Electron correlation in doubly excited S states: large-Z limit

The intrashell states of two-electron atoms are studied using the diagonalisation of the electron interaction r 12 -1 . The relationship between the spatial structure of the electron density and electron correlation is established. (r 12 -1 ) is obtained as an asymptotic function of the principal quantum number n for the lower (r 12 -1 ) 0 =(1-2/ pi )n -2/5 +(8 pi )n -3 and the higher (r 12 -1 ) n-1 =0.5 ln 4n+0.6 states. It is shown that the screening approximation can be used for reliable non-relativistic estimation of lower state energies along the whole isoelectric series. It is predicted that for any positive ion there exists an n 0 , at which intrashell multiplets start to overlap.