Determination of Ion-Neutral Three-Body Association Rates

When three particles collide with each other, two of them may form a stable complex while the third particle removes the complexation energy from the collision. Such three-body association collisions are ubiquitous in many environments where they form the initial step of atomic and molecular condensation into clusters, droplets, or grains. These collisions are much more difficult to describe theoretically than collisions between only two atoms, molecules and ions. At the same time their collision rates are also difficult to measure accurately because of the very small magnitude in few-atom systems. In recent years, both semiclassical and quantum mechanical calculations were performed for three simple atoms, while full quantum mechanical calculations are still not available for collisions involving more than three atoms. In the present project, we will study several types of fundamental three-body collisions, in order to gain new insight into these collisions, identify the role of quantum effects, and clarify the importance of direct versus step-wise collision processes. For this we will combine measurements of temperature-dependent collision rates with different semiclassical and quantum calculations. The experiments will be carried out by the Innsbruck team using cryogenic radiofrequency ion traps, which provide high sensitivity to extremely low-rate coefficients. The theoretical work will be carried out by the Bordeaux team together with international partners, who will use statistical methods, quasiclassical trajectory simulations, and full quantum scattering calculations. This collaborative approach, which is founded on many years of successful joint work by our research groups, will allow us to unveil the specific mechanisms that govern three- body collisions.