Spectroscopic Characterization of Miniaturized Atmospheric-Pressure dc Glow Discharge Generated in Contact with Flowing Small Size Liquid Cathode
The miniaturized atmospheric pressure glow discharge (APGD) generated between a solid electrode and a flowing small size liquid cathode (dimension 2 mm) was investigated here using optical emission spectroscopy. The discharge was studied in an open air atmosphere, and the spectral characteristics of the plasma source was examined. Analysed APGD was operated at a discharge voltage of 1,100–1,700 V, a discharge current of 20 mA and gaps between a solid anode and a liquid cathode in the range from 0.5 to 3.5 mm. The emission intensities of the main species were measured as a function of various experimental conditions, including the solution flow rate, the gap between the electrodes, and the concentration of hydrochloric acid. The excitation temperature, the vibrational temperatures calculated from N2, OH, and NO bands, and the rotational temperatures determined from band of OH, N2 and NO, were found to be dependent on these experimental parameters. The electron number density was determined from the Stark broadening of Hβ line. Additionally, the ionization temperature and degree were calculated using the Saha–Boltzmann equation, with the ion to atom ratio for magnesium (MgII/MgI). The results demonstrated that Texc(H), Tvib(N2), Tvib(OH), Tvib(NO) and Trot(OH) were well comparable (~3,800–4,200 K) for selected plasma generation conditions (gap ≥2.5 mm, HCl concentration ≥0.1 mol L−1), while the rotational temperatures determined from band of N2 (~1,700–2,100 K) and band of NO (~3,000 K) were considerably lower. The electron number density was evaluated to be (3.4–6.8) × 1020 m−3 and the ionization temperature varied, throughout in the 4,900–5,200 K range.
KeywordsAtmospheric pressure glow discharge Optical emission spectrometry Plasma diagnostics Liquid cathode Miniaturized plasma source
Atmospheric pressure glow discharges (APGD’s) generated in contact with liquid are of great importance due to their potential applications in analytical spectrometry [1, 2, 3, 4, 5] as compact and low operating cost atomic emission micro-sources. These plasma sources may be also applied to the production of special kinds of materials (coatings, nanoparticles, etc.) [6, 7], to surface modification , to the purification of waste water from organic compounds [8, 9, 10], and to water sterilization [8, 9].
Usually the generation of the APGD between a solid anode and a liquid cathode requires application of a dc high voltage [10, 11, 12, 13, 14]. Stable APGDs with liquid electrodes have been generated by applying voltages between 0.4 and 1.8 kV and currents between 5 and 150 mA, over gaps ranging from 0.2 to 5 mm in the air atmosphere [7, 15, 17, 18]. On the other hand, by using a miniature argon or helium gas flow instead of a solid rod anode, stable glow discharge has been observed for gaps of up to 20 mm between the anode and the liquid cathode . Stable APGD was also generated in other gas atmospheres like N2, N2O, CO2 .
The main aim of this work was the spectroscopic characterization of miniaturized plasma source operated in an open air atmosphere between solid anode and a new construction of liquid cathode. As a small size liquid cathode the solution of hydrochloric acid spiked with Mg ions was applied. Various plasma parameters were measured at different experimental conditions. Investigated plasma parameters included the ionization, excitation, vibrational and rotational temperatures derived from various species, as well as electron number density. Experimental variables were the flow ratio of solution, the gap between electrodes, and the concentration of hydrochloric acid. In addition, we investigated the plasma equilibrium. Additionally, the range of plasma stability was described. It was found, that it is possible to generate stable plasma with flow of electrolyte and without, i.e., between solid anode and graphite tube. Advantageously, in this work a stable micro glow discharge was obtained at a lower solution uptake (0.6 mL min−1) than it has usually been reported in previous studies.
In the plasma device construction developed by us, the glow discharge was generated between a molybdenum rod anode and a small size flowing liquid cathode in an open air atmosphere. In order to improve the electrical contact with the electrolyte solution, an additional graphite tube was applied (Fig. 2). At the top of the graphite tube (4 mm) it was encountered with the electrolyte. In contrast with other studies described in a recent review paper , we found that a lower sample uptake (range of flow rates from 0.60 to 3.20 mL min−1) was required to maintain a stable glow discharge at atmospheric pressure. It was also possible generate stable plasma without liquid flow, i.e., between solid anode and graphite tube. This plasma device construction proposed here is considerably less sensitive for any instabilities and interferences than the former devices.
