Electronic structure of the radicals NF and NCl. III. The radiative lifetimes of the b 1Σ + and a 1Δ states

Abstract

The radiative lifetimes of the b 1Σ + and a 1Δ states have been evaluated by perturbation expansions including X 3Σ −, a 1Δ, b 1Σ +, 1 3,1Π, 2 3,1Π, 2 3Σ − and 2 1Σ + 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 a 1Δ 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(b 1Σ +)/τ(a 1Δ) 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, O 2 and S 2 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 (S 2) s −1.