Measurements of dipole and quadrupole polarizabilities of Rn-like Th 4 +

Measurements of the fine structure pattern in high-$L$, $n$ = 37 Rydberg levels of Th${}^{3+}$ have been extended and improved to allow more precise determinations of the dipole and quadrupole polarizabilities of the Rn-like Th${}^{4+}$ core of the Th${}^{3+}$ Rydberg system. The improved precision is partly due to the recent availability of theoretical calculations that limit the size of higher-order contributions to the fine structure pattern. The measured polarizabilities are ${\ensuremath{\alpha}}_{D}$ = 7.702(6) a.u. and ${\ensuremath{\alpha}}_{Q}$ = 29.1(1.9) a.u.