## 3 Results and Discussion
### The Effects of Pulse Duration on Reactive Species  
The SPS of the nitrogen molecule and the first negative system (FNS) of the nitrogen molecular ion (N\({}_{2}\)+) N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+ \(\rightarrow\) X\({}^{2}\Sigma\)\({}_{g}\)+) are collected to investigate the effects of pulse duration and gas composition on N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+) and N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)) in different positions of the packed bed reactor, and the results are shown in Figures 10 and 11, respectively. The spectra at 337 nm and 391.4 nm are chosen to represent the SPS of N\({}_{2}\) and FNS of N\({}_{2}\)+, respectively. In Figure 10, the increase rate of the emission intensity of FNS of N\({}_{2}\)+ at the bottom of the reactor is smaller than that at the top. However, the increase rates of the SPS of N\({}_{2}\) at the top and bottom of the reactor does not have obvious differences in Figure 11. The reason is that the production of N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+) depends on the existence of a streamer channel [35], and there are more streamer channels on the top of the reactor than those on the bottom, which can be seen in Figure 7. Hence, the increase rates of the emission intensity of FNS of N\({}_{2}\)+ are different in the top and bottom of the reactor. As for N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)), the excitation potential of N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)) is about 11 eV [36], which is much smaller than that of N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+). This means that the production of N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)) is not dependent on the high energy electrons produced in the streamer channel, and the discharges at the top and the bottom of the reactor can both play roles in generating N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)). As a result, there is no obvious difference in increase rate at the top and bottom of Figure 11. For the same reason, the emission intensity of the SPS of N\({}_{2}\)+ is stronger than the emission intensity of FNS of N\({}_{2}\)+. The intensities of FNS of N\({}_{2}\)+ in Figure 10 and the SPS of nitrogen molecule in Figure 11 both increase with the increase in pulse duration. This is because the discharge currents increase with the pulse duration (shown in Figure 3), which leads to an increase of the electron density, and more N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+) and N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)) can be generated. Moreover, the N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+) and N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)) produced in nitrogen are more than those produced in air. This is also due to the larger discharge current in pure nitrogen, which is discussed in Section 3.1. In addition, there are more nitrogen molecules participating in the reactions in pure nitrogen than in air, which also results in the larger generation of N\({}_{2}\)+ (B\({}^{2}\Sigma\)\({}_{u}\)+) and N\({}_{2}\) (C\({}^{3}\Pi\)\({}_{u}\)). Figure 12 aims to further investigate the effects of the pulse duration on the evolution process of the emission intensity of SPS of N\({}_{2}\) during one single voltage pulse. The exposure time of the ICCD camera during the measurement is set as 5 ns. It can be found that there are two main intensity peaks from the emission of SPS of N\({}_{2}\) that can be seen during one voltage pulse, and the stronger one is generated during the rising time of the voltage (in Figure 12a,c) while the weaker one is generated during the falling time of the voltage pulse (in Figure 12b,d). Furthermore, the first intensity peaks under three pulse durations are almost coincident whenever they are in pure nitrogen or in air. This means that changing the pulse duration has no effect on the discharge during the rising time of the voltage pulse. Additionally, the appearance time of the second intensity peaks has a corresponding delay with the increasing pulse duration. Figure 9: ICCD images of the final discharge morphology during the pulse rising time in nitrogen (**a**) without a filter and (**b**) with a filter.