IMPROVED NONADIABATIC CALCULATION OF EXCITED SINGLET STATES OF THE HYDROGEN MOLECULE

$^{1}$ P. Quadrelli, K. Dressler, and L. Wolniewicz. J. Chem. Phys. (in press).Author Institution: Physical Chemistry Laboratory, ETH-Zentrum.Avoided crossings of the electronic energy curves of the $1\sigma_{u}{^{2}}$ state and of the Rydberg series $ns, nd\sigma (n-3,4\ldots)$ of hydrogen give rise to the double minima in the EF and $GK {^{1}}\Sigma_{g}^{+}$ states and to strong coupling between the vibrations of the adiabatic $^{1}\Sigma_{g}^{+}$ states. From the analysis of ab initio calculations of the five rovibronically coupled states $(2s)EF + (3d\sigma)GK+(3s)H {^{1}}\Sigma^{+}_{g} + (3d\pi)I{^{1}}\Pi_{3} + (3d\delta)J{^{1}}\Lambda{^{J}},^{1}$ we know that still higher states contribute to the nonadiabatic energy shifts. We have included the three states $(4d\sigma)\rho {^{1}}\Sigma_{g}^{+}\Sigma_{g}^{+}+(4d\pi)R{^{2}}\Pi_{g}+(4ds)s{^{1}}\Delta_{g}$ 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 $H_{2}$ and $D_{2}$ 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