Type of discharge
Open air/atmospheric pressure
Gap between electrodes
Electrolyte flow rate
0.60–3.2 mL min−1
Solution of HCl, spiked with Mg ions, maximum diameter 2 mm
A JY TRIAX 320 scanning monochromator (grating 1,200 grooves mm−1, blazed at 250 nm) was used with a Hamamatsu R-928 photomultiplier biased at −700 V, to measure the emission coming from the plasma over the range from 200 to 850 nm. The output signal of the photomultiplier was amplified using a JY SpecAcq2 system. The SpectraMax/32 software was used for data processing and collecting. Plasma emission was measured near the liquid cathode region. A quartz achromatic lens (f = 75.6, diameter 2”) was used to project the plasma radiation onto the entrance slit of the monochromator. Calibration of the whole optical system was undertaken with a CL2 halogen lamp (Bentham), working at 8.5 A, 17.2 V, with a known spectral emissivity (Protection Engineering Ltd. certificate of radiation).
Experimental Results and Discussion
Emission Spectra of Atmospheric Pressure dc Glow Discharge
Atomic and molecular species identified in the region near the liquid cathode
Numerous bands of the C3Πu–B3Πg system
Bands of the A2Σ–X2Π system (0–0) (1–0) (2–0)
Numerous bands of the γ-system (A2Σ+–X2Π)
Band of the A3Π–X3Σ− system (0–0) at 336.0 nm
Lines at 486.1 nm (Hβ) and 656.2 nm (Hα)
12.74 eV; 12.09 eV
Lines at 777.2 and 844.6 nm
10.74; 10.99 eV
Line at 285.21 nm
Lines at 279.55 and 280.26 nm
The spectral lines resulted from excitation processes of species originating either from the saturated water vapour and/or from the surrounding air atmosphere. If the water in the system was replaced by a solution containing Mg ions, additionally strong atomic line of the Mg I at 285.21 nm, was observed. We also detected and measured weak lines of ionized magnesium, i.e., Mg II at 279.55 nm and Mg II at 280.257 nm (see Fig. 2b). As can be seen in the Table 2, the maximum energy of excited states of species was up to 13 eV.
The presence of magnesium atoms (or ions) in the plasma zone can be attributed to plasma sputtering of the water surface, similarly to the plasma sputtering processes in low pressure glow discharge, according to [18, 19]. Generally, emission intensities of atomic and ionic Mg lines decreased with lowering concentration of HCl in solution. By contrast, the decrease of HCl concentration in solution (and consequently, the decrease of the solution conductivity) caused the increase of the burning voltage from 1,300 to 1,700 V. A similar dependence was noted by Mezei et al. in a ELCAD source . At the HCl concentration of 0.01 mol L−1, the Mg atomic spectral line was very weak and the Mg II lines were not excited. A similar effect was also observed by Cserfalvi and Mezei , who also observed the metal atomic lines when the solution pH was higher than 2.5. In this work, the emission ion to atom ratio for Mg (MgII/MgI) was found to be between 0.027 and 0.080. It is two orders of magnitude lower than in the inductively coupled plasma (ICP) source working at atmospheric pressure . The maximum value of the ion to atom ratio for Mg, (0.080), was observed using a solution containing 1.0 mol L−1 HCl.
In contrast to others reports on emission spectra of APGD plasmas generated using significantly higher currents and slightly lower voltage [11, 25], we did not observe any molecular bands of N2+ or any ionic lines of oxygen (O II) in the near cathode region. The excited states of ionized molecular nitrogen (N2+) and ionized oxygen (O+) require threshold energies of approximately ~18 and ~25–35 eV, respectively. Anyway, these values seem to be large compared with the energies of electrons or other high energy species present in glow discharge plasmas at atmospheric pressure [5, 13].
The Behavior of Active Plasma Species
Emission from plasma was monitored versus experimental parameters, including solution flow rate, gap size (length of separation between the solid anode and the liquid cathode) and the HCl concentration in solution.
