4fnfdoubly excited autoionizing series in barium

In a two-step pulsed-laser atomic-beam experiment we have studied doubly exicted 4fnf series of barium with energy of {~}11 eV above the atomic ground state (6s²) and 6 eV above the first ionization limit (6s). Starting from 5d² ³{F}₃$,${}¹G₄ metastable states, 4f5/2n'f J=2--6 (9{≤}n'{≤}46) states and 4f7/2n''f J=3--6 (n''=19, 20, 21, 25, and 40) states were excited using intermediate 5dnf autoionizing Rydberg states. The 4f5/2n'f J=2--6 states strongly interact with 4f7/2n''f J=2--6 (8{≤}n''{≤}20) and 7pn'''f J=2--5 (6{≤}n'''{≤}10) states. The excitation spectra were analyzed and interpreted using multichannel quantum-defect theory as well as a limited Slater-Condon analysis. The quantum defects of the 4fnf series are large (0.5--1.4), which is an indication for considerable two-electron correlation effects.