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Revista mexicana de física

versión impresa ISSN 0035-001X

Rev. mex. fis. vol.63 no.4 México jul./ago. 2017



Plasma diagnostics of glow discharges in mixtures of CO2 with noble gases

P.G. Reyesa 

A. Gómeza 

J. Vergarab 

H. Martínezc 

C. Torresb 

aLaboratorio de Física Avanzada, Facultad de Ciencias, Universidad Autónoma del Estado de México, Estado de México, 50200, México.

bLaboratorio de Análisis y Sustentabilidad Ambiental, Escuela de Estudios Superiores de Xalostoc, Universidad Autónoma del Estado de Morelos, Morelos, 62717, México.

cLaboratorio de Espectroscopia, Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Morelos, 62210, México.


This study presents the plasma diagnostics of a glow discharge produced in two different mixtures, CO2/He and CO2/Ar, at a constant pressure of 1.5 Torr. The experiment was conducted to determine the carbon dioxide decomposition in the plasma by using the optical emission spectroscopy (OES) method through identifying lines and emission bands in the spectra. In addition, an electrical characterization of the mixture plasma was made by the determination of ion density (ni) and the electronic temperature (Te), estimated through a double Langmuir probe. The electronic temperature for the mixture plasma was in the range of 2.07-5.37 eV, and the ion density between 2.15 × 109 and 18.70 × 109 particles/cm3. The principal bands and lines identified in the OES correspond to CO2+, CO2, CO+, CO, O*, O2, O2+, C2, He* and Ar*.

Keywords: Langmuir probe; electron temperature (Te); ion density (ni); OES; gas mixture

PACS: 52.20.Fs; 52.25.Os

1. Introduction

Plasma is a partially ionized gas, where the electron temperature in the system has values lower than 10 eV with a density of ionized particles in the order of 109 particles/cm3. The species produced in the plasma can interact with each other, producing several species highly reactive in the plasma, with many technological applications 1, such as in semiconductor materials and materials technology (for example, thin film deposition and etching), new light sources, solar cells, water pollution, medical applications, and advanced chemical processes 2,3,4,5. Carbon dioxide (CO2) is the largest contributor to the greenhouse effect; small variations in the concentration of this gas in the atmosphere can cause adverse effects on the environment and the ecosystem. Therefore, researchers continue to try solving the problems that are caused due to the change in the concentration of CO2.

In literature, there are several types of studies that show different ways of plasma production to dissociate the molecule of CO2, which include dielectric barrier discharges, glow discharges, corona discharges, microwave discharges, RF discharges, and the use of mixtures with noble gases such as He and Ar 6. These noble gases are known to reduce the breakdown voltage in plasmas without changing their properties. For example, in Ar-CO2 plasma, the presence of Ar helps to sustain the Ar-CO2 discharge by providing electrons 7, and the energy changes by favoring vibrational processes in the CO2 molecule before the ionization and then the noble gas can be ionized. Because the Ar ionization potential is much lighter than that of CO2, the energy loss in the interaction process is reduced; thus, the impinging energy of the electrons increases.

CO2 is a major source of the greenhouse effect, and no systematic studies on the effect of the He and Ar addition on the CO2 conversion, including detailed attempts to explain the behavior based on the underlying mechanism, have been reported yet. In the present work, we investigate the effect of He and Ar addition on the CO2 conversion in a glow discharge over a broad concentration range of He and Ar in the mixture plasma. The decomposition products of CO2 were analyzed by OES and detailed electrical characterization of the plasma.

2. Experimental Setup

The system that is represented in Fig. 1, described in a previous work 8, comprises two circular electrodes of copper with diameters of 2.5 cm, in a vacuum chamber with a spacing of 20 mm between both electrodes. To evacuate background gases, the chamber (with a volume of 1.16 × 104cm3) was pumped down to a base pressure of 10-6 Torr (turbo-molecular pump Alcatel ATP80 and mechanical pump Varian DS302). Subsequently, the chamber was filled with a continuous dynamic flow of CO2, He, or Ar (ultra-pure gases, Infra 99.99%) through needle valves at the desired pressures. First, the CO2 gas was introduced in the chamber at a partial pressure, and then, the He (or Ar) gas was introduced in the system. The mixture was varied by changing the partial pressure of He (or Ar) and maintaining a total pressure of 1.5 Torr. Once the plasma was generated, the discharge current values were kept constant at 10 mA, independent of the mixture. The voltage (DC power supply Spellman SA4) was 300 V for 100 % of He, 500 V for 100 % of CO2 and 270 V for 100% of Ar.

Figure 1 Experimental system. 

The optical emission spectrum was measured using an Ocean Optics HR4000CG-UV-NIR spectrometer in the range of 200-1100 nm with a resolution of 0.75 nm FWHM. The signal was corrected by considering the spectral sensitivity of the spectrometer. The wavelength accuracy of the spectrometer was calibrated using an Ar source (Ocean Optics Inc.). Spectra data were obtained with a 10 s integration time. Spectral response of the CCD spectrometer was measured using a reference light source (deuterium lamp for UV, and Ocean Optics LS-1-CAL for visible and IR).

