Diagnostic studies of H2–Ar–N2 microwave plasmas containing methane or methanol using tunable infrared diode laser absorption spectroscopy

Abstract

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH4, CH3OH, C2H2, C2H4, C2H6, NH3, HCN, CH2O and C2N2, in H2–Ar–N2 microwave plasmas containing up to 7% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20% and 97%. The methyl radical concentration was found to be in the range 1012–1013 molecules cm−3. By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol (RF(CH4) = (2–7) × 1015 molecules J−1, RF(CH3OH) = (6–9) × 1015 molecules J−1) and the respective conversion rates to methane, hydrogen cyanide and ammonia (RCmax(CH4) = 1.2 × 1015 molecules J−1, RCmax(HCN) = 1.3 × 1015 molecules J−1, RCmax(NH3) = 1 × 1014 molecules J−1) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H2–N2–Ar–CH4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed.