This dissertation describes experiments performed in the MOTion trap, a hybrid atom--ion trap comprised of a magneto-optical trap (MOT) and a linear Paul trap with an integrated time-of-flight mass spectrometer (ToF-MS).We aim to sympathetically cool the vibrational and rotational degrees of freedom of a BaCl$^+$ molecular ion by overlapping a cold ($\\sim4$~mK) Ca MOT.\nThis sympathetic cooling can prepare a molecular ion in its rovibrational quantum ground state, providing a method for state preparation for applications in molecular ion quantum logic.\nWhile collisions between the molecular ion and the cold atoms can cool the internal degrees of freedom, chemical reactions are also possible for electronically excited atoms.\nAs such, we study these excited-state atom--ion reactions at low temperature $<1$~K and develop tools to control these collisions.\nWe make the first experimental observation of reaction blockading, an effect that suppresses excited-state reactions at low temperature, and develop a method to reverse this suppression if desired with the addition of a catalyst laser, providing a means to optically control such reactions.Beyond its intended purpose of studying and achieving sympathetic cooling, we find that this apparatus is a powerful tool for chemical studies.By imaging the ions, we can observe reactions occurring one atom at a time.\nWe can precisely tune the collision energy or temperature down to the mK regime to as high as $\\sim$10~K using techniques described in this dissertation, allowing controlled interrogation of these low-temperature reactions.\nFurther, using lasers to address electronic transitions, we can precisely control the quantum state of the reactants, and we can measure the reaction products with the ToF-MS.\nA testament to the value of this apparatus is the study of BaOCa$^+$ described in this dissertation.\nUsing the tools of the MOTion trap, we were able to identify the production of BaOCa$^+$, the first observation of a mixed hypermetallic oxide, and determine the path of reaction as BaOCH$_3^+$ + Ca($4s4p$~$^3P_J$) $\\rightarrow$ BaOCa$^+$ + CH$_3$.In addition to developing control of these atom--ion interactions towards sympathetic cooling of molecular ions, we also work towards further developing new quantum logic schemes for molecular ions, such as dipolar quantum logic, dipole--phonon quantum logic (DPQL), and electric-field gradient gates (EGGs).Specifically, we propose a class of molecules particularly well-suited for DPQL, the alkaline earth monoxide cations, that can also be added to existing atomic ion experiments with minimal additional experimental complexity.\nWe further develop the techniques for DPQL and propose methods for single- and two-qubit gates, outlining a path towards universal quantum computation with molecular ions.
This dissertation describes experiments performed in the MOTion trap, a hybrid atom--ion trap comprised of a magneto-optical trap (MOT) and a linear Paul trap with an integrated time-of-flight mass spectrometer (ToF-MS).We aim to sympathetically cool the vibrational and rotational degrees of freedom of a BaCl+ molecular ion by overlapping a cold (sim4mK) Ca MOT.\nThis sympathetic cooling can prepare a molecular ion in its rovibrational quantum ground state, providing a method for state preparation for applications in molecular ion quantum logic.\nWhile collisions between the molecular ion and the cold atoms can cool the internal degrees of freedom, chemical reactions are also possible for electronically excited atoms.\nAs such, we study these excited-state atom--ion reactions at low temperature <1K and develop tools to control these collisions.\nWe make the first experimental observation of reaction blockading, an effect that suppresses excited-state reactions at low temperature, and develop a method to reverse this suppression if desired with the addition of a catalyst laser, providing a means to optically control such reactions.Beyond its intended purpose of studying and achieving sympathetic cooling, we find that this apparatus is a powerful tool for chemical studies.By imaging the ions, we can observe reactions occurring one atom at a time.\nWe can precisely tune the collision energy or temperature down to the mK regime to as high as sim10K using techniques described in this dissertation, allowing controlled interrogation of these low-temperature reactions.\nFurther, using lasers to address electronic transitions, we can precisely control the quantum state of the reactants, and we can measure the reaction products with the ToF-MS.\nA testament to the value of this apparatus is the study of BaOCa+ described in this dissertation.\nUsing the tools of the MOTion trap, we were able to identify the production of BaOCa+, the first observation of a mixed hypermetallic oxide, and determine the path of reaction as BaOCH3+ + Ca(4s4p3PJ) rightarrow BaOCa+ + CH3.In addition to developing control of these atom--ion interactions towards sympathetic cooling of molecular ions, we also work towards further developing new quantum logic schemes for molecular ions, such as dipolar quantum logic, dipole--phonon quantum logic (DPQL), and electric-field gradient gates (EGGs).Specifically, we propose a class of molecules particularly well-suited for DPQL, the alkaline earth monoxide cations, that can also be added to existing atomic ion experiments with minimal additional experimental complexity.\nWe further develop the techniques for DPQL and propose methods for single- and two-qubit gates, outlining a path towards universal quantum computation with molecular ions.
