Electron-impact excitation of atoms in high-lying doubly excited states: Single-electron excitation between doubly excited states

It is theoretically pointed out that, even between strongly correlated doubly excited states, there exist single-electron--type transitions for electron-impact excitation processes of He atoms in doubly excited states under the condition {Δ}L=0, where L is the total orbital angular momentum of the atom. The propensity rules found for the ¹{P}ᵒ-¹{P}ᵒ$ and $¹Dᵉ{{{-}}}¹Dᵉ double-excitation processes are also presented and discussed based on the rovibrational interpretation of the collective motion of two atomic electrons.