Doubly excited states of the akaline-earth atoms

Electron correlations in the alkaline-earth atoms and alkali negative ions, especially as manifested by photoabsorption experiments, are studied using a hyperspherical-coordinate approach. A prototype quantitative calculation of the Be photoabsorption spectrum near the $2s$ and $2p$ thresholds, allowing for nonadiabatic transitions between potential curves for the first time and hence for autoionization, shows the formation of the " + "-type and "-"-type eigenmodes in the two-electron wave function. This has strong implications for the spectra of Be and other atoms, and permits a unified interpretation of their discrete and continuum oscillator strength.