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Solved and unsolved problems of near–threshold spectroscopy of N2 and CO

Rotationally resolved jet absorption studies of N2 at 800–850 Å have led to detailed identifications of the electronic structures associated with low–J rovibrational thresholds of ground–state N2. Of particular interest is a series of perturbed f complexes that interact with core–excited Rydberg levels and, possibly, with the b′ valence state. The latter assumes the role of a core–excited Rydberg state at high energies and small internuclear distances. A similarly complete picture of the Rydberg structure of CO is still lacking. The interpretation of ℓ–uncoupling effects and, thus, the identification of Rydberg complexes is difficult owing to widespread perturbations by non–Rydberg levels. Significant advances have been made in the understanding of the ∑–∑ Rydberg–valence interactions, but few hard facts have emerged concerning the ∏–∏ interactions where the analysis proceeds in the near–total absence of guidance by theoretical calculations.