Investigation of 4% carbon in hydrogen electron cyclotron resonance microwave plasmas using ethane as the source gas

Abstract

Supersonic pulse, plasma sampling mass spectrometry has been used to probe electron cyclotron resonance microwave plasmas consisting of 2% ethane in hydrogen and 2% ethane in deuterium. The overall hydrocarbon chemistry and interconversion of species within these plasmas were determined by comparing the composition of these two chemically equivalent plasmas. The ethane in hydrogen plasma is shown to consist of 58% unreacted ethane (C2H6), 16% ethylene (C2H4), 12% acetylene (C2H2), 9% methane (CH4), with the remaining 4% of the counts attributed to the ethylene radical species (C2H3) and the ethane radical species (C2H5). The mass spectrum of the analogous deuterium plasma reveals the ethane to remain entirely undeuterated, while the ethylene and acetylene exhibit significant deuteration. The observation of significantly deuterated ethylenes indicates a new reaction channel is available in these ethane-based plasmas, that is not available to hydrocarbon plasmas based on acetylene or ethylene. Specifically, the reaction of the ethane radical (C2H5) with a hydrogen atom results in the cleavage of the carbon–carbon bond forming two methyl radicals (CH3). Once formed, the methyl radicals may undergo repeated cycles of hydrogen (deuterium) atom additions and abstractions (analogous to those previously observed for acetylene) before recombining to yield the deuterated ethane radicals (C2DxH5−x) which then by abstraction of a hydrogen (or deuterium) forms the observed deuterated ethylenes. Overall, the chemistry of these hydrocarbon plasmas is shown to be completely consistent with the neutral molecule reactions previously observed in combustion chemistry literature.