A theoretical study of the electronically excited states in linear and cyclic C-6(+)
Résumé
The electronically excited doublet and quartet states of the linear (D-infinityh) and Cyclic (C-2v) C-6(+) ion were studied using high-level ab initio methods. For the linear ion it is found that the three lowest excited (2)Pi(g) states, which contribute to the n(2)Pi(g)-X(2)Pi(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 2(2)Pi(g) state. For the cyclic C-2v structures the lowest states have (2)A(1) and B-2(2) symmetries. At their equilibrium geometries both states are almost degenerate and their energies are 0.3 eV lower than the minimum of the linear X(2)Pi(u) state. The excitation energies are strongly affected by geometry relaxation effects. The adiabatic excitation energies of the cyclic 2(2)A(1) and the linear 1(2)Pi(g) states are predicted to be about 1.9 eV, i.e. close to the observed band origin. Several electronically excited states-2(2)A(2), 2(2)B(1), 2(2)B(2)-of the cyclic structure and the 2(2)Pi(g) state of the linear structure were calculated between 2.3 and 2.5eV, i.e. higher than the observed band origin at 2.17eV. 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.