Lay summary
Recent advances in the preparation of "cold" molecules and ions at very low temperatures T<<1 K in the gas phase have opened up perspectives to study chemical reactions in a new physical regime. In conjunction with recently established techniques for the simultaneous control of the internal (rotational-vibrational) motion of the cold molecules, it has become possible not only to investigate, but also to control chemical processes at a level of accuracy which has not been possible before.

The present project aims at investigating chemical reactions of atomic and molecular ions with neutral atoms at collision energies corresponding to the range 1 mK-1 K (often called the "cold regime") and thereby realising full control over the internal motion (quantum state) of the reaction partners. These objectives will be reached by extending a recently developed "hybrid trap" for the simultaneous trapping and cooling of atomic ions and neutral atoms by a recently developed method for the state-selected generation of molecular ions. Subsequent cooling of the motion of the ions will enable the study of reactive processes with ultracold Rb atoms down to mK energies.

In a first step, we will study the dynamics of reactive collisions of simple systems, i.e., atomic ions and ultracold atoms to characterise general features of cold ionic collisions. These include light-assisted processes such as radiative charge exchange and molecule formation. Our experiments will also serve to test ion-neutral chemistry models in the cold regime. Subsequently, we will enhance the scope of our studies to molecular collision partners and investigate reactive processes between state-selected molecular ions such as nitrogen and bromine molecular ions and ultracold Rb atoms. The aim is to to elucidate molecular phenomena in cold ionic collisions, in particular electron transfer and radiation-driven processes. Finally, we will take the first steps to extend our studies to reactions of polyatomic ions such as water, ammonia and carbon dioxide.