The energy shifts of high-angular-momentum Rydberg states in berylliumlike ${\mathrm{O}}^{4+}$, due to long-range interactions of the Rydberg electron with the ionic core, are measured by laser spectroscopy, using stimulated two-step recombination of free electrons with ${\mathrm{O}}^{5+}$ ions. The comparison of the accurate experimental results with theoretical shifts from the core polarization model and from ab initio atomic structure calculations shows considerable disagreement, demonstrating the need for an efficient method to include configuration interaction in a long-range interaction model.

• Received 11 April 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.802