Raising the solution flow rate from 0.60 to 3.2 mL min−1 led to a linear increase in the emission intensities of OH, H, O and N2 species (Fig. 4b), while the intensity of the Mg I line decreased and was lowest for solution flow rates of 1.9–2.3 mL min−1. A similar effect was also observed by Shaltout  for the Ca I emission line in a dc microplasma source, generated at the following conditions: current 20–80 mA, voltage 500 V, gap 0.5–2 mm. The decrease in the Mg I emission along with growing solution flow rate may also be a consequence of additional water vaporization. The presence of additional water (or products coming from water) may reduce the energy and number of free electrons responsible for the excitation of Mg atoms.
Intensities of the main species were also monitored as a function of HCl concentration. The intensities of OH, N2, Mg I and O I increased with the growth of the HCl concentration, while the emission intensity of the H emission line changed in a different manner (Fig. 4c). The dissociation of water vapour (H2O + e = OH + H + e) and dissociative recombination of H2O+ ions (H2O+ + e = OH + H) were found to be the dominant processes for the production of H and OH species in the central part of the plasma . However, the difference in behavior of the OH and H emission intensities as a function of experimental conditions can be treated as evidence for the presence of a multichannel process in the region near the liquid cathode. The excited state of nitrogen, N2(C), was produced by electron impact excitation of nitrogen in the surrounding air. In contrast, a stepwise reaction consisting of the dissociation of molecular oxygen followed by electron impact excitation of oxygen atoms was probably the main mechanism responsible for producing excited states of atomic oxygen . An alternative process, dissociative excitation of water by electron impact resulting in excited states of O, seems to be unlikely due to very high energy thresholds (17.6 eV) . The growth of Mg I intensity with HCl concentration is probably due to an enhanced sputtering rates of the liquid cathode at higher HCl solution concentrations.
Plasma temperatures, i.e., the ionization, excitation, vibrational, and rotational temperatures, and electron number density are very important parameters for description of a plasma source. The plasma temperatures of different species, permit a characterization of plasma equilibrium phenomena. Additionally, knowledge of the electron number density plays an important role in understanding the processes (excitation, ionization, etc.), which exist in a plasma source.
The Excitation, Vibrational and Rotational Temperatures in the APGD Source
As shown in Fig. 5, an increase of solution flow rate from 0.6 to 3.2 mL min−1 resulted in a growth of temperatures. The vibrational temperature determined from N2 bands changed from 3,400 to 4,600 K and the excitation temperature from 3,900 to 4,300 K. The Trot(OH) and Trot(N2) depended slightly on the solution flow rate and varied from 3,600 to 4,000 K and 1,900–2,100 K, respectively (Fig. 6). The maximum values of the vibrational and excitation temperatures (~4,200 K) were obtained for a gap equal to 1.5 mm (Fig. 7). For small gap sizes (0.5–1 mm) the excitation, vibrational and rotational temperatures were the lowest. The increase in the gap between electrodes caused a growth in these temperatures. The most significant change was noted for Trot(OH), namely from 2,700 to 4,000 K. This was probably associated with an increase of the discharge power from 22 to 34 Watts, especially taking into account that water vapour is a carrier medium. A similar effect was observed by Webb et al.  for the rotational temperature (from 2,700 to 3,200 K). The gap size only slightly affected the Trot(N2) (changes in the range ~1,700–1,900 K). Differences in the rotational temperatures indicate that mechanisms of the excitation process of these molecules are different. Changing the HCl concentration from 0.01 to 1 mol L−1 caused the Trot(OH) and Trot(N2) increase from 3,200 to 4,100 K and from 1,700 to 2,100 K, as shown in Fig. 8, respectively. For the HCl concentrations ranging from 0.1 to 1.0 M, the excitation temperature and the vibrational temperatures as well as the rotational temperature determined from OH band were nearly equivalent (~3,800–4,200 K), while for the solutions containing 0.01 M HCl, the Texc(H) and Tvib(N2) temperatures (~4,300 K) were higher than the Trot(OH), Tvib(NO) and Trot(OH) temperatures (3,300–3,500 K; Fig. 8).
