Formation of NaCl by radiative association in interstellar environments

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.

Non-adiabatic dynamics in collisions of sodium and chlorine atoms and their ions

“…2, where the cross section is shown from 0.03 eV to 4.6 eV (up to 0.8 eV, the cross section is taken from ref. 11). With increasing collision energy, resonances occur on the cross-section function more frequently until they cease around the threshold 1.4845 eV where the ionic channel opens.…”

“…3. For completeness, the results from 80 K to 750 K, taken from Šimsová-Zámečníková et al , 11 are also shown in the figure. The values of individual rate coefficients increase with increasing temperature.…”

“…The theory for the non-adiabatic dynamics employed here was described in detail by Gustafsson 10 and Šimsová-Zámečníková et al 11 and is based on the work of Tchang-Brillet et al 21 and Cooper et al 9…”

“…on the chemiionisation and mutual neutralisation. 1–9 Recently, Gustafsson 10 and Šimsová-Zámečníková et al 11 showed that the non-adiabatic dynamics has a profound effect on the radiative association of sodium atoms (Na) with chlorine atoms (Cl), where the differences in the calculated cross section, and consequently also in the calculated rate coefficient, were in orders of magnitude.…”

“…We have decided to revisit the charge transfer between Na and Cl (or Na + and Cl − ) and radiative association of Na and Cl making use of the state-of-the-art ab initio results of Giese and York, 20 which provided data for the two-state formulation in both the adiabatic and diabatic representations. The purpose of this study is thus twofold: first, to determine the cross sections for radiative associations 1 Σ + [Na( 2 S) + Cl( 2 P)] → NaCl( 1 Σ + ) + ħω , 1 Σ + [Na + ( 1 S) + Cl − ( 1 S)] → NaCl( 1 Σ + ) + ħω ,A 1 Π[Na( 2 S) + Cl( 2 P)] → NaCl(X 1 Σ + ) + ħω at higher energies so that the temperature range for radiative association could be extended up to 5300 K (Gustafsson 10 and Šimsová-Zámečníková et al 11 calculated the rate coefficients for radiative-association processes (1) and (3) up to 750 K). Secondly, for this temperature range to determine the rate coefficients for chemiionisation and mutual neutralisation 1 Σ + [Na( 2 S) + Cl( 2 P)] → 1 Σ + [Na + ( 1 S) + Cl − ( 1 S)], 1 Σ + [Na + ( 1 S) + Cl − ( 1 S)] → 1 Σ + [Na( 2 S) + Cl( 2 P)].…”