Theoretical spectroscopy of the NO radical. I. Potential curves and lifetimes of excited states

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Abstract
Multireference configurations interaction calculations are employed to study the low‐lying excited states of the NO radical up to the dissociation limits. The states of 2Π symmetry are characterized by a change from Rydberg to valence character with increased internuclear separation; the interaction takes place over a fairly narrow range of R and both, Rydberg and valence states, show potential wells. The 2Σ+ states exhibit only distinct potential minima in the Rydberg area and are otherwise characterized at larger bond lengths in the main by interacting valence states. The lifetimes of the Rydberg states A, C, and D are calculated in the order of 5×10−8 s, those of the valence states B and L lie around 300×10−8 s while τ for B’ 2Δ (v=0) is calculated to be 17×10−8 s. The results are interpreted in terms of the spatial extension of the orbitals, their deviation from the gerade and ungerade character relative to a homonuclear molecule and the Franck–Condon factors. The lowest 2Σ+ state is found to possess a potential barrier relative to dissociation, whereby the tunneling probability for its v=8 level is found to correspond to a line broadening of 20.8 cm−1. The results form the basis for a study of spin–orbit, Λ‐doubling, and predissociation effects to be reported in a consecutive paper.