Generally, the rotational temperatures determined from OH and N2 band changed in the range from 2,700 to 4,100 K and from 1,700 to 2,100 K, respectively. Comparable values of the rotational temperatures were noted by Bruggeman et al. [11, 22] in the region near the liquid cathode. Similar rotational temperature determined from band of OH was also measured Nikiforov et al.  by LIF and Webb et al.  by optical emission spectroscopy (OES) techniques. The vibrational temperature and excitation temperature were consistent with the previous studies [12, 22]. Throughout all experiments, the Trot(N2) was considerably lower than the Trot(OH). The difference between these temperatures ranged from 1,000 to 2,100 K. Nevertheless, a relatively good linear correlation (R2 = 0.85) was found between Trot(OH) and Trot(N2). The rotational temperature calculated from band of NO was equal to ~3,000 K and was slightly dependent on experimental conditions. It was always observed that Trot(OH) > Trot(NO) > Trot(N2). It can be explained by fact that APGD, generated in contact with liquid cathode, mainly operates in saturated water vapour, which contains H2O+ ions  and nitrogen diffuses into the plasma from surrounding air  and thus may be excited in the “colder” part of plasma outside the central part (core of the glow discharge). The presence of NO in the plasma is probably due to the reaction of high energy states of nitrogen with oxygen . The residual energy from excitation of N2 may be converted into additional rotational energy of NO and thus the Trot(NO) may be higher than Trot(N2). The good correlation between Trot(N2) and Trot(OH) may also indicate that heat transfer from central parts of plasma (core of glow discharge) to “colder” parts can play an important role in the distributions of rotational temperatures in the near cathode region.
The Electron Number Density, Ionization Temperature and Ionization Degree
The experimental profile of the Hβ line was fitted using the Voigt algorithm, for deconvolution of a Lorentzian and a Gaussian line profile. This was done using the Galactic Grams/32 computer program. In order to measure the Stark broadening precisely, the Hβ half width was corrected to account for Van der Waals broadening . Based on the standard deviation, the uncertainty of ne was estimated to be up to 10%.
The APGD plasma is not in the equilibrium state. However, at the assumption of the partial LTE state, the ionization temperature may be evaluated for the investigated plasma using the Boltzmann-Saha equation. A similar procedure was recently applied for the determination of the ionization temperature in atmospheric pressure glow discharges generated in air  and He , and an atmospheric pressure ICP [27, 34].
Spectroscopic constant of Mg lines applied for the determination of the ionization temperature and degree of ionization 
Aji (108 s−1)
The experimental degrees of ionization for Mg were found to be: 9–14 and 19–21% for 0.1 and 0.5–1.0 mol L−1 solutions of HCl, respectively. It is almost four-five times lower than in the low power capacitively coupled plasma (CCP)  and ICP  sources. The ionization degrees (αion) were compared to those calculated from the Saha equation at assumption of plasma equilibrium state. For the ionization temperature equal to 5,000 K and the electron number density assumed to be 4 × 1020 m−3, the theoretical ionization degree for Mg, (14%), was consistent with experimental values.
The values of the ionization temperature and the ionization degree are surprisingly low in our investigated plasma source and may indicate that ionization processes play an insignificant role in the production of additional electrons.
The investigated source with a new construction of the liquid cathode can be operated at lower flow rates (range 0.6–3.2 mL min−1). A stable atmospheric pressure glow discharge was obtained under these conditions using the electrode gap and the concentration of HCl within 0.5–3.5 mm and 0.01–1 mol L−1, respectively. The burning voltage in the range of 1,100–1,700 V and the current equal to 20 mA was applied to generate stable APGD plasma. The emission spectra of glow discharges were dominated by OH, N2, H, NH, NO and O species. Strong Mg atomic lines were also observed. Weak ionic lines (Mg II) could be measured, but other ionic species as N2+ emission bands and O II lines were not observed Experimental parameters, such as the solution flow rate, the size of the gap between electrodes, and the concentrations of HCl, significantly affected the observed emission intensities. An increase in solution uptake and in concentration of HCl caused a growth of OH, H, O and N2 intensities. Maximum spectral intensities were observed for a gap of 2.5 mm between the solid anode and the water surface.
The electron number density and ionization degree of Mg changed over the range (3.4 – 6.8) × 1020 m−3 and 9–21%, respectively.
For comparison, in an atmospheric pressure ICP the electron density is one to two orders higher, the Mg ionization degree reaches 95–99% while the excitation and ionization temperatures are usually between 5,000 and 10,000 K , so the APGD plasma is much more far from the LTE state than the ICP. However, the results indicate that deviations from local thermal equilibrium are not large and rather depend on plasma generation conditions.
Our results are comparable to other temperature measurements for similar types of discharges generated in the contact with liquid electrodes. Differences between values reported in the literature are likely caused by different conditions of the plasma generation.
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