Formation of NaCl by radiative association in interstellar environments

¹ Luleå University of Technology, Department of Engineering Sciences and Mathematics, Division of Materials Science, Applied Physics 971 87, Luleå, Sweden
e-mail: magnus.gustafsson@ltu.se
² Charles University, Faculty of Mathematics and Physics, Department of Chemical Physics and Optics Ke Karlovu 3, 121 16 Prague 2, Czech Republic

Received: 21 December 2021
Accepted: 17 May 2022

Abstract

Context. Radiative association is a possible way of sodium chloride (NaCl) formation in interstellar and related environments. Theoretical studies are essential since laboratory experiments are unavailable and difficult to perform.

Aims. The total rate coefficient was calculated for the formation of NaCl by radiative association at 30–750 K.

Methods. We included two contributing processes for the total rate-coefficient computation. One of them takes the nonadiabatic coupling between the two lowest ¹Σ⁺ states, Χ¹Σ⁺ and Β¹Σ⁺, into account. The other one was calculated conventionally as a single channel and started in the continuum of the A¹Π state. The individual rate coefficients were calculated from cross sections obtained up to 0.8 eV, which enabled us to calculate the rate coefficients up to 750 K. The cross section was also calculated for a one-state process within the Χ¹ Σ⁺ state.

Results. The nonadiabatic coupling enhances the formation of NaCl by radiative association by two orders of magnitude at about 30 K and by around one order of magnitude at about 750 K. The single-channel process starting in the continuum of the A¹ Π state starts to contribute above around 200 K. The one-state transition model, within the Χ¹Σ⁺ state, is not an adequate approximation for collisions in ¹Σ⁺ symmetry. Instead, these collisions are treated in the diabatic representation in the total rate-coefficient calculation.

Conclusions. The calculated total rate-coefficient function at 30–750 K can improve the astrochemical reaction networks for the CRL 2688, IRC+10216, and Orion SrcI environments, where NaCl was detected before.