## 3 Results and Discussion
### Plasma Characterization
Wide emission spectra of the DBD plasma generated at different applied voltage frequencies were recorded for both working gas mixtures, He and He with ethylene glycol vapors (Figure 5a). From the assigned emission features, it can be noticed that atomic (such as He and oxygen spectral lines) and molecular plasma species (such as OH radicals, N\({}_{2}\) and N\({}_{2}^{+}\)) are constituted in both DBD working environments. The spectral region showing the neutral N\({}_{2}\) (\(C_{3}\prod_{u}\to B_{3}\prod_{g}\)) and ionic N\({}_{2}^{+}\) (\(B^{2}\sum_{u}\to X^{2}\sum_{g}^{+}\)) molecular nitrogen species and involved transitions [35] is presented in Figure 5b. As expected, irrespective of the working gas composition, the intensities of all DBD plasma species increase monotonically with discharge frequency because more discharge pulses are caught during the gate width for which the optical signal was recorded. Instead, the intensities of the most representative plasma species plotted as a function of discharge frequency (Figure 5c,d) follow a drop trend with the addition of a monomer, except for the intensities of the He spectral lines that remain constant. A possible route for the formation of the excited species (i.e., N\({}_{2}\) and He) is electron impact excitation, while N\({}_{2}^{+}\) is created by Penning ionization. The preponderant ionization process changes to electron impact ionization when monomer vapors are introduced [29; 36; 37; 38; 39]. Figure 5: Space-integrated OES obtained results: (**a**) wide emission spectra recorded for 2 kHz applied voltage frequency; (**b**) nitrogen vibrational emitted bands,N\({}_{2}\) (\(C_{3}\prod_{u}\to B_{3}\prod_{g}\)) and N\({}_{2}^{+}\) (\(B^{2}\sum_{u}\to X^{2}\sum_{g}^{+}\)), with involved transitions; intensity variations of the most representative plasma species with frequency, recorded for DBD plasma operated in (**c**) He and (**d**) He with ethylene glycol (EG) vapors. Following the procedure described in Section 2.2, the recorded vibrational and rotational spectra were used for plasma temperature calculation. Figure 6 depicts the frequency variations of rotational (T\({}_{\text{rot}}\)) and vibrational (T\({}_{\text{vib}}\)) temperatures. The maximum plasma temperature of 2850 K (0.25 eV) found for the DBD plasma operating in He is much lower than the energy required to break the C-C bond (3.59 eV) in the EG monomeric unit, preventing the monomer from completely decomposing. As the time gap between individual voltage pulses decreases, no significant effects are observed on the plasma temperature, although the previously presented variation of the discharge current with the frequency of the applied voltage (Figure 3) suggests a change in the plasma parameters. This insensitivity of the plasma temperature is probably due to the way the spectral measurements were performed, i.e., the optical signal was time-averaged over the used gate width. Instead, these dependencies clearly show that the introduction of monomer vapors into the discharge led to a decrease in T\({}_{\text{rot}}\) and T\({}_{\text{vib}}\) all over the investigated frequency range, along with the optical signal associated with almost all plasma species (except for the He-assigned spectral features; see corresponding curves presented in Figure 5c,d). For instance, the T\({}_{\text{rot}}\) drops from approximately 475 K to 350 K (Figure 6a), and the T\({}_{\text{vib}}\) falls from 2850 K to 2650 K (Figure 6b) with monomer vapor addition. These results agree with the previously presented data related to DBD current recordings (see the discussions related to Figure 3) when the introduction of monomer vapors causes at high frequencies a significant decrease in the discharge current. This drop in electrical current was attributed to an enhancement of the recombination rate of the charged particles coming from a plasma-induced monomer fragmentation process.