Multichannel quantum-defect theory of n=2 and 3 gerade states in H2: Rovibronic energy levels.

Multichannel quantum-defect theory is applied to the gerade [ital n]=2 and 3 states of H[sub 2]. In the first paper in this series [Phys. Rev. A 49, 4353 (1994)] we obtained the quantum-defect matrix of the strongly interacting [sup 1][Sigma][sub [ital g]][sup +] double-minimum states of H[sub 2] by fitting to the [ital ab] [ital initio] clamped-nuclei electronic energies of Wolniewicz and Dressler. This matrix was used in the second paper [Phys. Rev. A 49, 4364 (1994)] to calculate the [ital N]=0 vibronic energies of the corresponding electronic states. In this paper we calculate the [ital N][ge]0 rovibronic energies of the singlet and triplet gerade [ital n]=2 and 3 states of H[sub 2]. This [ital ab] [ital initio] treatment accounts for both singly and doubly excited channels and reproduces the rovibronic energies with almost the same accuracy as the more traditional coupled-equations approach. Additionally, the experimental singlet-triplet splittings for the [ital f] levels, and the evolution of these splittings with vibrational and rotational quantum numbers, are reproduced very well.