Interactions of anions with ultracold atoms in a hybrid trap

At very low temperatures, molecules behave very differently compared to the temperatures of our natural environment. This offers new perspectives for the investigation of fundamental phenomena that are governed by quantum mechanics, such as cold collisions or chemical reactions. One also finds a wide range of exciting applications e.g. for high precision spectroscopy or quantum computation. During the last decade, an increasing number of techniques have been developed for preparing or creating cold and ultracold molecular samples. Nevertheless, an efficient method for cooling a whole class of molecules, negatively charged molecular ions, into the millikelvin regime is completely absent up to now. The present project aims at the investigation of the interaction dynamics between ultracold atoms and negative molecular ions. Our central goal is to demonstrate cooling of the motion of the molecular ions and of the rotations and vibrations that hot molecules undergo with ultracold atoms. Using the cold molecules, we seek to measure reaction probabilities in order to better understand reaction dynamics at low temperatures. For the planned experiments a special hybrid trap for cold atoms and ions will be employed that we have recently set up together. This project builds upon the well-established collaboration between our two groups at the University of Heidelberg, Germany, and the University of Innsbruck, Austria, respectively, combining their mutually complementary expertise in the physics of ultracold quantum gases, ion trapping and molecular reaction dynamics.

Within this project we have investigated the interaction of a cloud of ultracold atoms with trapped negatively charged ions. The aim was to find out if the negative ions could be cooled by collisions with the cold atoms to temperatures near or even below the temperature of liquid helium. To pursue this experiment, we have combined a magneto-optical trap for laser-cooled rubidium atoms with an octupole radiofrequency ion trap that is loaded with an ensemble of mass-selected negative ions. We have implemented laser-based photodetachment to determine the spatial density distribution of the trapped ions. From the time-of-flight distributions of the extracted ions we could determine the translational temperature of the ions in the trap. This analysis was based on extensive numerical modeling of the ion motion in the trap and upon extraction to the time-of-flight detector. Studying O- and OH- ions we were finally able to observe cooling due to collisions with the ultracold rubidium atoms. Cooling of the O- anions was also achieved using forced photodetachment of the fastest ions in the trap, followed by re-thermalization. The cooling experiments with OH- were hindered by chemical reactions with the rubidium atoms, which lead to ion loss from the trap. We studied these competing processes and identified interesting reaction dynamics, which are distinctly different for rubidium in the ground state or the optically excited state. In comparison with theoretical calculations we also discovered that the reaction of OH- with Rb only proceeds for a well-defined range of orientations of the negative ion. Finally, we extended the experiments to negatively charged water clusters in order to address the transition from gas to condensed phase environments. Our experiments show that cooling of negative ions with ultracold atoms is indeed feasible, in particular if the reactivity is suppressed as for the case of O-. This may open up new perspectives for experiments on cold collisions or high precision spectroscopy of cold negative ions. The project has been carried out in close collaboration between two groups at the University of Heidelberg, Germany, and the University of Innsbruck, Austria. This collaboration and the associated exchange of researchers was essential for the success of this project.