  We present a precise measurement of the rubidium ionic core polarizability.
The results can be useful for interpreting experiments such as parity violation
or black-body radiation shifts in atomic clocks since the ionic core electrons
contribute significantly to the total electrical polarizability of rubidium. We
report a dipole polarizability $\alpha_d$ = $9.116 \pm 0.009$ $a_0^3$ and
quadrupole polarizability $\alpha_q$ = $38.4 \pm 0.6$ $a_{0}^{5}$ derived from
microwave and radio-frequency spectroscopy measurements of Rydberg states with
large angular momentum. By using a relatively low principal quantum number ($17
\leq n \leq 19$) and high angular momentum ($4 \leq \ell \leq 6$), systematic
effects are reduced compared to previous experiments. We develop an empirical
approach to account for non-adiabatic corrections to the polarizability model.
The corrections have less than a 1\% effect on $\alpha_d$ but almost double
$\alpha_q$ from its adiabatic value, bringing it into much better agreement
with theoretical values.
