When two electrons are excited far away from the nucleus, their motion results from the subtle balance between electronic repulsion and the Coulomb attraction of the residual ion. The strong electron correlations give rise to complex and fascinating two-electron dynamics that range from chaotic motion to quasi-stable orbits associated with sharp resonances. We report the experimental and theoretical observation of resonances belonging to a new and general type of doubly-excited states with a distinctive two-electron motion. Such states form a regular Rydberg-like series converging to the double-ionization threshold. Experimentally, ground-state Sr atoms are excited by sequential multiphoton resonant excitation to doubly-excited states of high angular momentum (L~15) lying below the Sr+(N=9-15) ionization thresholds - less than 0.5 eV below the double-ionization threshold. The excitation spectra reveal the presence of a regular series of lines that persists even at high energy, where most other spectral features merge into an unstructured continuum. To identify the origin of these lines, we combine large-scale theoretical calculations from first principles with simple modelling. The analysis reveals that the states of the series emerge from a surprising binding mechanism: the electronic repulsion creates, through a Stark-like avoided crossing, an effective, adiabatic potential-energy well in which the outermost electron is localized. Such wells appear at each principal quantum number N and give rise to the series, whose properties scale with powers of N that are different from usual Rydberg-state scaling laws. We also discuss the existence of such series in systems other than Sr.
Génévriez et al. (Wed,) studied this question.