Coulomb-crystallised molecular ions in traps: methods, applications, prospects

Accurate diabatization based on combined-hyperbolic-inverse-power-representation: 1,2 2A' states of BeH2

A diabatic potential energy matrix (DPEM) for the two lowest states of BeH2+ has been constructed using the combined-hyperbolic-inverse-power-representation (CHIPR) method. By imposing symmetry constraints on the coefficients of polynomials, the complete nuclear permutation inversion symmetry is correctly preserved in the CHIPR functional form. The symmetrized CHIPR functional form is then used in the diabatization by ansatz procedure. The ab initio energies are reproduced with satisfactory accuracy. In addition, the CHIPR-based DPEM also reproduces the local topology of a conical intersection. Future work will focus on a complete four-state diabatic representation with emphasis on the long-range interactions and spin-orbit couplings, which will enable accurate quantum scattering calculations for the Be+(2P) + H2 → BeH+(X1Σ+) + H(2S) reaction.

Trapping and Sympathetic Cooling of Conformationally Selected Molecular Ions

We report the generation, trapping, and sympathetic cooling of individual conformers of molecular ions with the example of cis- and trans- meta-aminostyrene. Following conformationally selective photoionization, the incorporation of the conformers into a Coulomb crystal of laser-cooled calcium ions was confirmed by fluorescence imaging, mass spectrometry, and molecular dynamics simulations. We deduce the molecules to be stable in the trap environment for more than ten minutes. The present results pave the way for the spectroscopy and controlled chemistry of distinct ionic conformers in traps.

Cold Ion-Molecule Reactions in the Extreme Environment of a Coulomb Crystal

Coulomb crystals provide a unique environment in which to study ion-neutral gas-phase reactions. In these cold, trapped ensembles, we are able to study the kinetics and dynamics of small molecular systems. These measurements have connections to chemistry in the Interstellar Medium (ISM) and planetary atmospheres. This Feature Article will describe recent work in our laboratory that uses Coulomb crystals to study translationally cold, ion-neutral reactions. We provide a description of how the various affordances of our experimental system allow for detailed studies of the reaction mechanisms and the corresponding products. In particular, we will describe quantum-state resolved reactions, isomer-dependent reactions, and reactions with a rarely studied, astrophysically relevant ion, CCl+.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers

Much attention has been paid to the dynamics of both activated gas-phase bimolecular reactions, which feature monotonically increasing integral cross sections and Arrhenius kinetics, and their barrierless capture counterparts, which manifest monotonically decreasing integral cross sections and negative temperature dependence of the rate coefficients. In this Perspective, we focus on the dynamics of gas-phase bimolecular reactions with submerged barriers, which often involve radicals or ions and are prevalent in combustion, atmospheric chemistry, astrochemistry, and plasma chemistry. The temperature dependence of the rate coefficients for such reactions is often non-Arrhenius and complex, and the corresponding dynamics may also be quite different from those with significant barriers or those completely dominated by capture. Recent experimental and theoretical studies of such reactions, particularly at relatively low temperatures or collision energies, have revealed interesting dynamical behaviors, which are discussed here. The new knowledge enriches our understanding of the dynamics of these unusual reactions.

Single molecule infrared spectroscopy in the gas phase

Spectroscopy is a key analytical tool that provides valuable insight into molecular structure and is widely used to identify chemical samples. Tagging spectroscopy is a form of action spectroscopy in which the absorption of a single photon by a molecular ion is detected via the loss of a weakly attached, inert 'tag' particle (for example, He, Ne, N2)1-3. The absorption spectrum is derived from the tag loss rate as a function of incident radiation frequency. So far, all spectroscopy of gas phase polyatomic molecules has been restricted to large molecular ensembles, thus complicating spectral interpretation by the presence of multiple chemical and isomeric species. Here we present a novel tagging spectroscopic scheme to analyse the purest possible sample: a single gas phase molecule. We demonstrate this technique with the measurement of the infrared spectrum of a single gas phase tropylium (C7H7+) molecular ion. The high sensitivity of our method revealed spectral features not previously observed using traditional tagging methods4. Our approach, in principle, enables analysis of multicomponent mixtures by identifying constituent molecules one at a time. Single molecule sensitivity extends action spectroscopy to rare samples, such as those of extraterrestrial origin5,6, or to reactive reaction intermediates formed at number densities that are too low for traditional action methods.

Conformational and state-specific effects in reactions of 2,3-dibromobutadiene with Coulomb-crystallized calcium ions

Recent advances in experimental methodology enabled studies of the quantum-state- and conformational dependence of chemical reactions under precisely controlled conditions in the gas phase. Here, we generated samples of selected gauche and s-trans 2,3-dibromobutadiene (DBB) by electrostatic deflection in a molecular beam and studied their reaction with Coulomb crystals of laser-cooled Ca⁺ ions in an ion trap. The rate coefficients for the total reaction were found to strongly depend on both the conformation of DBB and the electronic state of Ca⁺. In the (4p)²P_1/2 and (3d)²D_3/2 excited states of Ca⁺, the reaction is capture-limited and faster for the gauche conformer due to long-range ion-dipole interactions. In the (4s)²S_1/2 ground state of Ca⁺, the reaction rate for s-trans DBB still conforms with the capture limit, while that for gauche DBB is strongly suppressed. The experimental observations were analysed with the help of adiabatic capture theory, ab initio calculations and reactive molecular dynamics simulations on a machine-learned full-dimensional potential energy surface of the system. The theory yields near-quantitative agreement for s-trans-DBB, but overestimates the reactivity of the gauche-conformer compared to the experiment. The present study points to the important role of molecular geometry even in strongly reactive exothermic systems and illustrates striking differences in the reactivity of individual conformers in gas-phase ion-molecule reactions.

Capture Theory Models: An overview of their development, experimental verification, and applications to ion-molecule reactions

Since Arrhenius first proposed an equation to account for the behaviour of thermally activated reactions in 1889, significant progress has been made in our understanding of chemical reactivity. A number of capture theory models have been developed over the past several decades to predict the rate coefficients for reactions between ions and molecules-ranging from the Langevin equation (for reactions between ions and non-polar molecules) to more recent fully quantum theories (for reactions at ultra-cold temperatures). A number of different capture theory methods are discussed, with the key assumptions underpinning each approach clearly set out. The strengths and limitations of these capture theory methods are examined through detailed comparisons between low-temperature experimental measurements and capture theory predictions. Guidance is provided on the selection of an appropriate capture theory method for a given class of ion-molecule reaction and set of experimental conditions-identifying when a capture-based model is likely to provide an accurate prediction. Finally, the impact of capture theories on fields such as astrochemical modelling is noted, with some potential future directions of capture-based approaches outlined.

Conformer-specific polar cycloaddition of dibromobutadiene with trapped propene ions

Diels–Alder cycloadditions are efficient routes for the synthesis of cyclic organic compounds. There has been a long-standing discussion whether these reactions proceed via stepwise or concerted mechanisms. Here, we adopt an experimental approach to explore the mechanism of the model polar cycloaddition of 2,3-dibromo-1,3-butadiene with propene ions by probing its conformational specificities in the entrance channel under single-collision conditions in the gas phase. Combining a conformationally controlled molecular beam with trapped ions, we find that both conformers of the diene, gauche and s-trans, are reactive with capture-limited reaction rates. Aided by quantum-chemical and quantum-capture calculations, this finding is rationalised by a simultaneous competition of concerted and stepwise reaction pathways, revealing an interesting mechanistic borderline case.

Identifying a concerted or stepwise mechanism in Diels–Alder reactions is experimentally challenging. Here the authors demonstrate the coexistence of both mechanisms in the reaction of 2,3-dibromobuta-1,3-diene with propene ions, using a conformationally controlled molecular beam reacting with trapped ions and ab initio computations

Quantum Dynamics Studies of the Significant Intramolecular Isotope Effects on the Nonadiabatic Be+(2P) + HD → BeH+/BeD+ + D/H Reaction

Quantum time-dependent wave packet dynamics studies on the nonadiabatic Be⁺(²P) + HD → BeH⁺/BeD⁺ + D/H reaction are performed for the first time employing recently constructed diabatic potential energy surfaces. Strong intramolecular isotope effects and unusual results are presented, which are attributed to the dynamic effects of shallow wells induced by avoided crossing on the diagonal V₂₂^d surface. The BeH⁺ + D and BeD⁺ + H channels are dominated by high-J and low-J partial waves, respectively. The BeD⁺/BeH⁺ branching ratio is larger than 10 at low energy and gradually decreases with increasing collision energy. The BeH⁺ product is primarily distributed at low vibrational states, whereas there exists an obvious population inversion of vibrational states on the BeD⁺ product. The results of differential cross sections suggest that the formation of the BeH⁺ + D channel favors a direct reaction process, while the BeD⁺ + H channel is mainly generated by the complex-forming mechanism.

