## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Species**} & **Spectral** & **Absorption Line** & **Limit of** & \\  & **Position** & **Strength** & **Detection** & **Ref.** \\  & **(cm\({}^{-1}\))** & **(cm\({}^{-1}\)/molecules cm\({}^{-2}\))** & **(molecules cm\({}^{-3}\))** & **Ref.** \\ \hline CH\({}_{4}\)(Q) & 1356.4868 & 1.784 \(\times\) 10\({}^{-20}\) & 2 \(\times\) 10\({}^{13}\) & [19] \\ CH\({}_{4}\)(Q) & 1356.5974 & 1.190 \(\times\) 10\({}^{-20}\) & 2 \(\times\) 10\({}^{13}\) & [19] \\ NH\({}_{3}\)(Q) & 1388.0552 & 2.726 \(\times\) 10\({}^{-22}\) & 2 \(\times\) 10\({}^{14}\) & [20] \\ NH\({}_{3}\)(Q) & 1767.5181 & 6.090 \(\times\) 10\({}^{-21}\) & 2 \(\times\) 10\({}^{13}\) & [20] \\ H\({}_{2}\)O(Q) & 1387.9337 & 8.769 \(\times\) 10\({}^{-23}\) & 5 \(\times\) 10\({}^{14}\) & [18] \\ H\({}_{2}\)O(Q) & 1388.3483 & 9.843 \(\times\) 10\({}^{-24}\) & 2 \(\times\) 10\({}^{15}\) & [21] \\ H\({}_{2}\)O(Q) & 1781.9619 & 1.167 \(\times\) 10\({}^{-21}\) & 1 \(\times\) 10\({}^{14}\) & [21] \\ C\({}_{2}\)H\({}_{2}\)(Q) & 1356.8305 & 5.899 \(\times\) 10\({}^{-22}\) & 5 \(\times\) 10\({}^{14}\) & [22] \\ C\({}_{2}\)H\({}_{2}\)(Q) & 1356.8881 & 8.920 \(\times\) 10\({}^{-21}\) & 2 \(\times\) 10\({}^{13}\) & [22] \\ HCN(Q) & 1356.9389 & 4.636 \(\times\) 10\({}^{-23}\) & 4 \(\times\) 10\({}^{14}\) & [23] \\ HCN(Q) & 1388.3225 & 3.592 \(\times\) 10\({}^{-22}\) & 1 \(\times\) 10\({}^{14}\) & [24] \\ CO(T) & 2150.3409 & 1.840 \(\times\) 10\({}^{-21}\) & 2 \(\times\) 10\({}^{11}\) & [25] \\ CO(T) & 2150.8560 & 1.826 \(\times\) 10\({}^{-19}\) & 2 \(\times\) 10\({}^{13}\) & [25] \\ CO\({}_{2}\)(T) & 606.2771 & 2.713 \(\times\) 10\({}^{-21}\) & 5 \(\times\) 10\({}^{13}\) & [5] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Species, spectral positions, room temperature line strengths, used for IRLAS measurements and their estimated limits of detection. The laser source for the given spectral position is denoted with a Q for EC-QCL (external-cavity quantum cascade lasers) and a T for TDL (lead salt tunable diode laser) sources, respectively. Data taken from the HITRAN database [18]. (H\({}_{2}\):Ar = 3:1) at a total flow rate of 60 slh. After reaching the target temperature, the target gas composition was set to 84% N\({}_{2}\), 10% H\({}_{2}\), and 6% O\({}_{2}\) and 87% N\({}_{2}\), 10% H\({}_{2}\), and 3% O\({}_{2}\) with a total gas flow rate of 80 slh. For the reference treatment without O\({}_{2}\), the gas composition was set to 90% N\({}_{2}\), and 10% H\({}_{2}\). The measurements were performed at a temperature of 540 \({}^{\circ}\)C (813 K) at a constant pressure of 3 mbar for a treatment duration of 8 h followed by a cooling step in pure H\({}_{2}\) (\(\Phi\) = 20 slh) until room temperature was reached. The power at the carbon AS during the treatment time was approximately 6 kW. Spectroscopic sampling of the current process gas composition occurred approximately every 20 min for the 8 h duration of an ARMCO(r) iron specimens' treatment. Therefore, 15 mm \(\times\) 15 mm specimens were cut out of a 2 mm thick plate. Before the process, the samples' surfaces were polished to 1 \(\upmu\)m surface finishing, cleaned in ethanol, and placed in the center of the vacuum reactor. The formed compound layers were analyzed by glow discharge optical emission spectroscopy (GDOES) on a LECO SDP 750 spectrometer in order to quantify the element concentrations of nitrogen and carbon as well as their diffusion depths, and to examine the nitride layers with respect to a possible oxygen enrichment. Cross-sections of the treated samples were nickel-plated, metallographically prepared, and etched with sodium picrate etchant. Images of the compound layers were obtained with an optical microscope Carl Zeiss Neophot 30 equipped with a JVC TK C1381 CCD-camera.