The near-threshold absorption spectrum of N2

<jats:p>A new comprehensive multichannel quantum defect study of the near-threshold absorption of N214 has been carried out over the energy range 118 720–125 425 cm−1. A nearly complete understanding of the rotationally cold spectra reported earlier [K. P. Huber and Ch. Jungen, J. Chem. Phys. 92, 850 (1990); K. P. Huber et al., ibid. 100, 7957 (1994)] has been achieved in the region where core-excited s and d Rydberg levels built on the A 2Πu state of the ion interact with the series of p and f complexes converging to the lowest vibrational levels of X 2Σg+. The interactions reduce to a purely electronic quantum defect matrix which, after suitable transformations, accounts for the observed perturbed structures and intensities arising from vibronic coupling, rotational l uncoupling, and the different geometries of the X and A ion cores. The final calculations converged with 42 nonzero quantum defect parameters reproducing the 597 upper-state rovibronic levels with a standard deviation of 1.12 cm−1. The results have been used to calculate the R(0) line oscillator strengths in terms of eight nonvanishing electronic dipole transition moments, the latter treated as parameters that were fitted to photoelectrically measured band absorption f values. The calculations satisfactorily reproduce the observed oscillator strength distribution. Using ab initio calculated core properties for ground state N2+, the long-range model for a nonpenetrating Rydberg electron interacting with a quadrupolar and polarizable ion core predicts the diagonal f quantum defects in reasonable agreement with the results of the least-squares fits. Similar to NO, deviations from predictions by the same model for the diagonal d quantum defects arise primarily from the strong sσ∼dσ interchannel coupling and from the intrachannel interaction of the dπg Rydberg with the 1πg valence orbital, which, in contrast to 2π of NO, is occupied not in the ground state of N2, but in the electronically excited precursor states a′ 1Σu−, w 1Δu, and b′ 1Σu+.</jats:p>