The electronically excited doublet and quartet states of the linear (D∞ h ) and cyclic (C2 v ) ion were studied using high-level ab initio methods. For the linear ion it is found that the three lowest excited 2Π g states, which contribute to the n 2Π g –X2Π u transitions between 1.88 and 2.73 eV, are strongly coupled and form avoided crossings if the bond distances are varied. This leads to a centrosymmetric double minimum potential of the 22Π g state. For the cyclic C2 v structures the lowest states have 2A1 and 2B2 symmetries. At their equilibrium geometries both states are almost degenerate and their energies are 0.3 eV lower than the minimum of the linear X2Π u state. The excitation energies are strongly affected by geometry relaxation effects. The adiabatic excitation energies of the cyclic 22A1 and the linear 12Π g states are predicted to be about 1.9 eV, i.e. close to the observed band origin. Several electronically excited states— —of the cyclic structure and the 22Π g state of the linear structure were calculated between 2.3 and 2.5 eV, i.e. higher than the observed band origin at 2.17 eV. Due to strong electronic and vibronic couplings a reliable prediction of relative intensities is presently not possible, and therefore the calculated transition moments cannot be used for the interpretation of the experimental spectrum.