Fine structure of perturbed Rydberg states in berylliumlikeS12+

The fine structure of 1$s^2$2snl states with high angular momentum l has been measured for the first time in a high-Z four-electron system by studying the n=6-7 transition in the ${\mathrm{S}}^{12+}$ ion. Strong perturbations by 1$s^2$2pn’l’ states are observed and are interpreted using results of new multiconfiguration Dirac-Fock calculations. Implications for the spectroscopy of highly ionized atoms and the determination of high-Z atomic core polarizabilities are discussed.