Diagnostic studies of H2–Ar–N2microwave plasmas containing methane or methanol using tunable infrared diode laser absorption spectroscopy

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH 4 , CH 3 OH, C 2 H 2 , C 2 H 4 , C 2 H 6 , NH 3 , HCN, CH 2 O and C 2 N 2 , in H 2 –Ar–N 2 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 10 12 –10 13 molecules cm −3 . By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH 3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol ( R F (CH 4 ) = (2–7) × 10 15 molecules J −1 , R F (CH 3 OH) = (6–9) × 10 15 molecules J −1 ) and the respective conversion rates to methane, hydrogen cyanide and ammonia ( R Cmax (CH 4 ) = 1.2 × 10 15 molecules J −1 , R Cmax (HCN) = 1.3 × 10 15 molecules J −1 , R Cmax (NH 3 ) = 1 × 10 14 molecules J −1 ) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H 2 –N 2 –Ar–CH 4 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.