Question

Why is the potential minimum of a molecule shifted towards greater nucleii separation for excited electron states?

Answer 1

I know it has to do with symmetry of the wave function, but I am having trouble piecing it all together. For a positive H ion we have a symmetric wave function ψ+ψ+, which base functions ϕaϕa and ϕbϕboverlap. This means that the bond between the nucleii will be stronger and hence closer together. For the anti-symmetric wave function ψ−ψ− these do not. The bond is weaker and the nucleii therefore farther apart. Now back to the first picture: How does an excited electron state (angeregter Zustand) relate to an anti-symmetric wave function? Or what is then the reason for the shift toward greater nucleii separation

Answer 2

This is due in fact that nuclei are much more massive than electrons and the electronic ... If we picture the vertical transition from ground to excited electronic state as occurring ...