## 3 Results and Discussion
### Optical Emission Spectra of APPJ
For wet argon discharges when the O\({}_{2}\) percentage increased to 1.5%, as depicted in Figure 14a,b, at the two observed locations (nozzle and 10 mm), IES was measured for a wettability of 1.5% for O\({}_{2}\) at 10 mm from the nozzle (Figure 14b), giving the same lines and bands as those observed at the nozzle, but IES increased by 37% for (OH) band and decreased by 33% for (N\({}_{2}\)) bands. Figure 13: Optical emission spectra of APPJ using oxygen/argon admixture discharge at an applied flow rate of 0.25% of O\({}_{2}\) for (**a**) nozzle and (**b**) 10 mm. Generally:  
* IES decreases along the jet length and decreases apart from the nozzle due to the presence of the strong electric field near the nozzle and because of the high rate of charge carrier generation by electron impacts near the nozzle. * As the admixture of oxygen increases, IES decreases due to the decrease in electron density and temperature resulting from significant dissociation of the oxygen molecules in the discharge processes [34]. * As the admixture of oxygen increases, IES for the concentration of O, OH, and NO radicals increases far away from the nozzle, compared with the dry argon case. * There are many reactions for dry argon discharge, as follows [35]:  
* Argon reacts with an energetic electron (e*) to produce metastable argon (Ar\({}^{\text{m}}\)): \[\text{Ar}+\text{e}^{*}\rightarrow\text{Ar}^{\text{m}}+\bar{\text{e}}\] (3)  
Figure 14: Optical emission spectra of APPJ using oxygen/argon admixture discharge at an applied flow rate of 1.5% of O\({}_{2}\) for (**a**) nozzle and (**b**) 10 mm. * Ar\({}^{\rm m}\) reacts with e\({}^{\bullet}\) to produce excited argon (Ar*): \[\rm{e}\ ^{*}+Ar^{\rm m}\to Ar^{\bullet}+\bar{e}\] (4)
* Ar\({}^{\star}\) reacts with water to generate OH\({}^{\bullet}\): \[\rm{H_{2}O}+Ar^{\star}\to H^{\bullet}+OH^{\bullet}+Ar\] (5)
* For the oxygen admixture in wet argon, there are many dissociation reactions in addition to the reactions for dry argon [36; 37]: \[\rm{O_{2}+e^{-}\to 2O+e^{-}}\] (6) \[\rm{N_{2}+e^{-}\to 2\,N+e^{-}}\] (7) \[\rm{N+O_{2}\to NO+O}\] (8) Figure 11:0 (9) Figure 11:1 (10)  
From the dissociation reactions (6) to (10), it is concluded that: (i) The dissociation reactions, the degree of gas ionization, and the production of ozone, \(\rm{O_{3}}\), play an important role in the inactivation process. (ii) Due to the oxygen admixture in wet argon, there are dissociation reactions that produce radicals and charged particles under high-energy electron bombardment [38]. (iii) The bonding energy B.E\({}_{\rm N=N}\) (9.79 eV) > B.E \(\rm{O_{O}=O}\) (5.15 eV) and the dissociation and concentration of active species of \(\rm{O_{2}>N_{2}}\) lead to an inactivation process by \(\rm{O_{2}}\) higher than for \(\rm{N_{2}}\)[39]. (iv) OH\({}^{\bullet}\) and O\({}^{\bullet}\) radical emission results due to impurities in the gas or due to the entry of air into the discharge zone [40]. (v) The presence of OH, O, and \(\rm{N_{2}}\) bands and lines is attributed to the interaction of ambient air with excited argon species, as well as high-energy electrons in the plasma. Reactive species, such as O, OH, and NO, are the most effective agents in biomedical applications; the plume interacts with ambient gases and molecules. The spectral lines of other elements will be investigated in our future experimental study on APPJ impact parameters that affect the microbial inactivation process of bacteria [41].