Kinetic mechanisms and species densities in N2/(0–5%)H2 afterglows

Abstract

N atoms absolute concentrations were obtained in the afterglows of N2/(x = 0–5%)H2 mixtures by optical emission spectroscopy (OES) coupled with NO titration in the 2–6 Torr pressure range and at distances to the discharge varying between 20 and 50 cm. Band intensity measurements were then used to obtain absolute concentrations of N2(A) and N2(X, v > 13) metastable molecules in the same conditions. From these data, absolute NH and H-atoms densities were deduced from a set of kinetic mechanisms. Independently, absolute N and H atoms densities were also measured using the TALIF technique. The comparison between TALIF and OES results suggests that the NH species can be generated outside the discharge (i.e. in the afterglow) by the 3-body recombination process N+H+N2→NH+N2, with a rate coefficient of 1.0 10−33 cm6 s−1, the NH(A) state being itself produced by the reaction N2(X,v>13)+NH→NH(A)+N2, with a rate equal to 5 10−11 cm3 s−1. The agreement between the TALIF and OES measurements is further improved considering the role of N(2D) metastable atoms in the H and NH kinetics.