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

Abstract

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.

Fig. 1. Schematic illustration of the different types of resonances present in the Penning ionisation and elastic scattering pathways following collisions between metastable He and D₂. Note that metastable He, denoted He* in the figure, is a triplet radical species. At collision energies that coincide with the location of partial waves (with the squared resonance wavefunctions shown shaded in black and green on the central panel), peaks are seen in the rate coefficient plots. Adapted from Paliwal et al., copyright 2021, under exclusive licence to Springer Nature Limited.

Fig. 2. Experimental (a–c) and theoretical (d–f) differential cross sections are presented as three-dimensional contour plots of the product velocity, at three collision energies (top: 1.56 meV, middle: 6.93 meV, bottom row: 9.97 meV). Below the differential cross section plots, the features of the reaction pathways for the reaction of H₂(j = 0) and (j = 1) with F are shown schematically. Feschbach resonance states can be seen in the exit channel (after the reaction barrier). Reproduced from Yang et al., copyright 2019, under exclusive licence to Springer Nature Limited.

Fig. 3. Velocity map scattering image recorded following inelastic collisions between NO(X²Π_3/2, j = 3/2) and Ne. Oscillatory patterns due to matter wave diffraction can be observed. As can be seen in the right hand side plot, excellent agreement was found between the experimental and simulated results. Adapted with permission from Plomp et al., copyright American Institute of Physics, 2020.

Fig. 4. Schematic illustration of a travelling wave Zeeman decelerator and magnetic trap apparatus, for the co-deceleration and trapping of atomic and molecular radicals. Cold paramagnetic species enter the apparatus (as indicated by the blue arrow on the left) and are decelerated by a series of 480 co-moving magnetic traps (illustrated in bronze and black), formed by the application of current pulses to the coils. Slow-moving species at the end of the decelerator can be loaded into a superconducting trap. The trapped species are probed after a selected delay time using laser-based detection methods. Reproduced from Segev et al., copyright 2019, under exclusive licence to Springer Nature Limited.

Left to right: Chloé Miossec, Brianna R. Heazlewood and Lok Yiu Wu