Towards chemistry at absolute zero

The prospect of cooling matter down to temperatures that are close to absolute zero raises intriguing questions about how chemical reactivity changes under these extreme conditions. Although some types of chemical reaction still occur at 1 μK, they can no longer adhere to the conventional picture of reactants passing over an activation energy barrier to become products. Indeed, at ultracold temperatures, the system enters a fully quantum regime, and quantum mechanics replaces the classical picture of colliding particles. In this Review, we discuss recent experimental and theoretical developments that allow us to explore chemical reactions at temperatures that range from 100 K to 500 nK. Although the field is still in its infancy, exceptional control has already been demonstrated over reactivity at low temperatures.

Ion-Molecule Reactions below 1 K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He^{+}+CH_{3}F

The reaction between He^{+} and CH_{3}F forming predominantly CH_{2}^{+} and CHF^{+} has been studied at collision energies E_{coll} between 0 and k_{B}·10  K in a merged-beam apparatus. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH_{3}F and He(n) Rydberg atoms with principal quantum number n=30 and 35 were merged and their relative velocity tuned using a Rydberg-Stark decelerator and deflector, allowing an energy resolution of 150 mK. A strong enhancement of the reaction rate was observed below E_{coll}/k_{B}=1  K. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar CH_{3}F molecules by the Stark effect as they approach the He^{+} ion. The enhancement of the reaction rate at low collision energies is primarily attributed to para-CH_{3}F molecules in the J=1, KM=1 high-field-seeking states, which represent about 8% of the population at the 6 K rotational temperature of the supersonic beam.

Thermometry in a Multipole Ion Trap

We present a characterization of the ions’ translational energy distribution in a multipole ion trap. A linear mapping between the energy distribution of the trapped ions onto the ions’ time-of-flight (TOF) to a detector is demonstrated. For low ion temperatures, a deviation from linearity is observed and can be attributed to the emergence of multiple potential minima. The potential landscape of the trapped ions is modeled via the finite element method, also accounting for subtleties such as surface-charge accumulation. We demonstrate the validity of our thermometry method by simulating the energy distribution of the ion ensemble thermalized with buffer gas using a Molecular Dynamics (MD) simulation. A comparison between the energy distribution of trapped ions in different multipole trap configurations—i.e., with hyperbolic rods, cylindrical rods, and cylindrical wires—is provided. With these findings, one can map the temperature of the trapped ions down to the Kelvin regime using their TOF distributions. This enables future studies on sympathetic cooling and chemical reactions involving ions in multipole traps.

Quantum-nondemolition state detection and spectroscopy of single trapped molecules

Trapped atoms and ions, which are among the best-controlled quantum systems, find widespread applications in quantum science. For molecules, a similar degree of control is currently lacking owing to their complex energy-level structure. Quantum-logic protocols in which atomic ions serve as probes for molecular ions are a promising route for achieving this level of control, especially for homonuclear species that decouple from blackbody radiation. Here, a quantum-nondemolition protocol on single trapped [Formula: see text] molecules is demonstrated. The spin-rovibronic state of the molecule is detected with >99% fidelity, and a spectroscopic transition is measured without destroying the quantum state. This method lays the foundations for new approaches to molecular spectroscopy, state-to-state chemistry, and the implementation of molecular qubits.

Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions

Isotopic substitution has long been used to understand the detailed mechanisms of chemical reactions; normally the substitution of hydrogen by deuterium leads to a slower reaction. Here, we report our findings on the charge transfer collisions of cold [Formula: see text] ions and two isotopologues of ammonia, [Formula: see text] and [Formula: see text]. Deuterated ammonia is found to react more than three times faster than hydrogenated ammonia. Classical capture models are unable to account for this pronounced inverse kinetic isotope effect. Moreover, detailed ab initio calculations cannot identify any (energetically accessible) crossing points between the reactant and product potential energy surfaces, indicating that electron transfer is likely to be slow. The higher reactivity of [Formula: see text] is attributed to the greater density of states (and therefore lifetime) of the deuterated reaction complex compared to the hydrogenated system. Our observations could provide valuable insight into possible mechanisms contributing to deuterium fractionation in the interstellar medium.

Cold and controlled chemical reaction dynamics

State-to-state chemical reaction dynamics, with complete control over the reaction parameters, offers unparalleled insight into fundamental reactivity.

Off-axis parabolic mirror relay microscope for experiments with ultra-cold matter

A new optical system is introduced for the imaging of Coulomb crystals held in a cryogenic ion trap where there are space limitations preventing the placement of an objective close to the fluorescing ions. The optical system features an off-axis parabolic (OAP) mirror relay microscope that will serve to acquire images of a lattice of fluorescing ions confined within an ultra-high-vacuum vessel operating at temperatures below 10 K. We report that the OAP mirror relay setup can resolve features smaller than the separation between neighboring ions in Coulomb crystals. The setup presented here consists of two 90-degree OAP mirrors arranged into a relay from which standard microscope optics deliver the image to a camera. This design allows the first element in the imaging setup-an OAP mirror-to be located as close as possible to the ion trap, achieving high resolution without the need for a direct line-of-sight to the trap center or for a view port to be located in close proximity to the ion trap. Such an arrangement would not be possible with a standard microscope objective, which is the approach commonly adopted by the field. OAP mirrors represent a novel solution for delivering polychromatic images with micrometer-scale resolution over extended distances.

Long-range versus short-range effects in cold molecular ion-neutral collisions

The investigation of cold interactions between ions and neutrals has recently emerged as a new scientific frontier at the interface of physics and chemistry. Here, we report a study of charge-transfer (CT) collisions of Rb atoms with N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}^{+}$$\end{document}2+ and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}^{+}$$\end{document}2+ ions in the mK regime using a dynamic ion-neutral hybrid trapping experiment. We observe markedly different CT kinetics and dynamics for the different systems and reaction channels studied. While the kinetics in some channels are consistent with classical capture theory, others show distinct non-universal dynamics. The experimental results are interpreted with the help of classical-capture, quasiclassical-trajectory and quantum-scattering calculations using ab-initio potentials for the highly excited molecular states involved. The theoretical analysis reveals an intricate interplay between short- and long-range effects in the different reaction channels which ultimately determines the CT dynamics and rates. Our results illustrate salient mechanisms that determine the efficiency of cold molecular CT reactions.

Studies on reactions between cold molecular ions and neutral atoms provide insights into intermolecular interactions. Here the authors explore the kinetics and dynamics of charge-transfer collisions between the cold N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}^{+}$$\end{document}2+ and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}^{+}$$\end{document}2+ ions and neutral Rb atoms and discuss the role of long- and short-range effects.

State-selective coherent motional excitation as a new approach for the manipulation, spectroscopy and state-to-state chemistry of single molecular ions

We present theoretical and experimental progress towards a new approach for the precision spectroscopy, coherent manipulation and state-to-state chemistry of single isolated molecular ions in the gas phase. Our method uses a molecular beam for creating packets of rotationally cold neutrals from which a single molecule is state-selectively ionized and trapped inside a radiofrequency ion trap. In addition to the molecular ion, a single co-trapped atomic ion is used to cool the molecular external degrees of freedom to the ground state of the trap and to detect the molecular state using state-selective coherent motional excitation from a modulated optical-dipole force acting on the molecule. We present a detailed discussion and theoretical characterization of the present approach. We simulate the molecular signal experimentally using a single atomic ion, indicating that different rovibronic molecular states can be resolved and individually detected with our method. The present approach for the coherent control and non-destructive detection of the quantum state of a single molecular ion opens up new perspectives for precision spectroscopies relevant for, e.g., tests of fundamental physical theories and the development of new types of clocks based on molecular vibrational transitions. It will also enable the observation and control of chemical reactions of single particles on the quantum level. While focusing on N2+ as a prototypical example in the present work, our method is applicable to a wide range of diatomic and polyatomic molecules.

