3.1.1. CO₂

CO2+ is formed through the collision of CO₂ with electrons, photons, and ions/neutral particles or a combination of them, such that they can excite the molecule above its ionization energies. The most common reactions are excitation, ionization, and dissociation.

The presence of the First Negative System (B²Σ- X²Σ) for CO⁺, the Chamberlains Airglow System (C³Δ_u - a¹ Δ_g) for O₂, the Swan System (A³Π_g - X³Π_u) for C₂, the Triplet Bands (d³Δ - a³Π) for CO, and the FDB System (A²Π_u - X²Π_g) for CO2+, among others, is observed in the optical emission spectrum of CO₂, as shown in Fig. 2. The bands identified in the optical emission spectrum of CO₂ are presented in Table I ^9,10. The spectra related to CO₂ discharge in Fig. 2.a and 2.b is displays in the range of 250 to 425 nm and from 425 to 1100 nm, respectively.

Figure 2 a) Optical emission spectrum of CO₂ in the range of 250 to 425 nm. b) Optical emission spectrum of CO₂ in the range of 425 to 1100 nm. 

From the identified elements, it can be deduced that the main electronic processes in the plasma of CO₂ are excitation (1) and ionization (2).

The dissociation processes in the molecule of CO₂ generally follows two paths by electron impact ¹¹.

Whether the pathway follows (3) or (4) depends on the vibrational excited state of CO₂. It is also possible to observe the presence of the recombination process like carbon (5) and oxygen (6) molecules.

Once formed, CO2+ may react in different ways, including any method of CO₂ dissociation already discussed:

3.1.2. He

Figure 3 shows the spectrum observed in a He glow discharge. The main electronic process identified is He excitation, which is principally due to electron impact process (Eq. 10).

Figure 3 Optical emission spectrum of He. 

The energy required to remove the first electron of the He atom is 24.6 eV. The energy for ionization of the He atom is very large; therefore, it is easier to obtain excited levels 3s, 3p, 3d, and 4d because these processes involve less energy consumption.

The most intense emission lines of He are listed in Table II.

3.1.3. Mixture of CO₂ - He

Figure 4 displays the optical emission spectrum of the CO₂ -He [33.3% and 66.6%] mixture in two different ranges of wavelengths, from 200 to 500 nm (Fig. 4a) and from 500 to 1100 nm (Fig. 4b), to observe in detail the transition present in the spectra. It is possible to observe small widenings of molecular bands due to vibrational structures. However, when He is added to the CO₂ gas, the effects diminish due to the presence of the following processes

Figure 4 4a) in the range of 200 to 500 nm. 4b): in the range of 500 to 1100 nm. Optical emission spectrum of the mixture CO₂- He [33.3% and 66.6%]. 

Figure 5(a) displays the OES of the CO₂-He mixture plasma, as a function of He concentrations. It can be observed the presence of CO⁺, CO2+, O₂, CO₂, CO, C₂ and He species. The intensity of the He* species decreases, while the intensity of the CO⁺, CO2+, O₂, CO₂, CO, C₂ increases as the He concentration decreases. To understand the effect of the intensity variation of species present in the mixture plasma as a function of the He percentage, we analyzed the evolution of each species as a function of He percentage, by normalizing their intensities (Ix/ΣIx). The line intensity indicates the peak value. Figure 5(b) shows the normalized intensity of CO⁺, CO, CO₂, CO2+, O₂ C₂ and He species as a function of the He percentage. It was observed that the normalized intensity of the species CO⁺, CO₂, CO2+, O₂ and C₂ decreases by approximately 56%, while that for CO decreased by 30%, when 33% of He is present in the CO₂-He plasma mixture. Moreover, the intensities of the species coming from CO₂ decrease monotonically as the He concentration increases. CO specie is the most intense in the CO₂-He plasma mixture. This may be because, the ionization and dissociation processes of CO₂ (6.23 eV) are favored because the energy requirement is lower compared to the energy required to excite the Helium atom (∼23 eV).

Figure 5 a) Optical emission spectra of CO₂-He at different concentrations and; b): Normalized intensity of principal species as a function of He concentration. 

3.1.5. Mixture CO₂/Ar

For Ar, lines of Ar* are observed in the range of 700-850 nm, Fig. 7, as shown in Fig. 7b). By comparing the optical emission spectrum of CO₂ with the spectra of the CO₂-Ar mixtures, a marked decrease is observed in the intensity of the emission bands and CO2+ and CO (Fig. 7b).

Figure 7 a). Optical emission spectrum of the mixture CO₂/Ar [33.3%/66.6%] in the range of 200 to 500 nm; b). in the range of 500 to 1100 nm. 

The presence of CO₂ affects the system configuration, causing changes in the values of electron temperature and the density of ionized particles. When, the molecular gas is introduced into the system, the energy changes, thus favoring the vibrational processes and facilitating the process of Ar ionization ¹².

