Electric field effects on barium autoionizing spectra

Stark spectra for atomic barium in the autoionizing region below the 5${\mathit{d}}_{3/2}$ ionization threshold are studied by photoionization of the laser-excited 5d6p ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_1^0$ bound level, in a combined experimental and theoretical study. The electric field dramatically increases the complexity of the resonance pattern beyond that of the already complex zero-field spectrum. A quantum defect and R-matrix calculation describes 86 relevant zero-field channels. A frame transformation couples these channels together nonperturbatively in the presence of the field, reproducing most of the observations.