## 3 Results and Discussion
### Effects on the Production of Reactive Species  
Figure 4 presents an overview of the emission spectra, without additional O\({}_{2}\) in the gas admixture, obtained using (a) the Avantes spectrometer, for the 200-750 nm wavelength range and (b) the Horiba one, for wavelengths between 730 nm and 840 nm. The employed He flow rate was 2.0 L/min and the distance between the plasma outlet and the target was  
Figure 3: Waveforms of applied voltage, capacitor charge and current for various concentrations of additional O\({}_{2}\). The He flow rate was 1.5 L/min and the distance **d** was 7 mm. Vertical scales are constant for each parameter: Voltage: \(\pm\)12 kV; Charge: \(\pm\)70 nC; Current: \(\pm\)40 mA. d** = 3 mm. The main emitting species observed in Figure 4a are NO, from 200 nm to 270 nm, OH at 288 nm, 296 nm and 308 nm, (the last two being jeopardized by N\({}_{2}\) emissions), N\({}_{2}\) from 298 nm to 450 nm and a He line emission at 587 nm. The emitting species observed in Figure 4b are the O I triplet at 777 nm and N\({}_{2}\) and N\({}_{2}^{+}\) in second order emissions. To investigate how the addition of O\({}_{2}\) to the working gas affects the production of RONS in the plasma we performed measurements for different O\({}_{2}\) contents obtaining the intensity of the light emitted by the following species: NO(A \(\rightarrow\) X) emitting at 247 nm (to be referred as NO\({}_{247}\) hereafter), OH(A \(\rightarrow\) X) emitting at 288 nm (OH\({}_{288}\)) and O I at 777 nm (O\({}_{777}\)). Although the emission at 288 nm is not the most intense coming from the OH molecule, it was chosen because it is isolated from emissions of other species, mainly those from N\({}_{2}\). Since the Avantes spectrometer, used for the measurements in the 200-750 nm wavelength range, has low resolution it would not be possible to separate emissions from different species emitting at close wavelengths. Figure 4: Overview of the emission spectrum of the plasma jet obtained using (**a**) the Avantes spectrometer and (**b**) the Horiba one with 2.0 L/min of He flow rate without additional O\({}_{2}\). The distance between the plasma outlet and the target was **d** = 3 mm. The asterisks in the notations in (**b**) indicate second order emissions from N\({}_{2}\) and N\({}_{2}^{+}\). In Section 3.1 we have found that although the discharge duration starts to decrease for more than 1% of O2 in the working gas, the \(P_{dis}\) values do not change considerably for an O2 percentage lower than 2%. Thus, since most applications require higher \(P_{dis}\) values and a longer discharge duration, we decided to analyze in more details the production of the NO, OH and O species only in the range from 0 to 2.5% of extra O2 in the gas admixture. Figure 5 shows the emission intensities of the O777 triplet and the NO247 and OH288 bands as a function of the additional O2 percentage for different He flow rates measured for a distance d = 3 mm in (a), and d = 5 mm in (b). By comparing the curves obtained for the O777 triplet in Figure 5a, for a distance of 3 mm between the plasma outlet and the target at different He flow rates, it can be seen that neither the position of maximal emission intensity nor the profiles of the curves change significantly when He flow rate scales from 1.0 to 2.0 L/min. The intensity values of the O777 emissions increase significantly only when the He flow rate is 2.5 L/min. Regarding the gas flow rate effect on the NO emission intensities as functions of the oxygen content, they exhibit almost the same behavior as O777 emissions. On the other hand, the intensities of OH emissions seemingly do not change if the He flow rate is kept in the 1.5-2.5 L/min range, but at low O2 concentration and 1.0 L/min gas flow rate the OH emission intensity decreases. Concerning the shape of the curves shown in Figure 5a, in general both NO and OH emissions show similar trends that are different from the ones obtained for O777.