Strong Angular Correlation of Bound Electrons Revealed by Resonant Two-Color, Three-Photon Ionization of Barium

The $5d7s{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ state of barium was resonantly prepared and photoionized. Branching ratios and photoelectron angular distributions were measured for the $6s$ and $5d$ core states of ${\mathrm{Ba}.\mathrm{}}^{+}$ Strong configuration mixing is indicated for the $5d7s{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ state. The $6s$ core is produced in 43% of the ionization events, and the $5d$-core photoelectron angular distributions show fourth and sixth spherical harmonics in violation of independent-particle predictions.