Rydberg states with anisotropic ion cores: Stark effect

We have measured the Stark spectra of 5$d_{3/2}$8l autoionizing states in barium. Because of the anisotropic 5d ion core, the Stark manifolds at higher fields are considerably more complex than those for the analogous 6snl bound states. The electrostatic coupling of the Rydberg electron with the anisotropic core gives rise to relatively large fine-structure splittings. For nonpenetrating orbitals ($l_{\mathrm{Ry}>l_{\mathrm{core}}}$), jK coupling is a useful representation and the number of fine-structure components due to this interaction with the core increases the number of eigenstates (2$j_{\mathrm{core}+1\mathrm{}}$)-fold. We present a theoretical model for the calculation of Stark spectra in jK coupling. Results for barium are in quantitative agreement with the experimental observations for both π and σ polarizations up to fields where there is extensive overlap between adjacent n manifolds. These fine-structure effects occur for any states with nonisotropic cores ($j_{\mathrm{core}>(1/2)\mathrm{}}$), i.e., the states of most atoms except for bound singly excited states of alkali-metal and alkaline-earth-metal atoms.