As the CO₂ concentration increases its percentage in the gas mixture, the intensity of the Ar emission lines in the spectra changes, suggesting that the amount of particles of excited Ar also changes. The identified lines for Ar* in the mixture were 696.50, 706.50, 727.50 738.00, 750.50, 763.50, 772.00, 794.50, 801.00, 811.50, 826.00, 842.00 and 912.00 nm. The identification of excited Ar lines is consistent with the amount of energy in the system because it is cold plasma, having at most the interaction process described in Eqs. 14 and 15.

Figure 8a) shows the OES of CO₂-Ar mixture plasma, as a function of the Ar percentage. Figure 8(a) shows the presence of C⁺, CO2+, O₂, CO₂, CO, C₂, Ar and Ar⁺ species. The intensity of the Ar and Ar⁺ species decreases and the intensity of the CO⁺, CO2+, O₂, CO₂, CO and C₂ species increases as the Ar concentration decreases. Figure 8b presents the normalized intensities of CO⁺, CO, CO₂, CO2+, O₂, C₂ and Ar species as a function of the Ar percentage. The emission spectra present a behavior similar to the CO₂-He plasma mixture case. That maybe due to the energy requirement to ionize or excite the Ar atom (∼15.76 and 11.55 eV respectively) are bigger than the CO₂ molecule, like the CO₂-He plasma mixture case.

Figure 8 a): OES of CO₂-Ar at different concentrations; b): Normalized intensity as a function of Ar concentration. 

The energy in the system is favorable to have the presence of the process of dissociation of the molecule of CO₂, mainly due to electronic processes through electron impact. The intensity ratio of CO-CO₂ mixtures plasma (Fig. 9) shows that the dissociative effect for electron impact increases with the presence of some noble gases being more efficient for He in this work, however the ratios between the intensities belonging to the molecular O₂ and CO₂, Fig. 10, remain close, which means that the O₂ production rate are constant, independently of the noble gas used in the mixture plasma. Secondly, due to values ratios for O₂ being greater than O₂ compared with those of CO₂, it is evident that the O₂ production is more efficient for both gas, i.e. Ar and He. Thus the path dissociation established in the Eq. 4 is favored in both cases, the dissociation process is more important than the excitation.

Figure 9 The intensity ratio of CO-CO₂ in mixture with Ar and He. 

Figure 10 The intensity ratio of O₂-CO₂ in mixture with Ar and He. 

3.2.1. Double Langmuir probe, electron temperature, and ion density

Figure 11 shows the I-V characteristic curve of the 33.3% CO₂-66.6% He mixture plasma, obtained by the double Langmuir probe, along with the corresponding theoretical curve (solid curve). The evaluation of electron temperature (T_e) and ion density (n_i) runs in the following way; first, the probe characteristic curve is differentiated two times. The electron temperature is calculated through the position of the extrema of the characteristic curve’s second derivative ¹³. Then a theoretical curve is fitted to the entire measured experimental I-V characteristic curve by adjusting only the ion density. This evaluation procedure gave an electron temperature of T_e and an ion density of n_i, at each mixture plasma used in this work.

Figure 11 Curve fit of double Langmuir curve of V vs. I. 

The electronic temperature (T_e) of the mixture is in the range of 4.54 to 5.37 eV, and the ion density (n_i) has values between 2.15 × 10⁹ and 12.00 × 10⁹ particles/cm³. The behavior of the aforementioned values is shown in Fig. 12, for different He concentrations. In this work, from ten measurements, the overall variation in the electron temperature and the ion density were found to be 10%.

Figure 12 T_e and n_i in a mixture of CO₂-He at different concentrations of He. 

Performing a similar procedure to obtain the electrical characterization make for before Ar and CO₂-Ar plasmas. In Fig. 13 characteristic (I-V) double Langmuir curve is shows. The electron temperature (T_e) values are in the range of 2.07 to 5.37 eV, and the ion density (T_e) values are between 1.87 × 10¹⁰ and 2.15 × 10⁹ particles/cm³ (Fig. 14). With the addition of Ar gas in the plasma, there is an increase in electron density, which leads to a high electron-electron collision frequency and always tends to deplete electrons. Thus, a decrease in the electronic temperature is caused.

Figure 13 I vs. V characteristic curve for CO₂-Ar. 

Figure 14 T_e and n_i in a mixture of CO₂/Ar at different concentrations of Ar. 

Figure 15 shows the observed behavior of the variables obtained by the electric Langmuir probe, electron temperature, and ion density, as a function of the percentage of molecular gas CO₂ present in the mixture.

Figure 15 T_e and n_i in mixtures of CO₂-He and CO₂-Ar at different concentrations of CO₂. 

Figure 15 also shows the behavior of the electron temperature and ion density as a function of the percentage of CO₂ in the mixture for both Ar-CO₂ and He-CO₂ mixtures plasma. Both behaviors are similar, the electron temperature decreases, while the ion density increases as the concentration of the noble gas increases. Besides, we can observe that with the addition of Ar, the ion density and electron temperature increased more as compared to when He was added.

The values of T_e and n_i in the mixture are present in Table IV.