The electronic spectrum of C5

Multi-reference configuration interaction (CI) calculations are employed to characterize the electronic states of linear C5 up to an energy of about 6 eV. It is shown that the transition at 2.43 eV, which has conventionally been assigned as 1Πu−X1Σg+ originates from excitation to 1Σu− and 1Δu states while the 1Πu state is calculated to lie around 2.9 eV with an absorption oscillator strength of f=0.03. The strong 1Σu+−X1Σg+ transition is found around 6 eV with f=2.2. Potential curves for cis- and trans-bending are calculated; all states are found to have their minima at linear geometry, but surfaces for 1Σu− and 1Δu states are very flat and transition moments for 1Δu(Au,Bu)−XΣg+(Ag) and 1Πg(B2)←X1Σg+(A1) increase strongly with deviation from linearity.