Ion kinetics in Ar/H2 cold plasmas: the relevance of ArH+

Abstract

The recent discovery of ArH⁺ in the interstellar medium has aroused an interest for this ion in chemistry. In this work, the ion-molecule kinetics of cold plasmas of Ar/H₂ is investigated in glow discharges spanning the whole range of [H₂]/([H₂] + [Ar]) proportions for two pressures, 1.5 Pa and 8 Pa. Ion concentrations are determined by mass spectrometry, and electron temperatures and densities, with Langmuir probes. A kinetic model is used for the interpretation of the results. The selection of experimental conditions evinces relevant changes with plasma pressure in the ion distributions dependence with the H₂ fraction, particularly for the major ions: Ar⁺, ArH⁺ and H₃⁺. At 1.5 Pa, ArH⁺ prevails for a wide interval of H₂ fractions: 0.3 < [H₂]/([H₂] + [Ar]) < 0.7. Nevertheless, a pronounced displacement of the ArH⁺ maximum towards the lowest H₂ fractions is observed at 8 Pa, in detriment of Ar⁺, which becomes restricted to very small [H₂]/([H₂] + [Ar]) ratios, whereas H₃⁺ becomes dominant for all [H₂]/([H₂] + [Ar]) > 0.1. The analysis of the data with the kinetic model allows for the identification of the sources and sinks of the major ions over the whole range of experimental conditions sampled. Two key factors turn out to be responsible for the different ion distributions observed: the electron temperature, which determines the rate of Ar⁺ formation and thus of ArH⁺, and the equilibrium ArH⁺ + H₂ ⇄ H₃⁺ + Ar, which can be strongly dependent of the degree of vibrational excitation of H₃⁺. The results are discussed and compared with previously published data on other Ar/H₂ plasmas.