Spatial and Temporal Detection of Ions Ejected from Coulomb Crystals.

Coulomb crystals have proven to be powerful and versatile tools for the study of ion-molecule reactions under cold and controlled conditions. Reactions in Coulomb crystals are typically monitored through a combination of in situ fluorescence imaging of the laser-cooled ions and destructive time-of-flight mass spectrometry measurements of the ejected ions. However, neither of these techniques is able to provide direct structural information on the positions of nonfluorescing "dark" ions within the crystal. In this work, structural information is obtained using a phosphor screen and a microchannel plate detector in conjunction with a Timepix3 camera. The Timepix3 camera simultaneously records the spatial and temporal distribution of all ions that strike the phosphor screen detector following crystal ejection at a selected reaction time. A direct comparison can be made between the observed Timepix3 ion distributions and the distributions established from SIMION simulations of the ion trajectories through the apparatus and onto the detector. Quantitative agreement is found between the measured Timepix3 signal and the properties of Coulomb crystals assigned using fluorescence imaging─independently confirming that the positions and numbers of nonfluorescing ions within Coulomb crystals can be accurately determined using molecular dynamics simulations. It is anticipated that the combination of high-resolution spatial and temporal data will facilitate new measurements of the ion properties within Coulomb crystals.

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 behavior 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 ultracold 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 modeling is noted, with some potential future directions of capture-based approaches outlined.

Charge Transfer Reactions between Water Isotopologues and Kr+ ions.

Astrochemical models often adopt capture theories to predict the behavior of experimentally unmeasured ion-molecule reactions. Here, reaction rate coefficients are reported for the charge transfer reactions of H2O and D2O molecules with cold, trapped Kr+ ions. Classical capture theory predictions are found to be in excellent agreement with the experimental findings. A crossing point identified between the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, further supports a capture-driven mechanism of charge transfer. However, ion-molecule reactions do not always agree with predictions from capture theory models. The appropriateness of using capture theory-based models in the absence of detailed experimental or theoretical studies is discussed, alongside an analysis of why capture theory is appropriate for describing the likelihood of charge transfer between Kr+ and the two water isotopologues.

Optimizing the intensity and purity of a Zeeman-decelerated beam.

A pure, state-selected beam of gas-phase radicals is an important tool for the precise study of radical reactions that are astrochemically and atmospherically relevant. Generating such a beam has proven to be an ongoing challenge for the scientific community. Using evolutionary algorithms to optimize the variable experimental parameters, the passage of state- and velocity-selected hydrogen atoms can be optimized as they travel through a 12-stage Zeeman decelerator and a magnetic guide. Only H atoms traveling at the target velocity are present in the beam that reaches the detection region, from a source containing a mixture of different species. All other species-including seed gases, precursor molecules, other dissociation products, and H atoms traveling outside the target velocity-are removed from the beam. The fully optimized parameters yield a pure H-atom beam containing twice as many target particles and a narrower velocity distribution compared to beams produced when only the Zeeman decelerator is optimized. These significant improvements highlight the importance of considering the passage of all target particles in the beam as they pass through all elements of the experimental apparatus.