Spectroscopy of Molecular Ions in Coulomb Crystals

In this Perspective, we examine the use of laser-cooled atomic ions and sympathetically cooled molecular ions in Coulomb crystals for molecular spectroscopy. Coulomb crystals are well-isolated environments that provide localization and long storage times for sensitive measurements of weak signals and cold temperatures for precise spectroscopy. Coulomb crystals of molecular and atomic ions enable the detection of single-photon molecular ion transitions at a range of wavelengths by a change in atomic ion fluorescence at visible wavelengths. We give an overview of the state of the art from action spectroscopy to quantum logic spectroscopy for a wide range of molecular transitions from rotational sublevels separated by 10^-7 cm^-1 to rovibronic transitions at 25 000 cm^-1. We emphasize how this system allows for unparalleled control of the molecular ion state for precision spectroscopy with applications in astrochemistry and fundamental physics. We conclude with an outlook of the use of this control in cold molecular ion reactions.

Optical Control of Reactions between Water and Laser-Cooled Be+ Ions

We investigate reactions between laser-cooled Be⁺ ions and room-temperature water molecules using an integrated ion trap and high-resolution time-of-flight mass spectrometer. This system allows simultaneous measurement of individual reaction rates that are resolved by reaction product. The rate coefficient of the Be⁺(²S_1/2) + H₂O → BeOH⁺ + H reaction is measured for the first time and is found to be approximately two times smaller than predicted by an ion-dipole capture model. Zero-point-corrected quasi-classical trajectory calculations on a highly accurate potential energy surface for the ground electronic state reveal that the reaction is capture-dominated, but a submerged barrier in the product channel lowers the reactivity. Furthermore, laser excitation of the ions from the ²S_1/2 ground state to the ²P_3/2 state opens new reaction channels, and we report the rate and branching ratio of the Be⁺(²P_3/2) + H₂O → BeOH⁺ + H and H₂O⁺ + Be reactions. The excited-state reactions are nonadiabatic in nature.

Observation of different reactivities of para and ortho-water towards trapped diazenylium ions

Water is one of the most fundamental molecules in chemistry, biology and astrophysics. It exists as two distinct nuclear-spin isomers, para- and ortho-water, which do not interconvert in isolated molecules. The experimental challenges in preparing pure samples of the two isomers have thus far precluded a characterization of their individual chemical behavior. Capitalizing on recent advances in the electrostatic deflection of polar molecules, we separate the ground states of para- and ortho-water in a molecular beam to show that the two isomers exhibit different reactivities in a prototypical reaction with trapped diazenylium ions. Based on ab initio calculations and a modelling of the reaction kinetics using rotationally adiabatic capture theory, we rationalize this finding in terms of different rotational averaging of ion-dipole interactions during the reaction.

Water molecules exist as two distinct nuclear-spin isomers denoted ortho and para. Here, the authors separate these two isomers in the gas phase to show that they exhibit different reactivities in a prototypical proton-transfer reaction.

NH2- in a cold ion trap with He buffer gas: Ab initio quantum modeling of the interaction potential and of state-changing multichannel dynamics

We present an extensive range of accurate ab initio calculations, which map in detail the spatial electronic potential energy surface that describes the interaction between the molecular anion NH₂^- (¹A₁) in its ground electronic state and the He atom. The time-independent close-coupling method is employed to generate the corresponding rotationally inelastic cross sections, and then the state-changing rates over a range of temperatures from 10 to 30 K, which is expected to realistically represent the experimental trapping conditions for this ion in a radio frequency ion trap filled with helium buffer gas. The overall evolutionary kinetics of the rotational level population involving the molecular anion in the cold trap is also modelled during a photodetachment experiment and analyzed using the computed rates. The present results clearly indicate the possibility of selectively detecting differences in behavior between the ortho- and para-anions undergoing photodetachment in the trap.

