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. Ab initio calculations complemented by transition state theory calculations were employed to interpret the measured branching ratios between the product exit channels, namely CH2+, and CH3+ for the Ar+ + CH4 reaction and C2H2+, C2H3+ and C2H4+ for the Ar+ + C2H4 reaction. The branching between the products is reasonably well explained by theoretical calculations.