This dissertation focuses on observing and controlling cold atom-ion collisions. Such collisions have numerous applications ranging from providing insight into the formation of the interstellar medium to presenting a potential platform for developing quantum information architectures. The experimental apparatus at the center of this work is a hybrid atom-ion trapping system in which laser-cooled Ca atoms held within a magneto-optical trap are spatially overlapped with ions localized within an ion trap. The ions studied in this work are both laser-cooled atomic ions as well as sympathetically cooled molecular ions. As detailed in the remainder of this thesis, by employing tools such as optical pumping, collision energy control, and mass spectrometry, reactions between these species can not only be studied with high precision but can also be controlled, leading to expanded tools for reaction engineering as well as the creation of exotic chemical species. Finally, this work also includes efforts to use inelastic collisions between room temperature BaCl^+} molecular ions and cold Ca atoms to create ro-vibrational ground state molecules, a precursor for developing a high-fidelity qubit based on rotational levels in polar molecules.
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.
This dissertation details of the construction of and experiments performed in a hybrid system consisting of a cold neutral atomic magneto-optical trap (MOT) contained within a linear radio-frequency quadrupole ion trap (LQT). The combination of these two workhorses of atomic physics facilitates a variety of developing science such as the controlled investigation of ion-neutral quantum chemistry and the production of ground-state molecular ions. While the primary focus of this work is the production of ultracold molecular ions via sympathetic cooling, there has been investigation of cold (T ~ mK) ion-neutral charge exchange processes in two different ion-neutral pair systems. In addition, a possible method of producing ultracold homonuclear molecular ions via a photo-associative ionization (PAI) pathway is studied in the MOT. The LQT traps ions in spatial overlap with a 40Ca MOT constituting an ultracold buffer gas purposed to sympathetically cool molecular ions.An effective, general method of producing ground-state molecular ions would open a field of physics allowing, for example, precise measurements of molecular transitions which are uniquely sensitive to parity violation, or the possible variation of fundamental constants. Another promising application of cold molecular ions is an implementation of a quantum computing architecture by coupling micro-fabricated strip-line resonators to the microwave rotational transition found in many diatomic molecular ions. The molecular ion BaCl+ is chosen to be used in proof-of-principle sympathetic cooling experiments. Ultimately, a vibrational internal state thermometry experiment shows that the Ca MOT performs very efficiently at quenching the vibrational motion of the BaCl+ molecular ion.As a product of overcoming experimental challenges, this thesis discusses several experiments to characterize trapping and reaction dynamics in the hybrid system. For example, charge transfer measurements are performed with two different laser-cooled atomic ion species, Yb+ and Ba+, which are allowed and disallowed ground-state reaction, respectively. Controlled measurements of these inelastic processes test the cutting edge of quantum theory of ion-neutral interaction in both the ground-state and under optical excitation.
In recent years there has been much progress in the field of cold and ultracold molecular physics and a variety of experimental techniques for producing cold matter now exist. In particular, the generation of trapped molecular ions at mK temperatures has been achieved by sympathetic-cooling with laser-cooled atomic ions. By implementing schemes to selectively prepare and control the internal quantum state of molecular ions, and developing detection techniques, it will be increasingly possible to study cold state-selected chemical collisions in an ion-trap. Most molecular species produced in a selected rovibrational state have a lifetime of a few seconds, before the population is redistributed across numerous rovibrational states by interaction with the ambient blackbody radiation (BBR). Consequently, the investigation of state-selected reaction dynamics at low temperatures in experiments where long time scales (minutes to hours) are required, is hindered. This thesis looks into developing strategies that maintain state selection in molecular ions, allowing one to observe state-selected reactions in cold environments, in particular the state-selected reaction between C2H+2 and ND3. Examining reactive ion molecule collisions under cold conditions provides insight into fundamental reaction dynamics, which are thermally averaged out at higher temperatures. A theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several 1Σ and 2Π diatomic species. The rotational cooling is particularly effective for DCl+ and HCl+, for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% more than that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the achievable rotational cooling is also studied. This approach is extended to consider the BBR interaction and rotational cooling of a linear polyatomic ion, C2H+2, which has a 2Π electronic ground state. The (1-0) band of the ν5 cis-bending mode is infrared active and strongly overlaps the 300 K blackbody