Series of Molecularlike Doubly Excited States of a Quasi-Three-Body Coulomb System

We investigated resonant multiphoton excitation of high-angular-momentum doubly excited states of the strontium atom, a quasi-three-body Coulomb system, both experimentally and theoretically. A series of highly doubly excited states converging to the double ionization threshold was identified and reproduced by first-principles calculations, which provided access to the two-electron wave functions of the resonances. In these states, the outer electron is dynamically localized at large distances by pendulumlike oscillations of the inner electron across the nucleus. A complementary simple molecularlike adiabatic model offers a clear interpretation of the correlated two-electron motion and suggests that the series is a general feature of highly doubly excited states of atoms and molecules.