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Mutual neutralization is the fundamental process where a pair of oppositely charged ions collide and—in a charge transfer process—form neutral products: A+ + B- → A + B. To be able to study this process at low (<100 meV) center-of-mass energy, also for molecular ions, DESIREE was constructed as a double electrostatic ion-beam storage ring. In the cryogenic environment, the ions can be stored for more than an hour with very little external disturbance due to the low temperature and exceptional vacuum, giving ions time to relax spontaneously to their lowest quantum states or to manipulate the internal energy distributions applying laser techniques. While DESIREE is constructed as a double ring in order to enable studies of mutual neutralization and other anion-cation collision processes, the conditions are also ideal for studies of the properties of individual ions. In the colloquium, DESIREE will be presented and a number of recent experiments, their results and applications will be discussed. Examples of recent results include: The role of mutual neutralization in stellar atmospheres1; high-precision atomic anion spectroscopy2; stability of small doubly charged negative ions3; stability of polycyclic aromatic hydrocarbons in the interstellar medium4,5, mutual neutralization in Earth’s atmosphere6, and mutual neutralization of “water ions” H3O+ + OH-. 7
1 J Grumer et al, Phys. Rev. Lett. 128, 033401 (2022)
2 M K Kristiansson et al, Nature Comm 13, 5906 (2022)
3 P K Najeeb et al, Phys. Rev. Lett. 131, 113003 (2023).
4 M Gatchell et al. Nature Comm 12, 6466 (2021).
5 M H Stockett et al. Nature Comm 14, 395 (2023).
6 M Poline et al. Phys. Rev. Lett. 132, 023001 (2024)
7 A Bogot et al, Science, 383, 285 (2024)
About the speaker:
Henning Schmidt is a Professor of Physics at Stockholm University and Director of the DESIREE Research Infrastructure. His research is within the field of experimental atomic, molecular and optical physics and is focused, more specifically, on the properties of atomic and small molecular systems and their interactions with light and other ions, often with astrophysical applications in mind. Most of Schmidt’s research involves stored ion beams, making use of the fact that the same ion ensemble may be studied over an extended period of time. He was involved in the development of the cryogenic double ion-beam storage-ring facility DESIREE from its beginning.
He received his PhD in 1994 from Aarhus University, Denmark, and came to Stockholm as a postdoc in 1995. After positions as Assistant Professor and Senior Lecturer, he became a Professor in 2011. He was appointed Infrastructure Director in 2018 when DESIREE was designated a National Research Infrastructure by the Swedish Research Council.