With the use of a double Langmuir probe, the electron temperature and ion density were determined. The voltage applied to the probe was provided by a programmable direct current power supply (GW INSTEK, PST-3202), with 1 volt steps, from -30 V to +30 V. The current generated from the collection of charges inside the plasma was measured by a Fluke multimeter. The current-voltage (I-V) curves were the result of an average of 15 data scans.

3. Results

3.1. Optical emission spectroscopy (OES)

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 (C3Δu - a1 Δg) for O2, the Swan System (A3Πg - X3Πu) for C2, the Triplet Bands (d3Δ - a3Π) for CO, and the FDB System (A2Πu - X2Πg) 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. 

Table I Species present in the OES of CO2

Species λ(nm) Upper configuration Lower configuration υ' - υ"
CO+ 289.98 X2Σ B2Σ 6,10
CO2+ 305.66 X2Πg А2Пu 6,2
CO2+ 314.16 X2Πg А2Пи 3,0
CO2+ 326.09 X2Πg А2Пи 3, 1a
CO2+ 338.27 X 2Πg А 2П и 2,0
CO2+ 351.75 X2Πg А2Пи 1,1a
CO2+ 368.11 X2Πg А2Пи 1,2
CO2 386.27 a1 Δg C3Δu 5,3
CO2+ 387.53 X2Πg А2Пи 2,4
CO2 391.53 1B2 X1 Σ+ -
CO2+ 392.05 X 2Πg А2Пи 1,1
CO2 403.08 1B2 X1Σ+ -
CO2+ 407.28 X2Πg А2Пи 1,4
CO+ 413.05 X2Σ А2П 1,7
CO2 425.88 1B2 X1Σ+ -
CO2 433.72 1B2 X1 Σ+ -
C2 436.08 X3Пu А2Пg 4,2
CO 448.61 а3П d3Δ 10,2
CO+ 471.01 X2Σ А2П 2,1
C2 473.61 X3Пu А3Пg 1,0
CO 496.72 а3П d 6,1
C2 516.40 X3Пu А3Пg 0,0
CO 555.86 а3П d 4,1
C2 563.86 X3Пu А3Пg 0,1
CO 603.55 а3П d3Δ 1,0
02+ 642.29 X4Пu b4Σg- 0,1
CO 646.36 а3П d3Δ 0,0
CO2 695.47 X3Σg- b1Σg+ 2,1
O* 777.48 2s22p3(4S0)3p 2s22p3(4S0)3s -

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

e- +CO2e-+CO2* (1)

e- +CO22e-+CO2+ (2)

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

e-+CO2e-+CO+O (3)

e-+CO2e-+ C+O2 (4)

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.

C+CC2 (5)

O+OO2 (6)

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

e-+CO2+CO2*+O (7)

e-+CO2+C+ O2 (8)

e-+CO2+CO2+hv (9)

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.

He+e-He*+e- (10)

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

Table II Species present in the OES of He. 

Species λ (nm) Upper Lower
He* 389.16 1s3p 1s2s
He* 501.90 1s3p 1s2s
He* 587.69 1s3d 1s2p
He* 668.17 1s3d 1s2p
He* 708.82 1s3s 1s2p
He* 728.44 1s3s 1s2p

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

He+e-He++2e- (11)

He++CO2CO2++He (12)

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. 

3.1.4. Ar

The Ar emission lines are shown in Fig. 6, which shows that the principal process in the discharge is the atomic excitation by electron impact.

Figure 6 Optical emission spectrum of pure Ar. 

The process of excitation of Ar is described in (13), in which the electron that comes mainly from the plasma interacts with the Ar atom giving sufficient energy to increase its electron levels to 4s and 4p, which are identified with more intensity in OES (Table III).

Ar+e-Ar*+e- (13)

Table III Species present in the OES of Ar. 

