## 3 Results and Discussion
### OES of Plasma with Addition of Ar and He  
The intensities of the characteristic spectral lines represent the relative concentrations of the corresponding species, and Figure S3 presents the OES for the cases of adding Ar  
Figure 6: Effect of pulse width on (**a**) the rate of ammonia synthesis and (**b**) the energy yield and effect of pulse rising time on (**c**) the rate of ammonia synthesis and (**d**) the energy yield.  
and He, highlighting the characteristic peaks at specific wavelengths. Figure 7 shows the OES of the N\({}_{2}\) second positive system (N\({}_{2}\) (C\({}^{3}\)Ti\({}_{u}\)\(\rightarrow\)\(\mathrm{B}^{3}\)Ti\({}_{g}\), 337.1 nm)) and the N\({}_{2}\)\({}^{+}\) first negative system (N\({}_{2}\)\({}^{+}\) (B\({}^{2}\Sigma_{u}\)\({}^{+}\)\(\rightarrow\)\(\mathrm{X}^{2}\Sigma_{g}\)\({}^{+}\), 391.4 nm)) for Ar and He, respectively. This suggests that electronic excitation (6) and ionization (7) reactions occur during the synthesis of ammonia. As can be seen in Figure 7, the intensities of the N\({}_{2}\)\({}^{*}\) and N\({}_{2}\)\({}^{+}\) spectral lines increase with increasing Ar and He content, a phenomenon that suggests that electronic excitation and ionization reactions occur more strongly with increasing noble gas addition. Combined with experimental results, increasing the number of reactive particles in the gas phase reaction improves ammonia synthesis, but OES was unable to quantify whether the reactive particles were more involved in the forward or reverse reaction. However, despite the higher strength of N\({}_{2}\)\({}^{*}\) and N\({}_{2}\)\({}^{+}\) at higher additions of noble gases, the experimental results show that the energy yield of ammonia synthesis with 50% Ar and 50% He is lower than that with 40% Ar and 40% He. Because the energy of metastable He exceeds the ionization energy of N\({}_{2}\), they generate He\({}^{+}\) particles through charge transfer between N\({}_{2}\) and metastable He\({}^{*}\). (8), and the He\({}_{2}\)\({}^{+}\) particles collide with N\({}_{2}\) to produce N\({}_{2}\)\({}^{+}\) (9) at a very high reaction rate [28]. Similarly, the metastable Ar\({}^{*}\) collides with N\({}_{2}\) to form N\({}_{2}\)\({}^{+}\)[48]. This collision process is thus facilitated with increasing noble gas additions, which also leads to an increase in the excited state N\({}_{2}\)\({}^{+}\) content with increasing noble gas additions.  
\[\mathrm{e}+\mathrm{N}_{2}\rightarrow\mathrm{e}+\mathrm{N}_{2}\text{${}^{*}$} \tag{6}\]  
\[\mathrm{e}+\mathrm{N}_{2}\  
under experimental conditions. Electrons gain energy from the electric field and transfer it to other particles through collisions; the vibrational excitation of N\({}_{2}\) helps to lower the dissociation potential barriers to produce more intermediate species [49]. Figure 8 shows the effect of noble gas addition on gas temperature. As can be seen from the figure, the rotational temperature decreases as the amount of noble gas added increases, indicating that more energy is being used to dissociate N\({}_{2}\). In particular, gas temperature also has an important effect on ammonia synthesis; in previous reports, elevated temperatures promote particle collisions, while excessive temperatures can cause decomposition reactions in the ammonia produced [50].