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( 4 S) and investigate the influence of methane introduced downstream from the discharge by varying the CH 4 mixing ratio from 0% up to 50%. The N ( 4 S) maximum density is about 2.2 × 10 15 cm −3 in pure N 2 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 ( 4 S) density in N 2 /CH 4 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 ( 4 S) atoms whatever the CH 4 mixing ratio. For high CH 4 mixing ratio, the destruction process through collisions with CH 3 , H 2 CN and NH becomes important and is responsible for the observed decrease of the N ( 4 S) density.