Theoretical studies on the electronic spectrum of linear C6

Large-scale multireference configuration interaction calculations were performed on singlet and triplet states of linear cumulenic C-6 in the range up to 6 eV. Vertical excitation energies Delta E and oscillator strengths f are reported. For the 1(3)Sigma(u)(-) <-- X(3)Sigma(g)(-) system, whose most intense band is observed at 2.68 eV, Delta E (vertical) = 2.66 was calculated, with f = 0.023. A very strong 2(1)Sigma(u)(-) - X(3)Sigma(g)(-) transition is calculated around 5.7 eV; a transition (3)Pi-X(3)Sigma(g)(-) slightly below 5 eV should also be observable. The lowest singlet state, 1(1)Delta(g), lies only 0.17 eV above X(3)Sigma(g)(-). Singlet states with large oscillator strengths (relative to (1)Delta(g)) are 1(1)Delta(u) at 3.15 eV, (1)Pi(u) and (1)Phi(u) states between 4 and 5 eV. (C) 1999 Elsevier Science B.V. All rights reserved.