State-selected collisions of cold molecular ions

Cold molecules are studied in many laboratories around the world, because they allow for new approaches to perform precision measurements and well-controlled chemical reactions. Charged molecular species are often investigated in radiofrequency ion traps where the cooling of the rotational and vibrational motion of the molecules relies on collisions with a cold gas. The efficiency for the cooling process, however, is in most cases not very well known. In this project we propose to measure and calculate cooling rates for selected molecular ions colliding with neutral helium atoms at temperatures between about 10 and 100 Kelvin. Furthermore, we will investigate reactions with molecular hydrogen that compete with the quenching collisions. Experimentally, laser-spectroscopy will be performed while the molecular ions undergo the collisions with helium or hydrogen. From these data the cooling efficiencies and reactivities will be extracted. Theoretically, quantum mechanical calculations will be performed to extract the cooling efficiencies numerically. The comparison between experiment and theory will provide us with a unique test bed for the underlying quantum mechanical description of these collision processes. In particular we will search for quantum scattering resonances, quantum effects that have no classical analogue. The project will be carried out at the Institute for Ion Physics and Applied Physics of the University of Innsbruck.

Within this project we studied collisions of positively and negatively charged molecules where the molecular rotational or vibrational excitation is reduced through the interaction with another atom. To do this we combined different experimental and theoretical methods. By trapping molecular ions in radiofrequency ion traps that are operated at low temperature we measured the probability for rotational and vibrational relaxation processes. To be sensitive to these processes, tunable laser light was used, which allowed us to neutralize molecular ions in specific rotational or vibrational quantum states of motion. Accurate theoretical calculations were carried out in two steps, first the interaction potential and the corresponding forces between a molecular ion and a separate atom were calculated. Then the probability for scattering that leads to a reduced rotational or vibrational excitation was determined. Comparison of the probabilities for relaxation processes in experiment and theory provides detailed insight into the quantum mechanics of the collision processes. Such probabilities are also of interest for modelling the behavior of molecular ions in naturally occurring environments such as the Earth's upper atmosphere or interstellar clouds of cold molecular gas and dust. Several different molecular ions were studied in the low temperature range up to about 50 Kelvin that is particularly relevant for quantum effects and for applications in interstellar clouds. Furthermore, we investigated ion-molecule reactions with molecular hydrogen that can compete with the quenching collisions. One of our findings was that for the hydroxyl cation the quantum mechanical spin rotation does not require an explicit treatment in the theoretical models. Another finding was that molecular amide anions, which populate two different rotational quantum states at low temperatures, can be state-selectively removed from the trap to leave only one quantum state trapped. As a third example, the knowledge of their collisions allowed us to measure the absorption frequencies of amide anions in the terahertz range with unprecedented accuracy. The results from this project contributed to the thesis work of several PhD and master students. It was carried out at the Institute for Ion Physics and Applied Physics of the University of Innsbruck.