Species À(nm) Upper configuration Lower configuration
Ar+ 283.00 3s23p4(1D)4p 3s23p4(3P)4s
Ar+ 309.00 3s23p4(3P)4d 3s23p4(3P)4p
Ar+ 336.50 3s23p4(3P)4d 3s23p4(3P)4p
Ar+ 357.00 3s23p4(3P)4d 3s23p4(3P)4p
Ar+ 380.00 3s23p4(1D)4d 3s23p4(3P)4p
Ar+ 405.00 3s23p4(1S)4p 3s23p4(1S)4s
Ar* 416.00 3s23p5(2P3/2)5p 3s23p5(2P3/2)4s
Ar* 420.00 3s23p5(2P3/2)5p 3s23p5(2P3/2)4s
Ar* 427.00 3s23p5(2P3/2)5p 3s23p5(2P3/2)4s
Ar+ 434.50 3s23p4(3P)4p 3s23p4(3P)4s
Ar+ 443.50 3s23p4(3P)3d 3s23p4(3P)
Ar+ 461.00 3s23p4(1D)4p 3s23p4(1D)4s
Ar+ 476.50 3s23p4(3P)4p 3s23p4(3P)4s
Ar+ 488.00 3s23p4(3P)4p 3s23p4(3P)4s
Ar* 560.50 3s23p5(2P3/2)5d 3s23p5(2P3/2)4p
Ar* 603.50 3s23p5(2P3/2)5d 33s23p5(2P3/2)4p
Ar+ 656.50 3s23p4(3P2)4f 3s23p4(1S)3d
Ar* 675.50 3s23p5(2P3/2)4d 3s23p5(2P3/2)4p
Ar* 696.50 3s23p5(2P1/2)4p 3s23p5(2P3/2)4s
Ar* 706.50 3s23p5(2P1/2)4p 3s23p5(2P3/2)4s
Ar* 727.50 3s23p5(2P1/2)4p 3s23p5(2P3/2)4s
Ar* 738.00 3s23p5(2P1/2)4p 3s23p5(2P3/2)4s
Ar* 750.50 3s23p5(2P1/2)4p 3s 23p5(2P1/2)4s
Ar* 763.50 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s
Ar* 772.00 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s
Ar* 794.50 3s23p5(2P1/2)4p 3s23p5(2P1/2)4s
Ar* 801.00 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s
Ar* 811.50 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s
Ar* 826.00 3s23p5(2P1/2)4p 3s23p5(2P3/2)4s
Ar* 842.00 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s
Ar* 852.00 3s23p5(2P1/2)4p 3s23p5(2P1/2)4s
Ar* 912.00 3s23p5(2P3/2)4p 3s 23p5(2P3/2)4s
Ar* 922.00 3s23p5(2P3/2)4p 3s23p5(2P1/2)4s
Ar* 965.50 3s23p5(2P3/2)4p 3s23p5(2P3/2)4s

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.

Ar +e-Ar++2e- (14)

Ar++CO2CO2++Ar (15)

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. Electrical characterization

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 (Te) and ion density (ni) 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 Te and an ion density of ni, at each mixture plasma used in this work.

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

The electronic temperature (Te) of the mixture is in the range of 4.54 to 5.37 eV, and the ion density (ni) 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 Te and ni 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 (Te) values are in the range of 2.07 to 5.37 eV, and the ion density (Te) 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 Te and ni 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 Te and ni 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 Te and ni in the mixture are present in Table IV.

Table IV Values of Te and ni for CO2-He and CO2-Ar mixtures. 

CO2-He CO2-Ar
% CO2 Te (eV) ni (× 109 particles/cm3) Te (eV) ni (× 109 particles/cm3)
0 4.54 12.00 2.07 18.70
33.3 4.79 7.60 3.19 14.12
66.6 5.30 3.11 4.78 6.41
100 5.37 2.15 5.37 2.15

4. Conclusions

A detailed OES and electrical characterization of glow discharges operating in CO2, and CO2-He and CO2-Ar mixtures to investigate the CO2 decomposition were performed. When, CO2 gas is mixed with a noble gas (Ar or He), the average kinetic energy of electrons decreases, while the amount of ionized particles increases. This effect can be explained by the fact that the concentration of a noble gas in the mixture increases, as the electron collisional frequency decreases, causing the electrons to be in the presence of an electric field longer, which accelerates them.

According to the analysis of the normalization intensities and, CO-CO2 and O2-CO2 ratios obtained by an OES, the intensity of the decomposition products of CO2 in the mixture of CO2-He is greater than CO2-Ar mixture at the same concentrations. This is consistent with the studies of these mixtures conducted with different experimental techniques to produce the plasma. CO2 could be efficiently decomposed by glow discharge of CO2-He and CO2-Ar mixtures. He gas was found to be more effective for the preferential decomposition of CO2 of CO + O at a 33% Ar/66% CO2 mixture.

The electron temperature was found to be in the range of 2.07 to 5.37 eV, and the ion density was between 2.15 × 109 and 18.70 × 109 particles/cm3. The species identified were CO2+ resulting from CO2+e-CO2++2e-;CO+ resulting from CO2 + e- → CO+ + O + 2e-; CO resulting from CO2 + e- → CO+ O+e-, and O* resulting from O+OO2,O2+ resulting from CO2+e- → C+O2 + + 2e-, and C2 resulting from C+C → C2. whereas the noble gases emission lines identified were He* and Ar*.


We are grateful to O. Flores (ICF-UNAM) for her technical assistance. This research was supported by DGAPA [IN-102916], CONACyT [268644], PROMEP [103.5/13/6626], PRODEP [DSA/103.5/15/6986], PII-43/PIDE/2013 and UAEM 4307/2017/CI.


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Received: January 09, 2017; Accepted: May 18, 2017

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