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.
In this thesis the dynamics of a trapped ion immersed in \na spatially localized buffer gas is investigated. For a homogeneous buffer gas, the ion’s energy distribution reaches a stable equilibrium only if the mass of the buffer gas atoms is below a critical value. This limitation can be overcome by using multipole traps in combination and/or a spatially confined buffer gas. Using a generalized model \nfor elastic collisions of the ion with the buffer gas atoms, the ion’s energy distribution is numerically determined for arbitrary buffer gas distributions and trap parameters. Three regimes characterized by the respective analytic form of the ion’s equilibrium energy distribution \nare found. One of these is a novel regime at large atom-to-ion mass ratios where the final ion temperature can tuned by adiabatically decreasing the spatial extension of the buffer gas and the effective ion trap depth (forced sympathetic cooling). The second part of the thesis presents a hybrid atom ion trap designed for sympathetic cooling of hydroxide anions. In this hybrid trap the anions are immersed in a cloud of laser cooled rubidium atoms. The translational and rovibrational temperatures of the anions is probed by photodetachment tomography and spectroscopy which shows the first ever indication of sympathetic cooling of anions by laser cooled atoms.
We are developing a new hybrid atom-ion trap to study the interaction of ultracold rubidium atoms with mass-selected OH- molecules. The ions are trapped inside an octupole rf-trap made of thin wires instead of the commonly used rods. This ensures good optical access to the center of the trap where the ions can be overlapped with laser cooled rubidium atoms stored in a dark spontaneous force optical trap (dark SPOT). This setup provides high collision rates since the density in a dark SPOT is about one order of magnitude higher than in a standard magneto-optical trap. Further, inelastic collisions with excited atoms are suppressed since almost all atoms are in the ground state. Numerical simulations of our setup using SIMION predict that cooling of the ions is feasible.
Abstract We present a theoretical investigation of reactive and non-reactive collisions of Li and Rb atoms with C 2 − molecular anions at low temperatures in the context of sympathetic cooling in hybrid trap experiments. Based on accurate potential energy surfaces for the singlet and triplet states of the Li– C 2 − and Rb– C 2 − systems, we show that the associative electronic detachment reaction is inefficient but that it occurs with a large rate for collisions involving excited states of either Li/Rb or C 2 − . We also investigate rotationally inelastic collisions in order to explore the cooling of the translational and rotational degrees of freedom of C 2 − in hybrid ion-atom traps. The effect of micromotion is taken into account by considering Tsallis distributions of collision energies. We show that the translational cooling occurs much more rapidly than rotational cooling and that the presence of excited atoms leads to losses of anions on a timescale comparable to that of rotational cooling.
We report close-coupling (CC) quantum dynamics calculations for collisional excitation/de-excitation of the lowest four rotational levels of OD − and of OH − interacting with Rb atoms in a cold ion trap. The calculations are carried out over a range of energies capable of yielding the corresponding rates for state-changing events over a rather broad interval of temperatures which cover those reached in earlier cold trap experiments. They involved sympathetic cooling of the molecular anion through a cloud of laser-cooled Rb atoms, an experiment which is currently being run again through a Heidelberg–Innsbruck collaboration. The significance of isotopic effects is analysed by comparing both systems and the range of temperatures examined in the calculations is extended up to 400 K, starting from a few mK. Both cross sections and rates are found to be markedly larger than in the case of OD − /OH − interacting the He atoms under the same conditions, and the isotopic effects are also seen to be rather significant at the energies examined in the present study. Such findings are discussed in the light of the observed trap losses of molecular anions.
This Dissertation details the development of state-of-the-art hybrid atom-ion trapping architecture and technique towards increasing the quantum control of matter and the detection of chemical processes at cold temperatures. Experimental work discussed herein is performed primarily using the second-generation of the MOTion trap, a hybrid atom-ion trap consisting of a co-located magneto-optical trap (MOT) and a linear quadrupole trap (LQT), with which
40Ca and a variety of atomic and molecular ions (Yb
+, Ba
+, BaCl
+, and others) can be co-trapped and cooled. Such a device enables the immersion of laser-cooled, ionic Coulomb crystals in diffuse gases of laser-cooled, highly-polarizable neutral atoms, allowing for studies in sympathetic cooling, cold atom-ion chemistry, and nonequilbrium collisional dynamics.In the first of these studies, I discuss our method of sympathetic cooling in which we use a laser-cooled gas of Ca atoms to sympathetically cool BaCl
+ ions confined in the LQT. Because of the large polarizability of Ca, this method is found to be extremely efficient at cooling the notoriously difficult vibrational degree of freedom. This technique opens the door to research with general, ultracold, highly-localized molecular ions in the quantum ground-state which are not able to be produced using the more traditional methods of Doppler-cooling or photoassociation.In the study of cold chemistry in the hybrid trap, I characterize a fundamental reaction, known as charge exchange, at the single-particle level in which an electron is transferred from a neutral atom to an atomic ion during an inelastic collision. This low-temperature process is thought to be foundational to the chemistry leading to the formation of stars, planets and interstellar clouds in our universe. Herein I discuss how improved experimental architecture has enabled the determination of reaction rate constants for individual electronic states of the reactants.Our theoretical study of the collisional processes in hybrid atom-ion traps has led to the discovery of surprising nonequilibrium physics which exists in the hybrid trap environment. Herein I outline the collisional dynamics between neutral atoms and atomic ions in the presence of a time-dependent confining potential and describe how these dynamics manifest themselves in the emergence of nonequilbrium phenomena as indicated by bifurcations in steady-state energies of trapped ions. I show how the these dynamics can potentially limit the efficiency and overall viability of using neutral buffer gases to sympathetically cool the translational motion of trapped ions to ultracold temperatures.In addition to these experimental studies, the development of architecture is discussed with focus given to the implementation of a novel time-of-flight mass spectrometer (TOF-MS) for use with hybrid atom-ion traps as well as standard LQTs. This simple, modular device couples radially with the LQT and enables the straightforward mass analysis of trapped species without complications derived from typical experimental difficulties, engineering constraints, and financial concerns. With this robust device we precisely characterize chemical processes which can take place in these hybrid atom-ion traps. Additionally, I describe how this new architecture is used to perform the action spectroscopy of several previously unstudied molecular ions in order to further establish the field of cold molecular ion research and the widespread use of hybrid atom-ion traps.
The detachment loss dynamics between rubidium atoms (Rb) and oxygen anions (O−) are studied in a hybrid atom–ion trap. The amount of excited rubidium present in the atomic ensemble is actively controlled, providing a tool to tune the electronic quantum state of the system and, thus, the anion–neutral interaction dynamics. For a ground state Rb interacting with O−, the detachment induced loss rate is consistent with zero, while the excited state Rb yields a significantly higher loss rate. The results are interpreted via ab initio potential energy curves and compared to the previously studied Rb–OH− system, where an associative electronic detachment reactive loss process hinders the sympathetic cooling of the anion. This implies that with the loss channels closed for ground-state Rb and O− anion, this system provides a platform to observe sympathetic cooling of an anion with an ultracold heavy buffer gas.