Knowledge Bank

Avoided crossings of the electronic energy curves of the $1\sigma u\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ state and of the Rydberg series $ns,nd\sigma (n-3,4\dots )$ of hydrogen give rise to the double minima in the EF and $GK\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+$ states and to strong coupling between the vibrations of the adiabatic $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+$ states. From the analysis of ab initio calculations of the five rovibronically coupled states we know that still higher states contribute to the nonadiabatic energy shifts. We have included the three states $(4d\sigma )\rho \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+\Sigma g++(4d\pi )R\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi g+(4ds)s\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta g$ in the eight-state coupled equations which describe the simultaneous vibronic and L-uncoupling interactions. The remaining deviations between calculated and experimental term values in $H2$ and $D2$ depend systematically on electronic state, on vibrational energy, and on isotopic mass; they are between two to ten times smaller than in the previous five-state calculation.