Several radiative data (e.g., transition moment Re′e″, oscillator strength fv′v″, Einstein emission coefficient A v′v″ and lifetime τv′) are derived for the B 1Δg-A 1Πu and B′ 1Σg + -A 1Πu infrared bands of C2 by using ab initio MRD-CI wavefunctions. The predicted f00 values are 0.75 × 10-3 (B←A) and 1.54 × 10-3 (B′←A), both results being somewhat smaller than the best literature estimate f00 ≈ 2.21 × 10-3 for the A←X (Phillips) band. The computed τ00 (B 1Δg) of 141.3 μs represents the longest radiative lifetime so far reported for any dipole-allowed C2 transition. The lifetime τ0 (B′ 1Σg +) is only 8 μs, a value which is approximately 50% shorter than the time range from 13 to 19 μs assigned to τ0(A 1Πu); however, for ν ≥ 2 the lifetimes of both B′ and A states differ by less than 3 μs. Differences between the radiative data for the A, B, B′ and b states are rationalized on the basis of excitation energies and Franck-Condon factors. According to the computed B and B′ lifetimes, the cascading process B′→ A → X might have been responsible for the longer A1Πu lifetimes reported experimentally.

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Journal Chemical Physics
Bruna, P.J. (Pablo J.), & Wright, J.S. (1991). Transition probabilities for the B 1Δg-A 1Πu and B′ 1Σg + -A1Πu infrared bands of C2. An ab initio study. Chemical Physics, 157(1-2), 111–121. doi:10.1016/0301-0104(91)87136-J