The electronic spectrum of C5

Multi-reference configuration interaction (CI) calculations are employed to characterize the electronic states of linear C-s up to an energy of about 6 eV. It is shown that the transition at 2.43 eV, which has conventionally been assigned as (1)Pi (u) - X(1)Sigma (+)(g) originates from excitation to (1)Sigma (-)(u) and (1)Delta (u) states while the (1)Pi (u) state is calculated to lie around 2.9 eV with an absorption oscillator strength of f = 0.03. The strong (1)Sigma (+)(u) - X(1)Sigma (+)(g) 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 (1)Sigma (-)(u) and (1)Delta (u) states are very flat and transition moments for (1)Delta (u)(A(u),B-u) - X Sigma (+)(g)(A(g)) and (1)Pi (g)(B-2) <-- X(1)Sigma (+)(g)(A(1)) increase strongly with deviation from linearity. (C) 2001 Elsevier Science B.V. All rights reserved.