Ab initio CCSD(T) and MRD-CI study of excited states and the electronic spectrum of linear C5+

Large-scale ab initio coupled cluster and multi-reference configuration interaction calculations (MRD-CI) are carried out to determine the equilibrium geometry and the vertical electronic spectrum of linear C5+. Contrary to prior theoretical estimates we find three low-lying states within an energy range of 0.3 eV: 2Σu+, 2Σg+ and 2Πg and a symmetric arrangement of nuclei. Transitions from 2Σu+ to these low-lying states are dipole-allowed; sizeable oscillator strengths are computed for the 2Π+g←X2Σu+ transition at 2.62 eV and the 2Σg←X2Σu+ transition at 3.36 eV and should give a guide to spectroscopic identification of linear C5+.