ECAMP 15

The cryogenically cooled ion beam storage ring facility DESIREE (Double ElectroStatic Ion Ring ExpEriment) is uniquely designed for studying mutual neutralization (MN) reactions in collisions between oppositely charged ions that are prepared in well-defined or narrow ranges of quantum states and with fine-control of the collision energy down to the sub-electronvolt regime [1,2]. In recent years, this has allowed a range of studies in which the final-state excitation energy distributions in atomic MN have been measured [3] and for the first ever MN studies with cooled molecular ions [4-6]. Such reactions are expected to be important for the ionization balance in any dilute environment where atomic or molecular anions are prominent negative charge carriers. Furthermore, the excellent vacuum conditions in DESIREE allows studying the cooling dynamics and stabilities of molecular ions on timescales exceeding minutes and in unprecedented detail [7-9].

This presentation will highlight recent DESIREE studies with a focus on complex molecular ions such as fullerenes, polycyclic aromatic hydrocarbons (PAHs), and biomolecules. These are important to benchmark theory and models that may be used for determining survival probabilities when molecules are exposed to different types of excitation agents in e.g. various interstellar environments, and for reliable predictions of MN rates that are expected to strongly influence the charge balance and hence the chemistry in dark interstellar clouds [10,11]. In addition, the MN studies reveal the importance of energy transfer, bond-breaking and bond-forming reactions in head-on collisions, which are driven by the Coulomb force between the oppositely charged ions. Here, support from quantum chemistry calculations is key to advance the understanding of such reactive charge transfer processes and to obtain accurate values of their rates.

References
[1] R. D. Thomas et al, Review Scientific Instruments 82, 0655112 (2011).
[2] H. T. Schmidt et al, Review Scientific Instruments 84, 055115 (2013).
[3] See e.g. J. Grumer et al, Physical Review Letters 128, 033401 (2022) and references therein.
[4] M. Poline et al, Physical Review Letters 132, 023001 (2024).
[5] A. Bogot et al, Science 383, 285-289 (2024).
[6] M. Gatchell et al, Astronomy & Astrophysics 694, A43 (2025).
[7] M. Gatchell et al, Nature Communications 12, 6646 (2021).
[8] P K Najeeb et al. Physical Review Letters 131, 113003 (2023).
[9] M. Stockett et al, Nature Communications. 14 395 (2023).
[10] S. Lepp and A Dalgarno, ApJ. 324, 553 (1988).
[11] V. Wakelam and E. Herbst, ApJ. 680, 371 (2008).