A theoretical study of the electronically excited states in linear and cyclic

Abstract

The electronically excited doublet and quartet states of the linear (D_{∞ h} ) and cyclic (C_{2 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 C_{2 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.