Resonance states of atomic anions

We study destabilization of an atom in its ground state with decrease of it s nuclear charge. By analytic continuation from bound to resonance states, we obtain complex energies of unstable atomic anions with nuclear charge th at is less than the minimum "critical" charge necessary to bind N electrons . We use an extrapolating scheme with a simple model potential for the elec tron, which is loosely bound outside the atomic core. Results for O2- and S 2- are in good agreement with earlier estimates. Alternatively, we use the Hylleraas basis variational technique with three complex nonlinear paramete rs to find accurately the energy of two-electron atoms as the nuclear charg e decreases. Results are used to check the less accurate one-electron model . (C) 2001 John Wiley & Sons, Inc.