Binding energies of high-L Rydberg levels of Th 3+ were measured using the resonant excitation Stark ionization spectroscopy technique. Analysis of the data with the long-range polarization model leads to determination of dipole and quadrupole polarizabilities of the free Th 4+ ion, α d = 7.61(6) a.u., α Q = 47(11) a.u.

The binding energies of high-L Rydberg levels of Pb + with n = 19 or 20 and 6 L 10 were measured with resonant excitation Stark ionization spectroscopy (RESIS). When combined with previous measurements of members of the nh Rydberg series in Pb + , and analyzed with the long-range polarization model, these determined the polarizability of the 5d 10 6s 2 ground state of Pb 2+ to be α d = 13.62(8) a.u. This value is substantially larger than the estimate published recently based on the measured lifetime of the 6s6p 1 P 1 resonance level of Pb 2+ , 7.9(6) a.u. The difference is mostly due to the polarizability of Pb 4+ , the 5d 10 core of the Pb 2+ ion. This was established by observation of resolved fine structure in the excitation of n = 39, high-L Rydberg levels of Pb 3+ , using the same RESIS technique. Analysis of this spectra determined the polarizability of Pb 4+ to be α d = 3.61(4) a.u. Reanalysis of optical spectra of nh and ng levels in Pb + and Pb 3+ in view of the polarizabilities determined in the RESIS studies indicates revised ionization energies of both ions, E I (Pb + ) = 121245.28(6) cm −1 and E I (Pb 3+ ) = 341 435.1(8) cm −1 .

Binding energies of 23 Rydberg fine-structure levels of Ni with n = 9 and L 5 were measured using resonant excitation Stark ionization spectroscopy. From this spectrum, the quadrupole moment and the scalar and tensor dipole polarizability of the 2 D 5/2 ground state of Ni + were determined to be Q = −0.474(2), α d,0 = 7.92(6), and α d,2 = 1.15(14) a.u. The electric hexadecapole moment was determined to be −0.33(21) a.u.

Binding energies of high-L Rydberg states (L 7) of Th 2+ with n = 27-29 were studied using the resonant excitation Stark ionization spectroscopy (RESIS) method. The core of the Th 2+ Rydberg ion is the Fr-like ion Th 3+ whose ground state is a 5 2 F 5/2 level. The large-core angular momentum results in a complex Rydberg fine-structure pattern consisting of six levels for each value of L that is only partially resolved in the RESIS excitation spectrum. The pattern is further complicated, especially for the relatively-low-L levels, by strong nonadiabatic effects due to the low-lying 6d levels. Analysis of the observed RESIS spectra leads to determination of five properties of the Th 3+ ion: its electric quadrupole moment Q = 0.54(4); its adiabatic scalar and tensor dipole polarizabilities α d ,0 = 15.42(17) and α d ,2 = -3.6(1.3); and the dipole matrix elements connecting the ground 5 2 F 5/2 level to the low-lying 6 2 D 3/2 and 6 2 D 5/2 levels, | 5 2 F 5/2 ||D||6 2 D 3/2 | = 1.435(10) and | 5 2 F 5/2 ||D||6 2 D 5/2 | = 0.414(24). All are in atomic units. These are compared with theoretical calculations.

High-resolution studies of the fine-structure pattern in high-L n = 37 levels of Th 3+ 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 Th 4+ is determined to be α D = 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.