A time resolved laser study of hydrocarbon chemistry in H2-CH4 surface wave plasmas

Time resolved tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four related stable molecules, CH4, C2H2, C2H4 and C2H6, in H2 surface wave plasmas (f = 2.45 GHz, power density ≈10-50 W cm-3) containing 10% methane under static conditions at different pressures (p = 0.1-4 Torr). For the first time, the time dependence of the conversion of methane to the methyl radical and three stable C-2 hydrocarbons was studied in a fixed discharge volume nearly up to a stationary state. The degree of dissociation of the methane precursor was found to increase by up to 96% in the stationary state, and the methyl radical concentration was measured to be in the range of 1012-1013 molecules cm-3. The concentrations of both C2H2 and C2H4 produced in the plasma showed a maximum at a distinct time before decreasing. In contrast, the C2H6 concentration was observed to increase with time to a nearly constant value between 6×1012 and 2×1014 molecules cm-3 varying with pressure. Based on time resolved concentrations, conversion rates to the measured C-2 hydrocarbons (RC(C2Hy) = 1011-1013 molecules J-1) could be estimated in dependence on pressure in a surface wave discharge. The influence of diffusion on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered. A qualitative model has been developed in order to describe the chemical processes and to identify the main plasma chemical reaction paths.