Rydberg spectroscopy of Fr-like thorium and uranium ions

Abstract

The binding energies of high L Rydberg levels of Th3+ were measured using the resonant excitation Stark ionization spectroscopy (RESIS) technique. When analyzed using the long range polarization model the measured energies determine the dipole and quadrupole polarizability of the Th4+ ion: αd = 7.61(6) a.u. and αQ = 45(4) a.u. The RESIS technique and apparatus constructed for this study are discussed in this work. Modifications to the original design of the detector are presented. The modifications to the detector increased the energy resolution of the beams in the detector. It was determined that a significant source of background present in the observations of the U5+ Rydberg fine structure is due to the presence of auto-ionizing Rydberg states attached to metastable excited ion cores. These auto-ionizing states severely limit the fine structure observations, preventing the successful observation of any U5+ Rydberg fine structure. Also discussed are future directions that could lead to an increase in the signal to noise in the Th3+ fine structure observations and to a successful measurement of the dipole polarizability of U6+.