spectrum. Hence the lifetimes of state-selected rotational levels are found to be short compared to the typical timescale of ion trapping experiments. Laser cooling schemes are proposed that could be experimentally viable, which involves simultaneous pumping of a set of closely spaced Q-branch transitions on the 2Δ5/2-2Π3/2 band together with two 2Σ+– 2Π1/2 lines. It is shown that this should lead to >70% of total population in the lowest rotational level at 300 K and over 99% at 77 K. In order to identify states of the acetylene ion that could be trapped sufficiently long enough for state-selected reactions in the ion trap with decelerated ND3, the theoretical work has been complemented by experimental investigations into the production of C2H+2 in selected states, and ion trapping of the same using sinusoidal and digital trapping voltages. Appropriate (2+1) REMPI (Resonance Enhanced Multiphoton Ionization) schemes are used to produce C2H+2 in different quantum states, with (1+1) Resonance Enhanced Multiphoton Dissociation (REMPD) employed to detect the ion thus produced. The concept of digital ion trapping for ejection onto MCPs is introduced. A comprehensive comparison between sinusoidal and digital trapping fields has been performed with respect to trap depth and stability regions. Programs have been developed to calculate the stability regions for different ions under varying experimental conditions. The trap depth has been derived for both digital and sinusoidal trapping fields. It is observed that as τ increases, the trap depth of a digital trap increases. For τ = 0.293, the trap depth and stability diagram for both sinusoidal and digital trapping fields would be equivalent. The trap depth at which the sinusoidal trap operates experimentally in our research group is ˜1.36 eV. In contrast, the experimental parameters at which the digital trap operates generates a trap depth of 1.21 eV. Ca+ Coulomb crystals have been formed, stably trapped and stored for extended periods of time in both sinusoidally and digitally time-varying trapping fields. The sympathetic cooling of a diverse range of ions into Ca+ Coulomb crystals is demonstrated, again using both sinusoidal and digital trapping fields. Mass spectrometric detection of ionic reaction products using a novel ejection scheme has been developed, where ejection is achieved by switching off the trapping voltage and converting the quadrupole trap into an extractor-repeller pair by providing the ion trap electrodes with appropriate ejection pulses. This technique is developed using a digital trapping voltage rather than the sinusoidal trapping voltage, as ejection with sinusoidal trapping voltages is not clean (resonance circuitry used in the electronics induces ringing after switching off the trapping voltage). Coulomb crystals, both pure Ca+ and multi-component crystals, are ejected from the ion trap and the TOF trace obtained is recorded on an oscilloscope. When the integrated, base-line subtracted TOF peak is plotted against the number of ions in a Ca+ crystal and sympathetically-cooled Ca+ – CaF+ crystal, a linear relationship is obtained. This technique is found to be well mass-resolved, with the signal arising from CaOH+ (57 amu) and CaOD+ (58 amu) resolvable on the TOF trace. This technique would enable one to monitor a reaction in a Coulomb crystal where the reactant and product species are both either lighter or heavier than calcium, such as the reaction between C2H+2 and ND3, something which has not been previously possible. It is, also, potentially a very important technique for reactions with many product channels.
We describe a versatile experimental apparatus designed for investigating ion-molecule reaction dynamics at low temperatures under ultra-high vacuum conditions (<8 × 10-11 Torr). The system integrates laser-cooled atomic ions confined in a linear Paul trap with a pulsed molecular beam featuring simultaneous quantum-state and velocity selection. Reaction products are identified using fluorescence imaging and a time-of-flight mass spectrometer. The setup accommodates a broad range of ionic and neutral species-including Be+, C+, H2, N2, O2, H2O, and others-enabling high-sensitivity studies of reaction dynamics. As a benchmark, we present measurements of the Be+ + H2 reaction across a range of collision energies, including reactions with H2 vibrationally excited to the v = 1 level. The apparatus offers fine control over both internal and external degrees of freedom, providing a general platform for cold ion-molecule reaction studies.
Trapping molecular ions that have been sympathetically cooled with laser-cooled atomic ions is a useful platform for exploring cold ion chemistry. We designed and characterized a new experimental apparatus for probing chemical reaction dynamics between molecular cations and neutral radicals at temperatures below 1 K. The ions are trapped in a linear quadrupole radio-frequency trap and sympathetically cooled by co-trapped, laser-cooled, atomic ions. The ion trap is coupled to a time-of-flight mass spectrometer to readily identify product ion species and to accurately determine trapped ion numbers. We discuss, and present in detail, the design of this ion trap time-of-flight mass spectrometer and the electronics required for driving the trap and mass spectrometer. Furthermore, we measure the performance of this system, which yields mass resolutions of m/Δm ≥ 1100 over a wide mass range, and discuss its relevance for future measurements in chemical reaction kinetics and dynamics.