A time resolved laser study of hydrocarbon chemistry in H2-CH4surface wave plasmas

Abstract

Time resolved tunable infrared diode laser absorption spectroscopyhas 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 understatic conditions at different pressures (p = 0.1-4 Torr). For the firsttime, the time dependence of the conversion of methane to the methyl radicaland three stable C-2 hydrocarbons was studied in a fixed discharge volumenearly up to a stationary state. The degree of dissociation of the methaneprecursor was found to increase by up to 96% in the stationary state, and themethyl radical concentration was measured to be in the range of1012-1013 molecules cm-3. The concentrations of bothC2H2 and C2H4 produced in the plasma showed amaximum at a distinct time before decreasing. In contrast, the C2H6 concentration was observed to increase with time to a nearly constantvalue between 6×1012 and 2×1014 molecules cm-3 varying with pressure.Based on time resolved concentrations, conversion rates to the measured C-2hydrocarbons (RC(C2Hy) = 1011-1013 molecules J-1) could be estimated in dependence on pressure in a surface wavedischarge. The influence of diffusion on the spatial distribution of thehydrocarbon concentration in the discharge tube was considered. A qualitativemodel has been developed in order to describe the chemical processes and toidentify the main plasma chemical reaction paths.