Mutual neutralization of C₆₀⁺ and C₆₀⁻ ions

Excitation energies and state-selective rate coefficients

Department of Physics, Stockholm University, 106 91 Stockholm, Sweden

^★ Corresponding author; gatchell@fysik.su.se

Received: 19 September 2024
Accepted: 4 December 2024

Abstract

Context. Mutual neutralization (MN) between cations and anions plays an important role in determining the charge balance in certain astrophysical environments. However, empirical data for such reactions involving complex molecular species have been lacking due to challenges in performing experimental studies, leaving the astronomical community to rely on decades-old models with large uncertainties for describing these processes in the interstellar medium.

Aims. Our aim is to investigate the MN reaction C₆₀⁺ + C₆₀⁻ → C₆₀^* + C₆₀ for collisions at interstellar-like conditions.

Methods. We studied the MN reaction between C₆₀⁺ and C₆₀⁻ at collision energies of 100 meV using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) and its merged beam capabilities. To aid in the interpretation of the experimental results, semiclassical modeling based on the Landau-Zener approach was performed for the studied reaction.

Results. We experimentally identified a narrow range of kinetic energies for the neutral reaction products. Modeling was used to calculate the quantum state-selective reaction probabilities, absolute cross sections, and rate coefficients of these MN reactions, using the experimental results as a benchmark. We compared the MN cross sections with model results for electron attachment to C₆₀ and electron recombination with C₆₀⁺.

Conclusions. Our results show that it is crucial to take mutual polarization effects, the finite sizes, and the final quantum states of both molecular ions into account in order to obtain reliable predictions of MN rates expected to strongly influence the charge balance and chemistry in environments such as dense molecular clouds.