On the hydrocarbon chemistry in a H2surface wave discharge containing methane

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four stable molecules, CH 4 , C 2 H 2 , C 2 H 4 and C 2 H 6 , in a H 2 surface wave discharge ( f = 2.45 GHz and power density ≈10-50 W cm -3 ) containing up to 10% methane under different flows (Φ = 22-385 sccm) and pressures ( p = 0.1-4 Torr). The degree of dissociation of the methane precursor varied between 20% and 85% and the methyl radical concentration was found to be in the range of 10 12 molecules cm -3 . The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C 2 H 2 , C 2 H 4 , C 2 H 6 , produced in the plasma, increased with an increasing amount of added CH 4 as well as with increasing pressure. For the first time, fragmentation rates of methane ( R F (CH 4 ) = 1 × 10 15 -2.5 × 10 16 molecules J -1 ) and conversion rates to the measured C-2 hydrocarbons ( R C (C 2 H y ): 5 × 10 13 -3 × 10 15 molecules J -1 ) could be estimated with dependence on the flow and pressure in a surface wave discharge. The influence of diffusion and convection on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered by a simple model.