On the ionic chemistry in DC cold plasmas of H2 with Ar

Abstract

An experimental diagnosis and kinetic modeling of the ionic chemistry in low pressure DC plasmas of H₂/Ar has been carried out. The studies were performed at pressures of 2 Pa and 0.7 Pa, in a hollow-cathode discharge reactor, using as plasma precursor a H₂/Ar mixture with 15% Ar content. Experimental measurements include distributions of ion fluxes to the cathode, as well as electronic temperatures and densities in the plasma glow. Besides the species resulting directly from electron impact ionization (H⁺, H₂⁺, Ar⁺ and Ar²⁺), the ions H₃⁺ and ArH⁺ were found to be formed in large amounts through protonation reactions in the glow. In spite of the not too large variation in the pressure of the two plasmas, the differences in the ion distributions are worth mentioning. In the 2 Pa discharge (but not in the 0.7 Pa), H₃⁺ was the dominant ion and ArH⁺ exceeded markedly the Ar⁺ signal. On the other hand, the appearance of Ar²⁺ in the two plasmas points at the relevance of high energy electrons. The experimental results can be accounted for by a simple kinetic model, after including corrections for the presence of a small fraction (<3%) of high energy (>50 eV) electrons and for the attenuation of the Ar⁺ ions in the sheath through asymmetric charge exchange with H₂.