A Fragile Hold on the Electron: Probing the Limits of Negative Ion Studies

Negative ions are unique quantum systems whose structures and dynamics are significantly influenced by electron correlation effects. Due to their weakly bound nature and the absence of long-range Coulomb potentials, these systems often lack optically allowed transitions, making high-resolution spectroscopic studies compared to those of neutral systems challenging.
Therefore, negative ion research has traditionally focused on bound continuum measurements, primarily determining electron affinity (EA) values with techniques such as laser photodetachment threshold spectroscope (LPTS). However, accurately determining the EA for ions with electrons detached into a p-wave continuum has been limited by the slow onset of the photodetachment just above the threshold and inherently smaller photodetachment cross sections. This thesis addresses these challenges by refining a combined method of LPTS and resonance ionization spectroscopy (RIS), enabling state-selective
measurements of partial photodetachment cross sections. A novel detection scheme to differentiate signal from background was implemented, greatly improving the measurement selectivity and overall experimental resolution.
The enhanced LPTS-RIS approach was demonstrated through improved EA measurements of cesium (Cs) and rubidium (Rb), significantly surpassing previous LPTS accuracies for determining p-wave detachment thresholds, thereby validating the method for future studies of more complex atomic systems. Extending these experimental investigations to radioactive negative ions
requires efficient production techniques. This thesis successfully demonstrated the feasibility of using charge exchange reactions to produce radioactive negative ions with uranium. A complementary theoretical study explored electron capture and energy loss processes in atom-ion collisions, highlighting critical factors that influence trajectory effects. By incorporating electron-nuclear coupling within the framework, the developed theoretical approach provides deeper insights essential for refining negative ion production models.
Most importantly, this work introduces an entirely new spectroscopic technique for probing previously inaccessible forbidden bound-bound transitions in negative ions. Leveraging the unique capabilities of a cryogenic ion storage ring, the first isotope shift (IS) measurement of an electric dipole forbidden transition was conducted using the tin anion (Sn−). The sensitivity of IS studies to electron correlation effects, particularly through the specific mass shift component, underscores the potential of this method. This work shows that a combined experimental and theoretical approach can overcome key limitations in negative ion spectroscopy, making it possible to probe systems and transitions that were previously out of reach. By extending high-precision methodologies to heavy ions and radioactive isotopes, developing simple yet sophisticated theoretical models, and pioneering studies of forbidden transitions, this thesis significantly expands the scope of negative ion research and opens new avenues for exploring atomic, nuclear, and quantum many-body physics.