Branching ratios and mechanisms of the reactions of Ar+ with CH4 and C2H4 at cryogenic temperatures

Interest in interstellar noble gas chemistry has been stimulated by the recent detection of ArH+ and HeH+ in a variety of astrophysical environments ranging from diffuse clouds to supernova remnants. In this context, it seems timely to explore or revisit chemical reactions involving noble gas ions, such reactions being an efficient way to form noble gas bearing molecules in the interstellar medium. The reaction of Ar+ ions with methane, CH4, and ethylene, C2H4, at low temperatures, i.e., 24.1 and 71.6 K, was investigated in the laboratory with the help of a dedicated instrument combining a uniform supersonic flow reactor with a mass selective ion source. Computational flow dynamics and ion trajectory simulations were conducted to assess the thermalization of the ions in the flow. Ab initio and transition state theory calculations were employed to complement, at a microphysical scale, the interpretations derived from the macroscopic measurements of the Ar+ + CH4 and Ar+ + C2H4 reactions. The branching between the products is reasonably well explained by theoretical calculations.