Rovibronic Spectroscopy of Sympathetically Cooled 40CaH. J Phys Chem A 2018. [PMID: 29521505 DOI: 10.1021/acs.jpca.7b12823]

We measure the rovibronic transitions X ¹Σ⁺, v″ = 0, J″ → A ¹Σ⁺, v' = 0-3, J' of CaH⁺ and obtain rotational constants for the A ¹Σ⁺ state. The spectrum is obtained using two-photon photodissociation of CaH⁺ cotrapped with Doppler cooled Ca⁺. The excitation is driven by a mode-locked, frequency-doubled Ti:Sapph laser, which is then pulse shaped to narrow the spectral bandwidth. The measured values of the rotational constants are in agreement with ab initio theory.

Rotational 'cooling' and 'heating' of OH+(3Σ-) by collisions with He: quantum dynamics revealing propensity rules under ion trap conditions

Multichannel scattering calculations are presented for the low-energy collisions of the OH+ cation and He atoms, using an ab initio evaluation of the interaction potential, which had been obtained in earlier work, and a time-independent, multichannel treatment of the quantum dynamics carried out in this study using our in-house scattering code ASPIN. Given the presence of spin-rotation coupling effects, within an essentially electrostatic formulation of the interaction forces with He atoms in the trap, the ensuing propensity rules which control the relative size of the state-changing cross sections and of the corresponding inelastic rates, also computed at the most likely temperatures in an ion trap, are presented and analysed in detail.

An ion trap time-of-flight mass spectrometer with high mass resolution for cold trapped ion experiments

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.

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

Stability Diagrams for Paul Ion Traps Driven by Two-Frequencies

In this paper, we present and discuss stability diagrams for Paul traps driven by two ac voltages. In contrast to a typical Paul trap, here we suggest a secondary ac voltage whose frequency is twice the frequency of the primary one. The ratio between their amplitudes can be used to expand the region of stability and to access different states of motion of trapped ions. This provides a further mechanism to trap, cool, and manipulate single ions and also to improve the experimental framework where ion clouds and crystals can be prepared and controlled. Such approach opens the possibility of designing more sophisticated trapping architectures, leading to a wide variety of applications on ion trap research and mass analysis techniques.

Rotational dynamics of a diatomic molecular ion in a Paul trap

We present models for a heteronuclear diatomic molecular ion in a linear Paul trap in a rigid-rotor approximation, one purely classical and the other where the center-of-mass motion is treated classically, while rotational motion is quantized. We study the rotational dynamics and their influence on the motion of the center-of-mass, in the presence of the coupling between the permanent dipole moment of the ion and the trapping electric field. We show that the presence of the permanent dipole moment affects the trajectory of the ion and that it departs from the Mathieu equation solution found for atomic ions. For the case of quantum rotations, we also evidence the effect of the above-mentioned coupling on the rotational states of the ion.

Observation of vibrational overtones by single-molecule resonant photodissociation

Molecular ions can be held in a chain of laser-cooled atomic ions by sympathetic cooling. This system is ideal for performing high-precision molecular spectroscopy with applications in astrochemistry and fundamental physics. Here we show that this same system can be coupled with a broadband laser to discover new molecular transitions. We use three-ion chains of Ca⁺ and CaH⁺ to observe vibrational transitions via resonance-enhanced multiphoton dissociation detected by Ca⁺ fluorescence. On the basis of theoretical calculations, we assign the observed peaks to the transition from the ground vibrational state, ν=0 to ν=9 and 10. Our method allows us to track single-molecular events, and it can be extended to work with any molecule by using normal mode frequency shifts to detect the dissociation. This survey spectroscopy serves as a bridge to the precision spectroscopy required for molecular ion control.

Studying the spectra of molecules typically requires large samples, which can be difficult to achieve for hard-to-generate ions. Here, the authors obtain spectra from single CaH⁺ molecules in a three-ion Columbic crystal, observing new molecular transitions.

Low-temperature kinetics and dynamics with Coulomb crystals

Coulomb crystals-as a source of translationally cold, highly localized ions-are being increasingly utilized in the investigation of ion-molecule reaction dynamics in the cold regime. To develop a fundamental understanding of ion-molecule reactions, and to challenge existing models that describe the rates, product branching ratios, and temperature dependence of such processes, investigators need to exercise full control over the experimental reaction parameters. This requires not only state selection of the reactants, but also control over the collision process (e.g., the collisional energy and angular momentum) and state-selective product detection. The combination of Coulomb crystals in ion traps with cold neutral-molecule sources is enabling the measurement of state-selective reaction rates in a diverse range of systems. With the development of appropriate product detection techniques, we are moving toward the ultimate goal of examining low-energy, state-to-state ion-molecule reaction dynamics.

