Time-resolved molecular beam mass spectrometry of the initial stage of particle formation in an Ar/He/C2H2 plasma.

The temporal evolution of the neutral plasma chemistry products in a capacitively coupled plasma from argon/helium/acetylene is followed via molecular beam mass spectrometry with a time resolution of 100 ms. Several chemistry pathways are resolved. (i) The formation of C2nH2 (n = 2-5) molecules proceeds via the following sequence: the production of highly reactive C2H radicals in electron impact dissociation of C2H2 is followed by C2H induced chain polymerization of C2nH2 (n = 1-4). (ii) CnH4 (n = 4, 5, 6) compounds are detected already at an early stage of the discharge excluding polymerization reactions with C2H radical being responsible for their formation. Instead, vinylidene reactions with acetylene or mutual neutralization reactions of ionic species are proposed as sources of their formation. (iii) Surface reactions are identified as the source of C8H6. The measured hydrocarbon molecules represents possible precursors for negative ion formation via dissociative electron attachment reactions and can hence play a crucial role in particle nucleation. On the basis of the comparison of our data with available experimental and modeling results for acetylene plasmas in the literature, we propose C2nH2 (n > 1) molecules as important precursors for negative ion formation.