A time resolved laser study of hydrocarbon chemistry in H-2-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 H-2 surface wave plasmas (f = 2.45 GHz, power densi ty approximate to 10-50 W cm(-3)) containing 10% methane under static condi tions 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 st able 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 rad ical concentration was measured to be in the range of 10(12)-10(13) molecul es 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 C2H 6 concentration was observed to increase with time to a nearly constant val ue between 6 X 10(12) and 2 X 10(14) molecules cm(-3) varying with pressure . Based on time resolved concentrations, conversion rates to the measured C-2 hydrocarbons (R-C(C2Hy) = 10(11)-10(13) molecules J(-1)) could be estimate d in dependence on pressure in a surface wave discharge. The influence of d iffusion on the spatial distribution of the hydrocarbon concentration in th e discharge tube was considered. A qualitative model has been developed in order to describe the chemical processes and to identify the main plasma ch emical reaction paths.