Vincent Richardson
University of Liverpool
The reactions of gas-phase ions with neutral molecules are fundamental processes that impact the chemical evolution of environments ranging from the dense, high-temperature plasmas within fusion reactors to the cold, low-density environments of the interstellar medium (ISM). Due to a combination of charge-dipole/induced dipole interactions and typically barrierless reaction pathways, the reactions of ions have repeatedly been observed to proceed faster at lower temperatures, in contradiction to the Arrhenius model of reactivity. Understanding how ions react, and how their reactivity is impacted by different chemical and physical conditions, is therefore not only a subject of interest for fundamental reaction dynamics and molecular quantum mechanics, but also a key component of astrochemical evolution.
Recently, the development of increasingly powerful observational tools such as the James Webb Space Telescope (JWST) and the Atacama Large Millimetre Array (ALMA) has enabled a wealth of new detections, with the number of molecular species observed increasing from 240 to ~350 over the past six years. Included in this chemical complexity are both prebiotic molecules essential for life of on Earth to have first evolved and potential biosignature molecules that are used in the ongoing search for extraterrestrial life. Importantly, as chemical complexity increases, so does the potential for multiple isomers (chemical species with the same constituent atoms but different structures) to be present. While different isomeric ions have long been observed in astrochemical environments, with improvements in observational capability our ability to determine isomeric abundances has also improved.
Unlike under terrestrial conditions, where the relative abundance of isomers typically reflects the thermodynamic equilibrium, the low temperatures present in most astrochemical environments mean that the barriers to interconversion are often energetically inaccessible. This leads to relative abundances that reflect the rates of isomer-specific formation and destructions processes. In addition to their presence in and impact on astrochemical environments, isomers also provide critical insight into chemical reactivity and dynamics. Seemingly small changes in chemical structure, such as the shift of a hydrogen between adjacent atoms, can lead to enormous changes in reactivity.
In my research, I use several different experimental techniques (guided-ion beam tandem mass spectrometry, action spectroscopy, and Coulomb crystals) to study astrochemically relevant ionic isomers. This involves both the identification of new methods to generate currently unstudied astrochemical ions, and the recording of kinetic parameters (reaction cross sections, rate constants, and branching ratios) at different kinetic and internal temperatures. These measurements not only represent valuable input data for the modelling of astrochemical environments and detection of ions in astrochemical environments but are also used to validate and refine the reactivity models used to predict kinetic parameters in the absence of relevant experimental data.