Two-Body and Three-Body Dynamics in Atom-Ion Experiments

During the last few decades, the two fields of ultracold neutral atoms and of single trapped ions have undergone an amazing development. The incorporation of both these systems into a single experiment exploits the complementary physical properties of the subsystems and opens up novel lines of research. In this thesis, I present experimental results obtained in one of the first hybrid atom-ion trap setups accessing the cold regime. Single Ba, Rb or Rb2 ions are brought to interaction with ultracold thermal or Bose-condensed samples of Rb atoms. The collision energies are set by heating mechanisms in the Paul trap and typically are on the order of mK·kB. Due to the polarizing effect that the excess charge of the ion has on the atoms, interactions are generally strong and longranged. We observe elastic atom-ion scattering in the semi-classical energy regime in which the cross section depends on the collision energy and favors scattering at small angles. Making use of our control over the atomic samples we were able to obtain a general good understanding of atom-ion two-body collisions. We observed sympathetic cooling of the ion over four orders of magnitude in energy through collisions with the ultracold atoms. Further, we utilized atom-ion scattering events to develop a novel and effective method to compensate excess ion micromotion, the primary source of ion heating in our experiments. Due to the long-range character of the interaction, three-body collisions between two atoms and the ion play an important role. We identified these processes through the large amounts of energy that they release. This makes them detectable despite their suppression with respect to two-body collisions. We also clearly showed that atom-atom-ion three-body collisions become more prominent as the collision energy decreases. Finally, we employed our atom-ion apparatus to gain novel insights into threebody recombination between ultracold atoms. In these experiments the ion trap acts as an extremely sensitive ion detector with essentially no background events. Through three-body recombination a Rb2 molecule is formed and can undergo resonanceenhanced multi-photon ionization by absorbing photons from a strong narrow-linewidth laser. By scanning the frequency of the ionization laser we obtained a molecular spectrum and identified the population of molecular levels with different vibrational, rotational, electronic and nuclear spin quantum numbers. This indicates that threebody recombination leads to a relatively broad distribution of final states.