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 ${\alpha }_D$ = 7.702(6) a.u. and ${\alpha }_Q$ = 29.1(1.9) a.u.

Figure 1

Plot of the scaled first-order energy differences measured between high-L n $=$ 37 Rydberg levels of Th${}^{3+}$. The solid line shows the fit to Eq. (1) of the text. The fitted intercept, shown as an open circle, determines the dipole polarizability of the core ion Th${}^{4+}$, while the initial slope determines its quadrupole polarizability.