On the hydrocarbon chemistry in a H2 surface wave dischargecontaining methane

Abstract

Tunable infrared diode laser absorption spectroscopy has been usedto detect the methyl radical and four stable molecules, CH4, C2H2,C2H4 and C2H6, in a H2 surface wave discharge (f = 2.45 GHz andpower density ≈10-50 W cm-3) containing up to 10% methaneunder different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr).The degree of dissociation of the methane precursor varied between 20% and85% and the methyl radical concentration was found to be in the range of1012 molecules cm-3. The methyl radical concentration and theconcentrations of the stable C-2 hydrocarbons C2H2, C2H4,C2H6, produced in the plasma, increased with an increasing amount ofadded CH4 as well as with increasing pressure. For the first time,fragmentation rates of methane (RF(CH4) = 1×1015-2.5×1016 molecules J-1) and conversionrates to the measured C-2 hydrocarbons (RC(C2Hy):5×1013-3×1015 molecules J-1) could be estimated withdependence on the flow and pressure in a surface wave discharge. The influenceof diffusion and convection on the spatial distribution of the hydrocarbonconcentration in the discharge tube was considered by a simple model.