Methane decomposition and active nitrogen in a N_2-CH_4 glow discharge at low pressures

摘要

Mass spectrometry and optical emission spectroscopy are used in N_2-xCH_4 glow discharge with x = 0.5-2%, at low pressures (1-2 Torr) and small flow rates (6 sccm), in order too determine the CH_4 and H_2 absolute concentrations and the N_2 (B~3#PI#_g) and N_2(C~3#PI#_u) relative concentrations. A kinetic model is developed based on the steady-state solutions t 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 CH_4 dissociation or as a result of reactions between radicals from N_2 and CH_4. It is observed that CH_4 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.