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 el ectric field to probe the Coulomb interaction between the two excited electrons. The spectra for the 6s --> 8s excitation with n - 13 (l = 1 2 to 6) show a strong dependence on the orbital momentum of the extern al electron, due to multipole effects. To explain the 5fn'l' resonance s appearing in the n = 13 and 14 l = 6 spectra, terms higher than the dipole interaction have to be considered in the multipole expansion.