Two types of combined-isotopologue analysis have been performed on an extensive spectroscopic data set for ground-state N2 involving levels up to v=19, which is bound by half the well depth. Both a conventional Dunham-type analysis and a direct-potential-fit (DPF) analysis represent the data within (on average) the estimated experimental uncertainties. However, the Dunham-type parameters do not yield realistic predictions outside the range of the data used in the analysis, while the potential function obtained from the DPF treatment yields quantum mechanical accuracy over the data region and realistic predictions of the energies and properties of unobserved higher vibrational levels. Our DPF analysis also introduces a compact new analytic potential function form which incorporates the two leading inverse-power terms in the long-range potential.

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The potential energy curve for the B 1Πu state of Li2 has a rotationless barrier which protrudes above its energy asymptote. A direct fit to spectroscopic data for all three isotopomers of this species, including Λ-doubling splittings and tunneling predissociation line widths, is used to determine an accurate analytic potential energy function plus Born–Oppenheimer breakdown and Λ-doubling perturbation radial strength functions for this system. This analysis introduces an analytic model for representing a potential function with a rotationless barrier, and shows that a radial perturbation function treatment can determine the symmetry of the perturbing state giving rise to Λ-doubling splittings.

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