Impact of N 2 addition on cation and anion chemistry in C 2 H 2 /Ar plasmas

The addition of (0.8%–31%) N 2 to C 2 H 2 /Ar radiofrequency glow discharges is investigated by mass spectrometry of neutrals and ions, and optical emission spectroscopy. During a ∼20 s initial transient after plasma ignition, C 2 n H 2 m molecules, C 2 and CH appear and decay quickly, in parallel with a steep C 2 H 2 dissociation and H 2 rise, irrespective of the N 2 initial content; N 2 decreases, and HCN, HC 3 N and CN increase gradually, evolving afterwards to stable concentrations. Comparison with previous results suggests that these effects are related with dust formation over this transient, after which, dust disappears and ions begin to grow. Cations and anions up to masses >200 u are detected in the steady-state. Protonated cations seem to dominate, together with those of pure hydrocarbons. Most cations decrease markedly with N 2 growth, except those containing N. Conversely, nitrile anions appear even for the lowest N 2 amount, and they increase by orders of magnitude with growing N 2 , while the hydrocarbon anions tend to disappear. Globally, a significant increase of plasma electronegativity with N 2 content is observed. Likely assignments of the molecular compositions are proposed, based on the dependences of given ion masses on N 2 content. Particular interest deserve the C 2 n H − and C 2 n −1 N − families, the only anions identified till now in space. Their relative signals at the lowest N 2 proportion agree with the column densities measured around a prototypical red giant star. The large sensitivity of the anion composition to small N 2 amounts found in this work, and the coincidence in mass of N and CH 2 , might contribute to corroborate that C 2 H − is the key ion for the main anion polymerization chain in C 2 H 2 plasmas, instead of C 2 H 2 − , proposed previously. Likely gas-phase reactions and the evidence of active surface chemistry are proposed for the rationalization of the results.