Ion chemistry in cold plasmas of H2 with CH4 and N2

The distributions of ions and neutrals in low-pressure (approximate to 10(-2) mbar) DC discharges of pure hydrogen and hydrogen with small admixtures (5%) of CH4 and N-2 have been determined by mass spectrometry. Besides the mentioned plasma precursors, appreciable amounts of NH3 and C2Hx hydrocarbons, probably mostly from wall reactions, are detected in the gas phase. Primary ions, formed by electron impact in the glow region, undergo a series of charge transfer and reactive collisions that determine the ultimate ion distribution in the various plasmas. A comparison of the ion mass spectra for the different mixtures, taking into account the mass spectra of neutrals, provides interesting information on the key reactions among ions. The prevalent ion is H-3(+) in all cases, and the ion chemistry is dominated by protonation reactions of this ion and some of its derivatives. Besides the purely hydrogenic ions, N2H+, NH4+, and CH5+ are found in significant amounts. The only mixed C/N ion clearly identified is protonated acetonitrile C2H4N+. The results suggest that very little HCN is formed in the plasmas under study.