Sympathetic cooling of a membrane oscillator in a hybrid mechanical-atomic system

Sympathetic cooling with ultracold atoms and atomic ions enables ultralow temperatures in systems where direct laser or evaporative cooling is not possible. It has so far been limited to the cooling of other microscopic particles, with masses up to 90 times larger than that of the coolant atom. Here, we use ultracold atoms to sympathetically cool the vibrations of a Si3N4 nanomembrane, the mass of which exceeds that of the atomic ensemble by a factor of 10(10). The coupling of atomic and membrane vibrations is mediated by laser light over a macroscopic distance and is enhanced by placing the membrane in an optical cavity. We observe cooling of the membrane vibrations from room temperature to 650 ± 230 mK, exploiting the large atom-membrane cooperativity of our hybrid optomechanical system. With technical improvements, our scheme could provide ground-state cooling and quantum control of low-frequency oscillators such as nanomembranes or levitated nanoparticles, in a regime where purely optomechanical techniques cannot reach the ground state.

Fluorescence imaging for visualization of the ion cloud in a quadrupole ion trap mass spectrometer

Laser-induced fluorescence is used to visualize populations of gaseous ions stored in a quadrupole ion trap (QIT) mass spectrometer. Presented images include the first fluorescence image of molecular ions collected under conditions typically used in mass spectrometry experiments. Under these "normal" mass spectrometry conditions, the radial (r) and axial (z) full-width at half maxima (FWHM) of the detected ion cloud are 615 and 214 μm, respectively, corresponding to ~6% of r0 and ~3% of z0 for the QIT used. The effects on the shape and size of the ion cloud caused by varying the pressure of helium bath gas, the number of trapped ions, and the Mathieu parameter q z are visualized and discussed. When a "tickle voltage" is applied to the exit end-cap electrode, as is done in collisionally activated dissociation, a significant elongation in the axial, but not the radial, dimension of the ion cloud is apparent. Finally, using spectroscopically distinguishable fluorophores of two different m/z values, images are presented that illustrate stratification of the ion cloud; ions of lower m/z (higher qz) are located in the center of the trapping region, effectively excluding higher m/z (lower qz) ions, which form a surrounding layer. Fluorescence images such as those presented here provide a useful reference for better understanding the collective behavior of ions in radio frequency (rf) trapping devices and how phenomena such as collisions and space-charge affect ion distribution.

Specific Chemical Reactivities of Spatially Separated 3-Aminophenol Conformers with Cold Ca+ Ions

Many molecules exhibit multiple rotational isomers (conformers) that interconvert thermally and are difficult to isolate. Consequently, a precise characterization of their role in chemical reactions has proven challenging. We have probed the reactivity of specific conformers by using an experimental technique based on their spatial separation in a molecular beam by electrostatic deflection. The separated conformers react with a target of Coulomb-crystallized ions in a trap. In the reaction of Ca+ with 3-aminophenol, we find a twofold larger rate constant for the cis compared with the trans conformer (differentiated by the O–H bond orientation). This result is explained by conformer-specific differences in the long-range ion-molecule interaction potentials. Our approach demonstrates the possibility of controlling reactivity through selection of conformational states.

Millikelvin reactive collisions between sympathetically cooled molecular ions and laser-cooled atoms in an ion-atom hybrid trap

We report on a study of cold reactive collisions between sympathetically cooled molecular ions and laser-cooled atoms in an ion-atom hybrid trap. Chemical reactions were studied at average collision energies <E(coll)>/k(B)>/~20 mK, about 2 orders of magnitude lower than has been achieved in previous experiments with molecular ions. Choosing N(2)(+)+Rb as a prototypical system, we find that the reaction rate is independent of the collision energy within the range studied, but strongly dependent on the internal state of Rb. Highly efficient charge exchange four times faster than the Langevin rate was observed with Rb in the excited (5p) (2)P(3/2) state. This observation is rationalized by a capture process dominated by the charge-quadrupole interaction and a near resonance between the entrance and exit channels of the system. Our results provide a test of classical models for reactions of molecular ions at the lowest energies reached thus far.