Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association

The deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H₃⁺ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H₃⁺, especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H₃⁺ only, considering that the protonated ions produced in the destruction of H₃⁺ by its main competitors present in the primitive nebula, i.e., H₂O, CO, and N₂, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H₂O, CO, and N₂, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10^-16 to 10^-19 cm³ s^-1, which is reasonable for such reactions and has to be compared to the rates of 10^-16 to 10^-18 cm³ s^-1, obtained with H₃⁺. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe.