Polarizability of Kr$^{6+}$ from High-L Kr$^{5+}$ Fine Structure Measurements

The transition between n=55 and n=109 Rydberg levels of Kr$^{5+}$ has been studied at high resolution using the RESIS method. Resolved excitation of L = 6, 7, 8, and 9 levels in n=55 lead to a determination of the fine structure energies of these levels. Interpreted with the long-range polarization model, this leads to a measurement of the dipole polarizability of Zn-like Kr$^{6+}$, $\alpha _{d}$ = 2.53(2) a$_{0}^{3}$. Considerations involved in deducing a value of the quadrupole polarizability from the data and factors contributing to the signal and noise levels in measurements of this type will be discussed.