Low-temperature reaction dynamics of paramagnetic species in the gas phase.

Radicals are abundant in a range of important gas-phase environments. They are prevalent in the atmosphere, in interstellar space, and in combustion processes. As such, understanding how radicals react is essential for the development of accurate models of the complex chemistry occurring in these gas-phase environments. By controlling the properties of the colliding reactants, we can also gain insights into how radical reactions occur on a fundamental level. Recent years have seen remarkable advances in the breadth of experimental methods successfully applied to the study of reaction dynamics involving paramagnetic species-from improvements to the well-known crossed molecular beams approach to newer techniques involving magnetically guided and decelerated beams. Coupled with ever-improving theoretical methods, quantum features are being observed and interesting insights into reaction dynamics are being uncovered in an increasingly diverse range of systems. In this highlight article, we explore some of the exciting recent developments in the study of chemical dynamics involving paramagnetic species. We focus on low-energy reactive collisions involving neutral radical species, where the reaction parameters are controlled. We conclude by identifying some of the limitations of current methods and exploring possible new directions for the field.

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.

A stand-alone magnetic guide for producing tuneable radical beams.

Radicals are prevalent in gas-phase environments such as the atmosphere, combustion systems, and the interstellar medium. To understand the properties of the processes occurring in these environments, it is helpful to study radical reaction systems in isolation-thereby avoiding competing reactions from impurities. There are very few methods for generating a pure beam of gas-phase radicals, and those that do exist involve complex setups. Here, we provide a straightforward and versatile solution. A magnetic radical filter (MRF), composed of four Halbach arrays and two skimming blades, can generate a beam of velocity-selected low-field-seeking hydrogen atoms. As there is no line-of-sight through the device, all species that are unaffected by the magnetic fields are physically blocked; only the target radicals are successfully guided around the skimming blades. The positions of the arrays and blades can be adjusted, enabling the velocity distribution of the beam (and even the target radical species) to be modified. The MRF is employed as a stand-alone device-filtering radicals directly from the source. Our findings open up the prospect of studying a range of radical reaction systems with a high degree of control over the properties of the radical reactants.

Evolutionary Algorithm Optimization of Zeeman Deceleration: Is It Worthwhile for Longer Decelerators?

In Zeeman deceleration, only a small subset of low-field-seeking particles in the incoming beam possess initial velocities and positions that place them within the phase-space acceptance of the device. In order to maximize the number of particles that are successfully decelerated to a selected final velocity, we seek to optimize the phase-space acceptance of the decelerator. Three-dimensional particle trajectory simulations are employed to investigate the potential benefits of using a covariance matrix adaptation evolutionary strategy (CMA-ES) optimization method for decelerators longer than 12 stages and for decelerating species other than H atoms. In all scenarios considered, the evolutionary algorithm-optimized sequences yield vastly more particles within the target velocity range. This is particularly evident in scenarios where standard sequences are known to perform poorly; simulations show that CMA-ES optimization of a standard sequence decelerating H atoms from an initial velocity of 500 ms-1 down to a final velocity of 200 ms-1 in a 24-stage decelerator produces a considerable 5921% (or 60-fold) increase in the number of successfully decelerated particles. Particle losses that occur with standard pulse sequences-for example, arising from the coupling of longitudinal and transverse motion-are overcome in the CMA-ES optimization process as the passage of all particles through the decelerator is explicitly considered and focusing effects are accounted for in the optimization process.