Knowledge Bank

The excited $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+$ states of the $H3$ molecule are affected by strong nonadiabatic coupling. The interaction of the doubly excited $i\sigma u\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ configuration with Rydberg configurations $\sigma g$ gives rise to the double-minimum electronic energy curves of the states $EF(2s\sigma g2\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\sigma g+),GK(3d\sigma g,3\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\sigma g+)$, and to the higher lying curves of the states $i\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\sigma g+(i=4,5,\dots )$ which continue the $ns\sigma$ and $nd\sigma$ series. The vibrations of all of these adiabatic states $i1\Sigma g+$ are mutually coupled via the operators which have been computed by Wolniewicz using ab-initio wavefunctions $(i,j)=(2,3)$ and (3,4), and by Quadrelli using an approximate diabatic model for the entire $nd\sigma$ and $nd\sigma$ Rydberg series interacting with $1\sigma u2$ The discrepancies between the nonadiabatic ab-initio results and experimetal term values increase with vibrational excitation energy from $2cm-1$ for $EF(v=0)$ to $50cm-1$ for $GK(v=5)$ and they can be apportioned approximately to (a) convergence errors of the adiabatic potential energy curves (up to $\simeq 20cm-1$) and (b) nonadiabatic shifts caused by the higher states $i>4$ (up to $\simeq 30cm-1$). This has been investigated with the aid of Quadrelli's model as well as by comparing the ab-initio results for $H2$ and $D2$. For the $v=0$ level of the EF state the nonadiabatic energy shift is smaller than $0.1cm-1$; the $2cm-1$; error of the adiabatic calculation is of magnitude comparable to ab-initio results on other excited states of $H2$.