  High resolution studies of the fine structure pattern in high-L n=37 levels
of Th3+ have been carried out using radio-frequency (rf) spectroscopy detected
with Resonant Excitation Stark Ionization Spectroscopy (RESIS). Intervals
separating L=9 to L=15 levels have been measured, and the results analyzed with
the long-range effective potential model. The dipole polarizability of Th4+is
determined to be aD= 7.720(7) a.u.. The quadrupole polarizability is found to
be 21.5(3.9) a.u. Both measurements represent significant tests of a-priori
theoretical descriptions of this highly relativistic ion.
