Impact of N₂ addition on cation and anion chemistry in C₂H₂/Ar plasmas

Abstract

The addition of (0.8%–31%) N2 to C2H2/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, C2nH2m molecules, C2 and CH appear and decay quickly, in parallel with a steep C2H2 dissociation and H2 rise, irrespective of the N2 initial content; N2 decreases, and HCN, HC3N 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 N2 growth, except those containing N. Conversely, nitrile anions appear even for the lowest N2 amount, and they increase by orders of magnitude with growing N2, while the hydrocarbon anions tend to disappear. Globally, a significant increase of plasma electronegativity with N2 content is observed. Likely assignments of the molecular compositions are proposed, based on the dependences of given ion masses on N2 content. Particular interest deserve the C2nH− and C2n−1N− families, the only anions identified till now in space. Their relative signals at the lowest N2 proportion agree with the column densities measured around a prototypical red giant star. The large sensitivity of the anion composition to small N2 amounts found in this work, and the coincidence in mass of N and CH2, might contribute to corroborate that C2H− is the key ion for the main anion polymerization chain in C2H2 plasmas, instead of C2H2−, proposed previously. Likely gas-phase reactions and the evidence of active surface chemistry are proposed for the rationalization of the results.