal-to 0.2 eV of the experime
ntal energies. Our results allow us to interpret the different feature
s observed in photoelectron spectra which are measured at the ground s
tate geometry of the neutral molecules as compared to the electronic a
bsorption spectra which are taken at the ground state geometry of the
cations.