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Studying Charge-Exchange in a Hybrid Ion-neutral Trap

Jonathan M. Kwolek

Published Jan 1, 2018• 0 citations
OpenAlex

Abstract

Interactions between neutral and ionized atoms are medium-range, and the spiraling nature of these collisions can lead to long interaction times and large cross-sections. The field of ion-neutral interactions is bolstered by methods from atomic physics, which allow us to accurately control and determine the quantum-states of the reactants. Trapping techniques allow us to study these interactions over a large range of energies, in order to observe interesting effects from the ultracold to hot temperature regimes. This control allows us to analyze and determine specific channels for the reaction, allowing for theoretical verification.\nThe hybrid-trap enables the concentric trapping of a sample of cold atoms and ions. This work discusses our exploration into the controlled reactions in the $\\text{Na}-\\text{Ca}^+$ system, including measurements of sympathetic cooling and charge-exchange rates which change as a function of energy and molecular state. The quantum-state distributions of Na in our magneto-optical trap is also explored, as well as its agreement and subsequent deviation based on a predictive model. The measurements of reaction rate reveal that there are reaction-energy thresholds in two separate reaction pathways, which can be attributed to either an endothermic reaction pathway or an activation barrier due to quantum-mechanical interactions.

Related papers

Measurement of charge exchange between Na and Ca+ in a hybrid trap

Jonathan M. Kwolek, D. S. Goodman, Benjamin Slayton +4 more

We present measurements of the charge-exchange reaction rate between neutral sodium (Na) and ionized calcium (${\mathrm{Ca}}^{+}$) in a hybrid atom-ion trap, which is comprised of a Na magneto-optical trap concentric with a linear Paul trap. Once the Na and ${\mathrm{Ca}}^{+}$ are co-trapped, the reaction rate is measured by continuously quenching the reaction product ${\mathrm{Na}}^{+}$ from the ion trap, and then destructively measuring the decay of the remaining ion population. The reactants' electronic state and temperature are experimentally controlled, allowing us to determine the four individual reaction rates between $\text{Na}[S\phantom{\rule{4.pt}{0ex}}\text{or}\phantom{\rule{4.pt}{0ex}}P]$ and ${\text{Ca}}^{+}[S\phantom{\rule{4.pt}{0ex}}\text{or}\phantom{\rule{4.pt}{0ex}}D]$ at different collision energies. With the exception of the largest reaction-rate channel ($\text{Na}[P]+{\text{Ca}}^{+}[D]$), our rates are consistent with the classical Langevin rate limit. We have also found evidence of reactant collision-energy thresholds associated with two of the four entrance channels.

arXiv:1812.02810•Physical Review A•May 24, 2019•8 citations

Scattering and Sympathetic Cooling within an Ion-Neutral Hybrid Trap

Douglas S. Goodman

Dilute ion-neutral gas mixtures are an interesting system to study due to the rich variety and enormous strength of their scattering interactions, especially in the cold and ultracold regimes. Ion-neutral collisions are dominated by long-range polarization potentials that lead to cross sections several orders of magnitude larger than typical neutral-neutral van der Waals interactions. These large ion-neutral interactions can play an important role in atomic and molecular optical physics, astrophysics, and quantum chemistry. Thanks to advances in theoretical modeling of ion-neutral scattering and the development of new ion-neutral hybrid trapping and cooling technology, interest in this previously unexplored area of cold atomic and molecular physics has surged over the past decade.\nThe ion-neutral hybrid trap, composed of a magneto-optical trap concentric within a linear Paul trap, offers the ideal playground for studying and manipulating ion-neutral interactions over a wide energy range (~ 1 μK - 104 K). This dissertation discusses the development of UConn's hybrid trap, compares experimental measurements of the Na+ - Na system and Ca+ - Na system's low-energy collision rates with previously reported theory, and reports on experimental and simulation results of the sympathetic cooling of Na+ ions by laser cooled Na atoms.

•Jan 1, 2015•0 citations

Action Spectroscopy of Molecular Ions and Studies of Cold Collsions in a Hybrid Atom-Ion Trap

Steven J. Schowalter

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 $^{40}$Ca 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.

•Jan 1, 2016•0 citations

Cold ion–neutral collisions in a hybrid trap

W. W. Smith, Oleg P. Makarov, Jian Lin

A dual hybrid ion–neutral trap has been built to study collisions between a cloud of ultracold atoms and small numbers of co-trapped atomic or molecular ions, in a common volume. Ultracold Na atoms are trapped in a vapour-cell magneto-optical trap (MOT), via a single-mode frequency-stabilized ring-dye laser, with repumping. A linear rf quadrupole Paul ion trap, centred on the MOT, co-traps selected atomic or molecular ions, for example Ca+. Control of the initial ion temperature involves laser cooling Ca+ ions via the resonant Fraunhofer K line at 397 nm. We showed the rf ion trap can operate simultaneously with the MOT without destroying the ultracold atom cloud. The goal is to investigate the physics of ion–neutral sympathetic cooling, the cooling by neutrals of translationally cold and vibrorotationally hot molecular ions (e.g. Na2 +(v*, J*)), and the observation of other ion–neutral processes near 0 K, of potential interest in astrophysics.

