Multichannel quantum-defect theory ofn=2 and 3 gerade states inH2: Rovibronic energy levels

Multichannel quantum-defect theory is applied to the gerade n=2 and 3 states of ${\mathrm{H}}_2$. In the first paper in this series [Phys. Rev. A 49, 4353 (1994)] we obtained the quantum-defect matrix of the strongly interacting ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ double-minimum states of ${\mathrm{H}}_2$ by fitting to the ab 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 N=0 vibronic energies of the corresponding electronic states. In this paper we calculate the N≥0 rovibronic energies of the singlet and triplet gerade n=2 and 3 states of ${\mathrm{H}}_2$. This ab 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 f levels, and the evolution of these splittings with vibrational and rotational quantum numbers, are reproduced very well.