Electronic structure of the radicals NF and NCl. III. The radiative lifetimes of the bΣ and aΔ states

The radiative lifetimes of the b¹Σ⁺ and a¹Δ states have been evaluated by perturbation expansions including X³Σ⁻, a¹Δ, b¹Σ⁺, 1^3,1Π, 2^3,1Π, 2³Σ⁻ and 2¹Σ⁺ states. All wavefunctions result from large MRD CI calculations. The b—X transition is dominated by the parallel transition moment; it is found to be much stronger than the a—X transition. The calculated radiative lifetimes of τ(¹Σ⁺)=18 ms, τ(¹Δ)=2.2 s for NF and τ(¹Σ⁺)=2.5–3.5 ms for NCl are in good accord with corresponding experimentally deduced values. The lifetime for the a¹Δ state in NCl is found to be τ(¹Δ)=1.1 s, ie. much longer than derived from a recent experiment. Its magnitude is consistent with the τ(b¹Σ⁺)/τ(a¹Δ) ratio of similar systems and with the decrease in lifetime from NF to NCl and is thus believed to be quite reliable. A detailed analysis of all contributions of the perturber states to the transition mechanism is made and comparison with the related data in SO, O₂ and S₂ is undertaken. The b-a transition probability dominated by the quadrupole transition is fairly constant in all the systems in the order of A = 0.013 (NF) - 0.0013 (S₂) s⁻¹.