Sympathetic cooling and rotational quenching of molecular anions in a hybrid atom ion trap

This dissertation reports sympathetic cooling of the molecular hydroxyl anion (OH– ) via collisions with ultra cold rubidium (Rb) atoms. The anion’s translational and rotational degrees of freedom were cooled simultaneously. A new generation hybrid atom-ion trap (HAITrap) has been set up, combining an octupole radio frequency (rf) trap for ion storage and a dark-spontaneous force optical trap (darkSPOT) for Rb atoms. While the octupole rf-trap provides a large rf-field-free region decreasing rf-induced collisional heating, the darkSPOT reinforces this effect by locating the atom cloud in the center of the ion trap. The darkSPOT also enables to generate large dark state fractions, suppressing associative detachment and thus ion losses. Evidence on translational sympathetic cooling in the HAITrap was provided using two methods of kinetic thermometry: photodetachment tomography (PD) and measurements of the time of flight (ToF) distribution. The ToF analysis has shown to be more conclusive and numerical simulations validated the experimental findings qualitatively. Based on a one dimensional ToF-temperature mapping, the narrowest measured energy spread would correspond to a temperature of 35 ± 5K. Deviations from thermal distributions were observed, as predicted from a theoretical model developed by our group. Rotational sympathetic cooling was observed via state selective photodetachment (SSPD). This was performed by pre-cooling the atoms with helium buffer gas at 295K and further cooling the ions with Rb at 300 μK. SSPD thermometry yielded a 70% decrease of the anions rotational temperature after interaction with Rb. These experimental findings could have direct applications in cold collision studies, quantum control of chemical reactions or sympathetic cooling of anti-protons.