## 3 Results and Discussion
Figure 3: _Cont._  
The spatial distributions of cycle-averaged Ar\({}_{2}^{+}\) ion density, Ar\({}^{+}\) ion density, and metastable Ar\({}^{*}\) atom density when varying the SEE coefficient from 0.001 to 0.01 are shown in Figure 4. They are symmetrically distributed in the discharge space, uniformly distributed in the plasma region and in the peaks of different degrees in the sheath regions. In quantity, Ar\({}^{+}\) ion density is the lowest. Although ionization produced many Ar\({}^{+}\) ions, the number of Ar\({}^{+}\) ions decreased due to the gradual dominance of reactions 5 and 13 in Table 1. Most of the Ar\({}^{+}\) ions are converted into Ar\({}_{2}^{+}\) ions. Ar\({}_{2}^{+}\) density is much higher than Ar\({}^{+}\) density, and the main positive argon ions are Ar\({}_{2}^{+}\) ions. This result is similar to the results of the numerical study on a pure argon discharge at atmospheric pressure, which also confirmed that, in an atmospheric-pressure argon discharge, the positive ions were largely Ar\({}_{2}^{+}\) ions [34]. From Figure 4 we can see that Ar\({}^{+}\) ions react more violently in the sheath region; the fluctuation from the plasma region to the sheath peak is greater than is the case for the other two particles. Additionally, the Ar\({}_{2}^{+}\) ion, Ar\({}^{+}\) ion, and metastable Ar\({}^{*}\) atom density increase when the SEE coefficient is increased. The change in the Ar\({}_{2}^{+}\) ion density is smaller. As shown in Figure 4a, the Ar\({}^{+}\) ion density decreases rapidly in the sheath regions, and the Ar\({}_{2}^{+}\) ions remain present on both electrode walls. Figure 5 shows the spatial distributions of various cycle-averaged oxygen particle densities as the SEE coefficient was increased from 0.001 to 0.01. In spatial distributions, the sheath thickness decreased slightly as the SEE coefficient increased, and particle density was symmetrically distributed. As evidenced by the numbers of particles, the metastable O\({}_{2}^{*}\) molecule and O atom density were not affected by the SEE coefficient, as shown in Figure 5a,d. Although secondary electron emissions resulted in increased electron density, the process of electron loss via dissociative electron attachment (Reaction 8 in Table 1) increased, resulting in more O\({}^{-}\) ions generated. The concentration of O\({}_{2}\) is 0.1% in the gas mixture, which means it was nearly equal to that in the pure O\({}_{2}\) discharge at a pressure in the double digits (measured in Torr). The main process of O\({}_{2}^{*}\) generation is the excitation of the O\({}_{2}\) molecule (Reaction 15 in Table 1), which balances the process of O\({}_{2}^{*}\) loss. As a result, the O\({}_{2}^{*}\) density is not affected by the SEE coefficient. The processes of O atom generation depend on the O\({}_{2}^{*}\) ion density, which is why the O atom density is also unaffected by the SEE coefficient. The O\({}_{2}^{+}\) ion density obviously increases across the entire discharge space as the SEE coefficient increases, and two peaks appear in the sheath region in Figure 5b, because the O\({}_{2}^{+}\) ions are mainly produced by the direct ionization of electrons and oxygen, which is closely related to the electron density. The effect of secondary electron emission on the O\({}^{-}\) ion density is observed mainly in the plasma region. The O\({}^{-}\) ion density increases slightly in the plasma region, as seen in Figure 5c. In addition, the O\({}^{-}\) ion cannot penetrate the sheath due to the influence of the strong electric field in the sheath; thus the O\({}^{-}\) ion  
Figure 3: Spatial distributions of cycle-averaged (**a**) electron density, (**b**) electron temperature, (**c**) electric field, with a SEE coefficient of 0.001, 0.005, and 0.01.  
density is almost zero near the electrode wall. Compared to the electron density (Figure 3a), the O\({}^{-}\) ion density is much higher than the electron density in the Ar/O\({}_{2}\) discharge and is nearly two orders of magnitude more than the electron density. Similar conclusions appeared in Refs.