Molecular reaction dynamics at low energies

Full text: Low-energy collisions of small molecules represent model systems for complex quantum reaction dynamics. The dynamics of negative ion reactions are particularly interesting, being characterized by a corrugated potential energy landscape that originates in the competition of long-range attractive and short-range repulsive forces. Scattering on such multi-dimensional potentials only leads to reaction products if the different molecular degrees of freedom are sufficiently coupled. This leads to unexpected dynamical features in the scattering cross sections. To study ion-molecule reaction dynamics, we have developed two complementary experimental approaches. Using velocity map imaging in combination with crossed beams at low energy, the differential scattering cross section of negative ion reactions is measured at a controlled relative energy. This setup has allowed us to image the nucleophilic substitution reaction, a prototypical reaction in organic synthesis, and we have observed several distinct reaction mechanisms as a function of collision energy. Currently we are preparing experiments to investigate the stereodynamics of reactions that take place in a strong laser field. Using a 22-pole ion trap, which allows for efficient buffer gas cooling of all degrees of freedom of trapped molecular ions, we study reaction rates of negative ions at cryogenic temperatures down to 8 Kelvin. Due to the recent detection of negative ions in interstellar molecular clouds, these measurements have become important for understanding the interstellar abundance of anions. Absolute cross sections for anion photodetachment, a significant destruction mechanism in photon dominated regions, are measured in our 22-pole trap with high systematic accuracy. In addition, we have observed unexpected temperature-dependences for a proton transfer reaction as well as for a ternary cluster stabilization reaction at low temperatures. In the future this work will be extended to ultracold ion-atom interactions. (author)