We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of ${\text{N}}_2$ and ${\text{H}}_2$ 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, ${\text{N}}_2$ and ${\text{H}}_2$ , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic $\text{N}$ and $\text{H}$ radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic $\text{N}$ and $\text{H}$ radicals to the surface. By optimizing the atomic $\text{N}$ and $\text{H}$ 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 $\text{N}$ and $\text{H}$ containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of $\text{N}$ and $\text{H}$ 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.

A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and $1958{\text{cm}}^{-1}$ has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the ${\nu }_2$ band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of $8.4\times {10}^{-4}{\text{cm}}^{-1}{\text{Hz}}^{-1/2}$ was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of $23to3.7\times {10}^{-5}{\text{cm}}^{-1}{\text{Hz}}^{-1/2}$ .

Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient ${\alpha }_{\mathit{m}\mathit{i}\mathit{n}}$ of 2.6 × 10^–8 cm^–1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH₄ in 15 millibars air for the R(3) transition at 1327.074 cm^–1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS.