Reactivity of methane in a nitrogen discharge afterglow

This work is devoted to the study of the reactivity of CH4 in a nitrogen afterglow. We show that CH4 reacts efficiently in the nitrogen afterglow producing the simple radicals CH3,CH2 or recombined species containing C-N bonds in the form of C2N2, CN or HCN. A reaction rate constant ranging between 1 x 10(-18) and 6 x 10(-18) m(3) s(-1) is found for the global reaction producing mainly CH4 and CH3 radicals. Investigations are also performed in order to determine which 'active' nitrogen species are involved in the dissociation mechanism of CH4 in the nitrogen afterglow. To do this we study elementary reactive processes between CH4 and specific nitrogen species such as N-2 (A (3)Sigma(u)(+), v) and N atoms. N-2 (A (3)Sigma(u)(+), v) is selectively produced by energy transfer from Ar(P-3(2)) metastable species to N, before reacting with CH4. We show that this reaction is efficient and leads mainly to the formation of CH3 and C2H6. According to the literature, the higher the vibrational level is the faster is the rate of the vibrational relaxation of N-2 (A (3)Sigma(u)(+), v). We also study the reaction of CH4 with N atoms in the nitrogen afterglow. We measure a reaction rate constant ranging between 2 x 10 18 and 6 x 10(-18) m(3) s(-1). This value is larger than the value reported elsewhere in the literature and close to the value given for the reaction of CHx<4 with N atoms. So we propose the following reaction mechanism for the dissociation of CH4 in a nitrogen afterglow. First CH4 reacts with vibrationally excited metastable species N-2 (A (3)Σ(+)(u), v) coming from the nitrogen discharge that produces CHx<4 simple radicals. Then these radicals react with N atoms producing hydrogen cyanide, cyanogen and hydrocarbons.