We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N 2 and H 2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N 2 and H 2 , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.

We measured the steady-state gas composition of plasmas produced from Ar-N 2 -O 2 mixtures and Ar-NO mixtures with quantitative mass spectrometry. In the former, mainly N 2 and O 2 , but also a significant amount of nitric oxide (NO) was formed, i.e. up to 5% of the background gas was NO. In the inverse experiment, in which NO was admixed to an argon plasma, up to 92% of the NO was converted into N 2 and O 2 . The observed molecules are mostly generated in wall association processes but also by gas phase reactions between N atoms and O 2 molecules leading to NO. The two types of plasmas show a strong mutual resemblance in the steady-state gas composition if substantial dissociation can be reached in the residence time of the gases in the plasma, i.e. ≈5% NO and ≈95% N 2 and O 2 , although the starting conditions are completely different. It seems that in first order the system prefers to produce the most thermodynamically stable molecules.

The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH 4 , N 2 and O 2 , is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H 2 O, N 2 and H 2 . The molecules CH 4 and O 2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH 3 , NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows (O 2 )/ (CH 4 ) > 0.5.

Abstract. A low pressure recombining Ar plasma to which mixtures of N 2 and O 2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N 2 , O 2 , NO, N 2 O and NO 2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry.A gas phase chemical kinetics model was developed in chemkin to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N 2 and O 2 . At a partial pressure of N 2 plus O 2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N 2 O and NO 2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N 2 plus O 2 higher than 40 Pa and a relatively high fraction of admixed O 2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably Natom induced) are more dominant than heterogeneous processes.

The visual appearance of an expanding nitrogen plasma with or without oxygen is shown. The interaction of the plasma with a substrate leads to the appearance of additional light, which is ascribed to the formation of excited molecules by association of N and/or O atoms at the substrate.