Multipole structure in asymmetrical double Rydberg states

We have investigated Ba high-l 8snl double Rydberg states produced by multistep laser excitation in the presence of a temporarily applied electric field to probe the Coulomb interaction between the two excited electrons. The spectra for the 6s→8s excitation with n=13 (l=12 to 6) show a strong dependence on the orbital momentum of the external electron, due to multipole effects. To explain the 5fn’l’ resonances appearing in the n=13 and 14 l=6 spectra, terms higher than the dipole interaction have to be considered in the multipole expansion.