Wall material interaction with the active species in a nitrogen–methane D.C. flowing discharge

• Study of the methane small amount influence on the behaviour of the active species in a (N2–CH4) D.C. flowing discharge.
• Study of the interaction of the far-off after-glow discharge tube wall with the nitrogen active species.
• Calculation of de-excitation probabilities of the nitrogen atoms and nitrogen – wall material interaction process rates.
• Identification of the chemical nature of the layer which could be formed on the device wall in the far-off after-glow zone.

The influence of a small quantity of methane on the behaviour of radiative species in a (N2–CH4) D.C. flowing discharge at values of pressures within the range (0.2–4) mbar has been studied by optical emission spectroscopy (OES). The active species identified in the discharge zone were the radiative species N2 (B) and N2 (C). A significant decrease of N2 (B) active species population, due to the quenching mechanisms was seen while the N2 (C) active species population increased. The atomic nitrogen concentration was strongly reduced on the addition of hydrogen. The main product of the plasma–chemical reactions appearing in the (N2–CH4) mixture D.C. flowing discharge was identified as being the cyano radical, whose concentration dependence on the experimental conditions was also studied. The influence of the wall material on the active species concentration, namely the nitrogen atoms in the post-discharge zone, was also studied by OES to determine the de-excitation process of the atomic nitrogen for Teflon, Aluminium, Plastics and Copper. Using a simple method based on the radiation emitted by the nitrogen first positive system, the de-excitation and the recombination probabilities at the wall were also calculated.