## 3 Results and Discussion
### Plasma Characterization  
A typical glow operational mode of a DBD plasma is described by two current characteristic peaks: one positive associated with the occurrence of a primary (1 ry) discharge and one negative linked to the generation of a secondary (2 ry) discharge (Figure 2a). The discharge current intensity was recorded within the frequency range of 2-10 kHz of an atmospheric-pressure DBD operating in He and a mixture of He and ethylene glycol (EG) vapors. In Figure 3, the variations of the current intensity peak with frequency for both cases are presented. The positive current peak, linked to the 1 ry structure, exhibits an increase until a value of 6 kHz is reached (Figure 3a). Subsequent increases in frequency result in a stagnation region of the dependence if the DBD operates only in He (see the red curve in Figure 3a) or lead to a slow decreasing part if the monomer vapors are added (see the black curve in Figure 3a). The initial increase in the current could be explained by an enhancement of the ionization processes occurring near the high voltage (HV) electrode during the first half-cycle of the discharge pulse. As the time gap between individual voltage pulses decreases, the charged particles (ions and electrons) that reach the surface of the electrodes diminish, especially in the case when the monomer vapors are added. This behavior can be explained by an augmentation of the recombination rate of the charged particles at high frequencies, when a variety of molecular and atomic species are very likely to be produced through a plasma-induced monomer fragmentation process [33]. The reversal polarity of the electrodes in the second half of the discharge cycle reveals a mirror evolution of the current associated with the 2 ry plasma discharge (Figure 3b). To investigate the dynamics of the two plasma regions, ICCD fast imaging measurements were performed. In Figure 2b are presented spectrally integrated ICCD fast images  
Figure 3: Variation with frequency of the current intensity peak of the 1 ry (**a**) and 2 ry (**b**) discharge in He and He with ethylene glycol (EG) vapors. of the 1 ry and 2 ry plasmas at different delays from the positive and negative discharge current peaks, respectively. Taking the axial cross-section of each image with the corresponding time sequences, the spatio-temporal evolution of the 1 ry and 2 ry plasmas is obtained (Figure 4). The spatio-temporal profiles corresponding to the first half-cycle of the DBD (i.e., 1 ry plasma) operated in He shows the propagation of this region toward the ground (GR) electrode. The maximum emission of this plasma structure is located at 0.4 mm from the HV electrode and at 0.44 us from the positive current peak reference (Figure 4a). With the introduction of ethylene glycol vapors into the discharge gap, the emission intensity of this region considerably diminishes, with no significant changes in the emission maximum (Figure 4b). Due to the \"reversal\" role of the electrodes acting during the second half-cycle of the applied discharge pulse [34; 35], a 2 ry plasma appears next to the GR electrode (at 0.5 mm) that propagates backward to the 1 ry plasma region (Figure 4c). Unlike the 1 ry plasma structure, the total intensity emitted by this region is less affected by the monomer vapor's introduction (Figure 4d), suggesting that the main processes influencing the properties of the polymerized film take place near the HV electrode. To get deeper insight into the kinetics of the plasma species, time-integrated OES measurements were performed within the investigated frequency range. Although most of the plasma optical signal intensity comes from the discharge region next to the HV electrode, the spectral measurements were performed by integrating the signal over the entire discharge space. Figure 4: Spatio-temporal evolution of the primary (1 ry) (**a**,**b**) and secondary (2 ry) DBD plasmas (**c**,**d**) in He and He with ethylene glycol (EG) vapors recorded for a voltage frequency of 5 kHz.