The radiative properties of the doubly-excited states Ε1Σg +(σu 2→σu 2) of C2 and 22Σg +(σu 2→σu 2) of C2 + are investigated theoretically by using MRD-CI wave functions. Due to a heavy mixing between bonding and antibonding configurations, the Ε state (with a maximum contribution of σu 2→σu 2 of 45%) does not show the typical features of strongly bound states (short Re and high ωe). Because of the mixing, the experimental Freymark system Ε1Σg + - A1Πu of C2 exhibits a moderate intensity though. At R = 2.43 bohr (an intermediate geometry between both minima), a calculated transition moment ΣRe′e″ 2 of 0.151 au2 disagrees with an experimental estimate of 2.26 ± 1.13 au2 from Cooper and Nicholls. The same authors assigned to the Mulliken band D1Σu + - X1Σg + of C2 a ΣRe′e″ 2 of 0.13 ± 0.05 au2, whereas this work and earlier data from the literature indicate a higher value of 0.41 au2 for that quantity. A total radiative lifetime τ0 ≈ = 15 ns for the Ε1Σg + state is mainly dictated by the radiative process E → A, the competitive decay E → D being about 10 times slower. The 22Σg +(σu 2→σu 2) state of C2 + is strongly bound, with a shortening ΔRe = 0.35 bohr and an increase Δωe ≈ 800 cm-1 (60%) relative to the X;4Σg - ground state. A lifetime τ0(22Σg +) ≈ 28 ns results from radiative decay into both 12Σ2 + and 22Πu states. On the basis of the present spectroscopic data for C2 +, an absorption band observed at 2.77 eV (assumed as belonging to C2) is tentatively reassigned to the 22Σg + → 12Σu + transition of C2 +, with a calculated ΔΕ00 = 2.81 eV.

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Journal Journal of Physical Chemistry
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Bruna, P.J. (Pablo J.), & Wright, J.S. (1992). Theoretical study of the transition probabilities of the doubly-excited states Ε1Σg + of C2 and 22Σg + of C2 +. Journal of Physical Chemistry, 96(4), 1630–1640.