Ultracold RbSr: Optical and magnetic spectroscopy, Feshbach resonances and molecular structure

This thesis reports on the experimental investigation of the RbSr molecule, which is a strongly polar, open-shell molecule. Such molecules are the subject of intense and ongoing experimental interest around the world, but have never been brought to the quantum degenerate regime thus far. Hopefully, the methods and results presented in this thesis will lead to a change of this situation in the future. Two major results led to peer-reviewed publications in the context of this PhD thesis. The first is the experimental observation of magnetic Feshbach resonances between alkali and closed-shell atoms, never achieved before. This result opens the door to the magnetic association of atom pairs into molecules of a new kind, among them RbSr, that have never been produced in the laboratory due to a lack of known magnetic resonances. The second major result is the understanding of the potential energy curve of ground-state RbSr, an achievement of high importance for the future of the RbSr laboratory. Based on this potential energy curve, one can predict the collisional properties of Rb-Sr mixtures of any isotopic combination, the binding energies of all molecular states in the electronic ground state and the position of yet-unobserved Feshbach resonances. This thesis also reports on the exploration of potential energy curves of electronically-excited RbSr, and on the ongoing experimental progress made towards the creation of ultracold RbSr molecules