The near-threshold absorption spectrum of N2

A new comprehensive multichannel quantum defect study of the near-threshold absorption of N-14(2) 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 , ibid. 100, 7957 (1994)] has been achieved in the region where core-excited s and d Rydberg levels built on the A (2)Pi(u) state of the ion interact with the series of p and f complexes converging to the lowest vibrational levels of X (2)Sigma(g)(+). 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 N-2(+), 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 ssigmasimilar todsigma interchannel coupling and from the intrachannel interaction of the dpi(g) Rydberg with the 1pi(g) valence orbital, which, in contrast to 2pi of NO, is occupied not in the ground state of N-2, but in the electronically excited precursor states a(') (1)Sigma(u)(-), w (1)Delta(u), and b(') (1)Sigma(u)(+). (C) 2003 American Institute of Physics.