Journal of Modern Optics•Nov 10, 2005•92 citations

The MOTion trap: a hybrid atom-ion trap system for experiments in cold-chemistry and the production of cold polar molecular ions

Scott T. Sullivan

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.

•Jan 1, 2013•0 citations

Ion-neutral chemistry at ultralow energies: Dynamics of reactive collisions between laser-cooled Ca^+ ions and Rb atoms in an ion-atom hybrid trap

Felix H. J. Hall, Pascal Eberle, Gregor Hegi +4 more

Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.

•Feb 19, 2013•0 citations
Interactions between neutral and ionized atoms are medium-range, and the spiraling nature of these collisions can lead to long interaction times and large cross-sections. The field of ion-neutral interactions is bolstered by methods from atomic physics, which allow us to accurately control and determine the quantum-states of the reactants. Trapping techniques allow us to study these interactions over a large range of energies, in order to observe interesting effects from the ultracold to hot temperature regimes. This control allows us to analyze and determine specific channels for the reaction, allowing for theoretical verification.\nThe hybrid-trap enables the concentric trapping of a sample of cold atoms and ions. This work discusses our exploration into the controlled reactions in the textNa−textCa+\\text{Na}-\\text{Ca}^+textNa−textCa+ system, including measurements of sympathetic cooling and charge-exchange rates which change as a function of energy and molecular state. The quantum-state distributions of Na in our magneto-optical trap is also explored, as well as its agreement and subsequent deviation based on a predictive model. The measurements of reaction rate reveal that there are reaction-energy thresholds in two separate reaction pathways, which can be attributed to either an endothermic reaction pathway or an activation barrier due to quantum-mechanical interactions.
We present measurements of the charge-exchange reaction rate between neutral sodium (Na) and ionized calcium (Ca+{\mathrm{Ca}}^{+}Ca+) in a hybrid atom-ion trap, which is comprised of a Na magneto-optical trap concentric with a linear Paul trap. Once the Na and Ca+{\mathrm{Ca}}^{+}Ca+ are co-trapped, the reaction rate is measured by continuously quenching the reaction product Na+{\mathrm{Na}}^{+}Na+ from the ion trap, and then destructively measuring the decay of the remaining ion population. The reactants' electronic state and temperature are experimentally controlled, allowing us to determine the four individual reaction rates between Na[SorP]\text{Na}[S\phantom{\rule{4.pt}{0ex}}\text{or}\phantom{\rule{4.pt}{0ex}}P]Na[SorP] and Ca+[SorD]{\text{Ca}}^{+}[S\phantom{\rule{4.pt}{0ex}}\text{or}\phantom{\rule{4.pt}{0ex}}D]Ca+[SorD] at different collision energies. With the exception of the largest reaction-rate channel (Na[P]+Ca+[D]\text{Na}[P]+{\text{Ca}}^{+}[D]Na[P]+Ca+[D]), our rates are consistent with the classical Langevin rate limit. We have also found evidence of reactant collision-energy thresholds associated with two of the four entrance channels.
Dilute ion-neutral gas mixtures are an interesting system to study due to the rich variety and enormous strength of their scattering interactions, especially in the cold and ultracold regimes. Ion-neutral collisions are dominated by long-range polarization potentials that lead to cross sections several orders of magnitude larger than typical neutral-neutral van der Waals interactions. These large ion-neutral interactions can play an important role in atomic and molecular optical physics, astrophysics, and quantum chemistry. Thanks to advances in theoretical modeling of ion-neutral scattering and the development of new ion-neutral hybrid trapping and cooling technology, interest in this previously unexplored area of cold atomic and molecular physics has surged over the past decade.\nThe ion-neutral hybrid trap, composed of a magneto-optical trap concentric within a linear Paul trap, offers the ideal playground for studying and manipulating ion-neutral interactions over a wide energy range (~ 1 μK - 104 K). This dissertation discusses the development of UConn's hybrid trap, compares experimental measurements of the Na+ - Na system and Ca+ - Na system's low-energy collision rates with previously reported theory, and reports on experimental and simulation results of the sympathetic cooling of Na+ ions by laser cooled Na atoms.
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 40^{40}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.
A dual hybrid ion–neutral trap has been built to study collisions between a cloud of ultracold atoms and small numbers of co-trapped atomic or molecular ions, in a common volume. Ultracold Na atoms are trapped in a vapour-cell magneto-optical trap (MOT), via a single-mode frequency-stabilized ring-dye laser, with repumping. A linear rf quadrupole Paul ion trap, centred on the MOT, co-traps selected atomic or molecular ions, for example Ca+. Control of the initial ion temperature involves laser cooling Ca+ ions via the resonant Fraunhofer K line at 397 nm. We showed the rf ion trap can operate simultaneously with the MOT without destroying the ultracold atom cloud. The goal is to investigate the physics of ion–neutral sympathetic cooling, the cooling by neutrals of translationally cold and vibrorotationally hot molecular ions (e.g. Na2 +(v*, J*)), and the observation of other ion–neutral processes near 0 K, of potential interest in astrophysics.
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.
Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.