Studies of high-$n, 120\lesssim n\lesssim 160$, strontium $5sng, 5snh$, and $5sni$ Rydberg states are reported. Measurements using microwave spectroscopy provide the quantum defects for these states and furnish a valuable test of quantum defects reported for lower-$\ell$ states in the high-$n$ limit. With increasing $\ell$ the relativistic spin-orbit interaction becomes comparable to the electron-electron interaction resulting in the breakdown of the standard $LS$-coupling scheme. Heavier alkaline-earth quasi-two-electron systems differ significantly from a true two-electron system such as helium because indirect spin-orbit effects are dramatically enhanced. We explore the high-$n$ limit of this transition between $LS$ and $jj$ coupling. For high-$\ell$ states the quantum defects are determined predominantly by the polarizability of the inner electrons.

• Received 28 May 2021
• Accepted 27 August 2021

DOI:https://doi.org/10.1103/PhysRevA.104.032817