Investigation of Ammonia Production from Small Admixtures of Nitrogen in a Hydrogen Plasma Using Long Time of Flight Mass Spectrometry

H₂/N₂ plasmas are used in a plethora of applications ranging from astrophysical comparisons to the generation of ammonia for industrial usage. Whilst the formation of ammonia is not quantitatively understood, it is widely accepted that this takes place via the progressive hydrogenation of nitrogen species at the surface through both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. This work used long time of flight mass (LTOF) spectrometry to measure relative ammonia densities in a hydrogen plasma with nitrogen admixtures of 500-5000 ppm. A significant level of ammonia was detected even at the lowest admixture of nitrogen, 500 ppm. The difficulty of ammonia extraction from the plasma to the LTOF was demonstrated by comparing bulk ammonia densities, as measured by laser diagnostics, with the extracted signals. An estimate for the maximum possible density of ammonia was also calculated by measuring the depletion of molecular nitrogen, this estimate was compared with a previously measured minimum value under identical experimental conditions and found to match well. Finally, the effects of heating the chamber were investigated and explained using a combination of thermally dependent ammonia retention coefficients and the increased efficiency of the LH mechanism at elevated temperatures.