Studies of U6+ with the RESIS method: difficulties and future directions

Abstract

This dissertation analyzes the details of previous resonant excitation Stark ionization spectroscopy (RESIS) measurements carried out on U5+ Rydberg states, aiming to determine properties of the ground state of U6+. These measurements were unsuccessful for apparently two reasons, a large background, and unexpectedly small signal sizes. It has been concluded that the background is a result of a prodigious amount of metastable states within the initial U6+ ion beam. Detailed simulations of the metastable population within the beamline showed the metastable hypothesis is plausible. Some reduction of the background within the RESIS technique was achieved with a redesign of the detection region in the apparatus. Detailed simulations of the RESIS signal size showed that the experimental resolved signals of the U5+ Rydberg states were no more than 1/4 the size expected from just ground state signal ions. A satisfactory explanation was proposed that metastable Rydberg ions bound to the lowest metastable level (J = 1) are forbidden to autoionize and can contribute to the measured signal. These metastable states compose 75% of the measured signal, diluting the ground state signal and preventing identification of the resolved signals. A possible future approach to the U5+ Rydberg state experiment is proposed that probes the Rydberg electron energies with a 1.5 μm laser, and looks like it could successfully measure the properties of the U6+ ion.