Methane decomposition and active nitrogen in a N2-CH4 glow discharge at low pressures

Mass spectrometry and optical emission spectroscopy are used in a N-2-xCH(4) glow discharge with x = 0.5-2%, at low pressures (1-2 Torr) and small flow rates (6 seem), in order to determine the CH4 and H-2 absolute concentrations and the N-2(B (3)Pi(g)) and N-2(C (3)Pi(g)) relative concentrations. A kinetic model is developed based on the steady-state solutions to the homogeneous electron Boltzmann equation coupled to a system of rate balance equations for the most populated neutral and ionic species produced, either from active nitrogen and CH4 dissociation or as a result of reactions between radicals from N-2 and CH4. It is observed that CH4 is very efficiently decomposed through a sequence of reactions in which at the end HCN and H-2 appear as the most abundant products in the discharge. A brown deposition on the tube walls has been detected which is attributed to HCN, in agreement with other investigations of Titan's atmosphere, since this species is poorly destroyed in volume. The accordance between theory and experiment is very satisfactory allowing an insight to be obtained into the basic elementary mechanisms in these discharges.