In-source high-resolution spectroscopy of holmium radioisotopes - On-line tailored perpendicular laser interaction at ISOLDE's Laser Ion Source and Trap LIST

The combination of resonant laser ionization and electromagnetic mass separation has become an extremely powerful and versatile tool for the study of atomic and nuclear physics properties of exotic species over the last decades. It exploits step-wise resonant excitation and subsequent ionization by laser light precisely tuned to unique, fingerprint-like resonances in the electronic shell as a technique, which is both highly efficient and inherently element-selective. It is applied at the majority of large scale radioactive ion beam facilities world-wide, directly coupled to the production of radioactive species which are presented to laser ionization in a hot atomic vapor cavity. This ion source type is an excellent environment for direct laser spectroscopic investigations on the atomic structure. Fundamental ground state properties of the nucleus can be derived from minute effects arising from interactions with the electronic shell. Most commonly, such hyperfine structure investigations are prevented by the limited spectral resolution due to Doppler broadening in the hot atomic vapor. This thesis comprises the development, characterization and first-time application of a laser ion source specialized for high resolution laser spectroscopy. It is based on the Perpendicularly Illuminated Laser Ion Source and Trap (PI-LIST), which was recently developed in the LARISSA group, and uses a perpendicular geometry between incident laser light and the atomic beam effusing from a hot cavity to overcome Doppler limitations. As this previous setup relies on lateral laser in-coupling through windows at the source vacuum vessel, the upgrade presented here is tailored for installation at radioactive ion beam facilities such as CERN-ISOLDE, where access to the highly radioactive front end area is impossible. With this novel tool, high resolution hyperfine structure studies on long-lived, radioactive holmium isotopes were performed at the RISIKO mass separator at Mainz University. The extremely dense spectra arising from the coupling of both high nuclear spin and total angular momenta of the atomic states demanded high spectral resolution far beyond the capabilities of a standard hot cavity laser ion source. With outstanding experimental line widths as low as 100 MHz FWHM, the first-time extraction of nuclear structure parameters of $^{166m}$Ho was possible. Its nuclear spin was confirmed as $I$ = 7. Additionally, the results on the magnetic dipole moment, the spectroscopic quadrupole moment and the first determination of the change in mean square charge radii towards the neutron-rich side of the holmium isotopic chain resolve inconsistencies in literature data and confirm the pronounced prolate deformation in this mid-shell region of the nuclear chart. The PI-LIST with internal reflection proved to be completely competitive to the previous mode with external lateral laser incoupling in all operational parameters. On dedicated characterization experiments, ultimate line widths of as low as 60 MHz were achieved, and an overall efficiency of the setup of up to the 10$^{-4}$ regime was confirmed. The PI-LIST is thus proven to be ready for installation at on-line facilities.