Time evolution of neutral and charged species in Ar/C2H2 capacitively-coupled RF discharges

The first stages of polymerization in plasmas of Ar/C2H2 have been studied both experimentally and using a kinetic model, by following the time evolution of the main neutral and ionic species in the discharge. The abundances of the different neutrals and ions have been measured by quadrupole mass spectrometry in conditions specifically chosen to avoid dust formation in the plasma. Maxima occurring at different times from the ignition of the discharge were detected for the density of the neutral species sampled, generally consistent with a stepwise polymerization mechanism. Ions were found to fundamentally follow the trend of their neutral counterparts, with the time evolution for negative ions stressing the difference in the chemical mechanisms responsible for the formation of the two primitive anions, C2H- and H2CC-. Positive and negative ion distributions were measured in the steady state of the discharge. Detection of anions with up to 22 carbon atoms was possible in mixtures with high acetylene content. A volume averaged kinetic model with a simple description of the Ar/C2H2 chemistry was used to identify the main reaction pathways and was able to satisfactory reproduce the experimental results.