Observation of an electronic correlation for double-Rydberg states of barium

Highly excited double-Rydberg states Nln'l' of barium have been observed for N=22 to 34 and n'=39 to 60. The experimental data trace the evolution from states in which the electrons are uncorrelated (N{}n') to those (N{~}0.6n') in which they are. These features can be interpreted as resulting from the polarization of the excited Ba⁺ (Nl) core by the outer (n'l') electron. A generalization of the isolated core excitation formalism taking into account dipole and quadrupole terms leads us to a quantitative understanding of the long-range electronic correlation observed.