Experimental study of configuration mixing in intermediate excited levels of barium

Resonant three-photon ionization of barium, via five closely spaced excited bound states---labeled 6s8s ¹S₀, 6p² ³P₀, 6p² ³P₁,6s7d ¹D₂, and 6p² ³P₂---has yielded both angular distributions of energy-resolved photoelectrons and branching ratios to the 6s, 5d, and 6p configurations of Ba⁺. In contrast to earlier ab initio and semiempirical theoretical analyses which attributed nearly pure configurations to these five bound states, our experimental findings are consistent with a significant degree of 6p² character in the 6s8s ¹S₀ and 6s7d ¹D₂ levels, more in accord with their original configuration labels in the tables of Moore. Branching to the 6p states of the ion predominates in each case, and the angular distributions of the corresponding photoelectrons from the 6p² ³P₂ and 6s7d ¹D₂ levels are described well by a parametric theoretical model approximating these states as pure 6p² configurations. For the 6s8s ¹S₀ level, inferences based on these observations must be made cautiously because of the possibility of an ``anomalously'' low photoionization amplitude for the 6s8s component of the state. Available oscillator strengths suggest the possibility of configuration interaction via spin-orbit coupling. Overall, strong configuration mixing and/or mislabeling of the configurations is implied; we suggest that the choice of configurational labels for these states be reexamined.