Cryogenic ion trap to study state- and velocity-selected radical-ion reactions Rahul Kumar Pandey, Lok Yiu Wu, Paul Regan, Brianna R. Heazlewood Department of Physics, University of Liverpool, UK Studying reactions at temperatures close to absolute zero has attracted considerable interest over the past several years. At these cold temperatures, chemical reactions are governed by quantum effects and their properties frequently cannot be accounted for by existing theoretical models [1,2,3]. Understanding how radical–ion reactions occur at low temperatures is directly relevant to the chemistry occurring in naturally cold environments, such as the interstellar medium, in addition to being of significant fundamental interest. A newly built cryogenic ion trap apparatus at the University of Liverpool offers an exciting opportunity to investigate chemical reactions in a carefully controlled environment, at temperatures down to <1 K. Maintaining low internal energies in molecular ions is technically challenging, as the vast majority of molecular ions interact with black body radiation at 300 K—causing state selectivity in trapped molecular ions to be rapidly lost (in a matter of seconds) [6]. To address this issue, a cryogenic ion trap has been developed and characterised [4]. At the centre of the apparatus is a linear Paul trap, where the ion–radical reactions of interest will be studied within Coulomb crystals. The cryogenic ion trap apparatus was recently combined with a neutral (atomic or molecular) beam source chamber, facilitating the study of cold radical–ion reactions with exceptional control over the reaction conditions [4]. The two components of this apparatus—a cold ionic target and a source of cold neutral species— have been developed and characterised independently [4,5]. A Zeeman decelerator, which slows down radicals from a supersonic beam using pulsed magnetic fields, combined with a magnetic guide (to filter out non-target species), is now attached to the cryogenic ion trap chamber [5]. The Zeeman decelerator and magnetic guide set-up can produce a pure beam of cold, state- and velocity-selected radicals, ideal for our intended radical–ion collision studies. A range of other (non-radical) internally cold reactants can also be introduced through the pulsed valve. A combination of complementary detection methods, including fluorescence imaging and time-of-flight mass spectrometry, will enable the reactions to be studied with exceptional sensitivity [7,8,9]. References
1. R. J. Shannon et al., Nat Chem . 2013 Sep;5(9):745-9. 2. T. Yang et al., Nat Chem. 2019 Aug;11(8):744-749. 3. M. A. Nichols et al., Phys. Rev. X 2022 Mar;12, 011049. 4. C. Miossec et al., Rev Sci Instrum. 2022 Mar;93(3):033201. 5. J Toscano et al., J. Chem. Phys. 2018 Nov;149 (17): 174201. 6. N. Deb et al., J. Chem. Phys. 2014 April;140, 164314. 7. A. Tsikritea et al., Chem. Sci. 2021 June;12, 10005. 8. L. S. Petralia et al., Nat Commun 2020 Jan;11, 173. 9. K. J. Catani et al., J. Chem. Phys. 2020 May;152, 234310.
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