Knowledge Bank

Major features of the Rydberg electronic structures and associated spectra for the rare gas dimers $He2,Ne2,Ar2,Kr2$, and $Xe2$ are summarized. Although the ground states of these molecules have only shallow van der walls minima, the Rydberg states built on at least the ground ionic core state, $X2\Sigma u+$, have significant minima. For $He2$, hundreds of these states have been observed experimentally and associated with the configurations $[1\sigma g21\sigma u]n\ell \lambda$. Recent spectroscopy of the heavier dimers indicate similar Rydberg structures associated with configurations in which the ionic portions (the orbitals in the square brackets) are suitably expanded. Rotationally resolved spectra for $He2,Ne2$, and $Ar2$ and partially resolved data for $Kr2$ and $Xe2$, provide an experimental basis for structural correlations both within the homologous series and between theory and experiment. Major unsettled questions include the prospects for channel coupling between Rydberg states built on core states other than $X2\Sigma u+$ with Rydberg states based on $X2\Sigma u+$.