Absolute ground-state nitrogen atom density in a N2/CH4 late afterglow: TALIF experiments and modelling studies

Abstract

Following a first study on a late afterglow in flowing pure nitrogen post discharge, we report new two-photon absorption laser-induced fluorescence (TALIF) measurements of the absolute ground-state atomic nitrogen density N(4S) and investigate the influence of methane introduced downstream from the discharge by varying the CH4 mixing ratio from 0% up to 50%. The N (4S) maximum density is about 2.2 × 1015 cm−3 in pure N2 for a residence time of 22 ms and does not change significantly for methane mixing ratio up to ∼15%, while above, a drastic decrease is observed. The influence of the residence time has been studied.A kinetic model has been developed to determine the elementary processes responsible for the evolution of the N (4S) density in N2/CH4 late afterglow. This model shows the same decrease as the experimental results even though absolute density values are always larger by about a factor of 3. In the late afterglow three-body recombination dominates the loss of N (4S) atoms whatever the CH4 mixing ratio. For high CH4 mixing ratio, the destruction process through collisions with CH3, H2CN and NH becomes important and is responsible for the observed decrease of the N (4S) density.