Spectroscopy study and modelling of an afterglow created by a low-pressure pulsed discharge in N₂-CH₄

Time-resolved emission spectroscopy is used to investigate the relaxation of N 2 (B 3 Π g ), N 2 (C 3 Π u ) and CN(B 2 Σ) states in the time afterglow of a low-pressure N 2 -CH 4 pulsed discharge, with time duration of 1 ms and in the range [CH 4 ]/[N 2 ] = 0-2%. The decays in the relative measured concentrations in the afterglow are interpreted by modelling the relaxation of a set of time-varying kinetic master equations for the various species produced in the discharge, with conditions at the beginning of the afterglow calculated from a time-dependent kinetic model for the pulsed discharge. It is observed that the N 2 (B 3 Π g ) state is populated in the afterglow mainly via the reaction N 2 (A 3 Σ u + ) + N 2 (X 1 Σ g + , 5⩽ v ⩽14)→N 2 (B 3 Π g ) + N 2 (X 1 Σ g + , v = 0), since the pulse duration is large enough to populate the N 2 (X 1 Σ g + , v ) levels at its end and, to a smaller extent, also by pooling of N 2 (A 3 Σ u + ). The N 2 (C 3 Π u ) state is populated by pooling of N 2 (A 3 Σ u + ) only, whereas the CN(B 2 Σ) state is created through reactions involving either N 2 (A 3 Σ u + ) states or N 2 (X 1 Σ g + , v ) levels in collisions with CN(X 2 Σ + ) molecules. The agreement between measured and calculated concentrations of N 2 (B 3 Π g ) and N 2 (C 3 Π u ) states is very good in pure N 2 and it may be considered satisfactory in the case of N 2 -CH 4 mixtures, and for the CN(B 2 Σ) state the agreement between theory and experiment is also reasonably good.