Binding Energies and Lifetimes in Negative Ions

Abstract

Negative ions hold significant interest because of their importance in understanding electron correlation. Further, they capture substantial focus for their role in, for example: stellar environments, medical applications, antimatter research, and accelerator mass spectrometry (AMS).

This thesis covers experimental studies of both the structure and dynamics of negative ions at the ion beam facilities DESIREE (Double ElectroStatic Ion Ring ExpEriment), CERN-ISOLDE (Isotope Separator OnLine DEvice) and GUNILLA (Gothenburg University Negative Ion and Laser LAboratory).

At DESIREE, the electron affinites (EA) for the three stable isotopes of silicon have been measured with high precision, using lasermanipulation of quantum-state populations followed by laser photodetachment threshold (LPT) spectroscopy. The corresponding isotope shifts in the EA have been calculated. Additionally, the hyperfine splitting of the ground state in 29Si− was measured.

The EA for two radioactive isotopes, 128I and 211At, have been determined using LPT spectroscopy with the GANDALPH (Gothenburg ANion Detector for Affinity measurements by Laser PHotodetachment) detector at ISOLDE. These are the first ever EA measurements of radioactive isotopes. This opens up a whole new field of experiments where the EA of even heavier ions can be measured, and giving the possibility of measuring isotope shifts in radioactive isotopes.

At the ion beam facility GUNILLA at the University of Gothenburg, the EA for rubidium has been measured using a state selective detection of the residual atom in the LPT processes. If this selective measurement technique is combined with the possibility of studying radioactive beams at ISOLDE, it can be applied to studies of rare anions, like francium.

In terms of dynamical properties, the radiative lifetimes of excited states in several atomic and molecular anions have been measured at DESIREE and some previously unobserved energy states have been detected. The methods used for detailed studies of the structure of negative ions used in this work lay the foundation for the ultimate goal to map out the lifetimes of all the excited states in negative ions.

The research presented in here shows that it is only by combining structural and dynamical experimental results that it is possible to obtain a complete picture of negative ions. The results shown so far have been used to benchmark theoretical methods, but there are still quite a few discrepancies. By applying the methods used in this work to the full range of elements in the periodic table, and comparing them with theoretical results, it will be possible to enhance our understanding of electron correlation. Of particular interest will be to study the very heavy systems, where relativistic effects play a decisive role.