XML Generation Notice This is an automatically generated XML version of the publication: "Probing the impact of oxygen negative ions on the self-organized pattern in 1 atm DC glow with liquid anode - https://doi.org/10.34657/41312", originally published under the license CC BY 4.0 Unported. The XML version was generated on 28-08-2026 by Technische Informationsbibliothek (TIB). In spite of high standards in the development of the script, we cannot outrule the possibility of mistakes in the process of file conversion. The authors are not responsible for possibly existing mistakes or differences in this XML-version. The link above directs you to the authorised version of the publication. If you spot mistakes in the XML-version, please contact us at oarepository@tib.eu. This XML-Version of the publication "Probing the impact of oxygen negative ions on the self-organized pattern in 1 atm DC glow with liquid anode - https://doi.org/10.34657/41312" is provided "as is", without any guarantee (“ausdrücklich oder stillschweigend”). TIB does not take responsibility for potential mistakes, missing passages and consequences resulting from this use. Hinweis zur XML-Generierung Dies ist eine durch die Technische Informationsbibliothek – TIB am 28-08-2026 automatisiert erstellte XML-Version der Publikation "Probing the impact of oxygen negative ions on the self-organized pattern in 1 atm DC glow with liquid anode - https://doi.org/10.34657/41312", die unter der Lizenz CC BY 4.0 Unported veröffentlicht wurde. Trotz sorgfältiger Entwicklung des Skripts können Fehler im Konvertierungsprozess nicht ausgeschlossen werden. Die Autor*innen der Originalpublikation sind für etwaige Fehler/Differenzen in der XML-Version nicht verantwortlich. Die oben verlinkte Datei stellt die autoritative Version der Publikation dar. Hinweise auf Fehler on der XML-Version nehmen wir unter oarepository@tib.eu gern entgegen. Die vorliegende XML-Version der Publikation "https://doi.org/10.1088/1361-6595/adac0a" wird "wie es ist" bereitgestellt, ohne jegliche Gewährleistung oder Garantie, sei es ausdrücklich oder stillschweigend. Die TIB übernimmt keine Verantwortung für etwaige Fehler, Auslassungen oder Folgen, die aus der Nutzung dieser Datei resultieren. xml_created 28-08-2026 dc.identifier.uri https://doi.org/10.1088/1361-6595/adac0a dc.rights.license CC BY 4.0 Unported dc.relation.doi https://doi.org/10.34657/41312 Probing the impact of oxygen negative ions on the self-organized pattern in 1 atm DC glow with liquid anode 29 01 2025 Probing the impact of oxygen negative ions on the self-organized pattern in 1 atm DC glow with liquid anode, Yang, ZimuYatom, ShurikFoster, John, , (2025-01-29). In an atmospheric DC glow discharge with a liquid anode, the plasma anode glow attached to the grounded liquid surface under certain conditions self-organizes into coherent patterns. Optical emission spectroscopy revealed that the emission consists primarily of the second positive system of nitrogen, N<inf>2</inf>(C-B), whose excitation energy is low and sensitive to changes in the electron energy distribution. In addition to electrons, negative ions can accumulate in the anode sheath and affect the local space charge. It has been speculated that these negative ions play a role in pattern formation at the anode surface. In this work, the role of oxygen negative ions was explored. It was found that the formation of anode patterns requires at least a 7% volume fraction of oxygen in the ambient gas. Results showed that O<inf>2</inf>⁻ is the dominant negative ion species in atmospheric DC glow discharge, with a density of ∼ 10 12 cm − 3 . While the presence of oxygen appears to be crucial for pattern formation, this study indicated that patterns still formed without geometric changes even when 62% of negative ions in the plasma were detached by a laser. This suggests that negative ions do not support the patterns, while oxygen’s heating effect may induce instability at the anode. IntroductionSelf-organization is ubiquitous in nature and widely observed in plasma over a broad range of temperatures, densities, and pressures. In DC glow discharges, the glowing region near the cathode or anode develops inhomogeneous structures that depend on space charge distribution, gas pressure, and inter-electrode spacing [1]. In atmospheric pressure dielectric barrier discharge (DBD) plasmas, self-organization is also observed, forming filamentary ionization waves that create symmetric patterns on the dielectric surface as well as in the discharge gap [2]. Interestingly, a very similar coherent pattern can be found on an atmospheric glow discharge with a liquid anode, although no filaments form in the air gap [3]. The selforganized patterns on the liquid anode are highly dynamic and capable of forming more complicated shapes [4].According to the theory of reaction-diffusion systems [5], coherent patterns result from phenomena in thermodynamically open systems where products flow out while new reactants are introduced. In such cases, the system is best described by non-equilibrium thermodynamics [6]. In these systems, energetic electrons [7], ions [6], and the sheath electric field [8] act as activators, while electronegative gases like oxygen [9,10], water vapor [11], and liquid as leaky dielectric [4] serve as inhibitors. The role of these species as drivers of selforganization has been discussed in the aforementioned studies and is well understood in analogous purely chemical systems. However, the exact cause of pattern formation in plasma systems remains elusive. In the gas phase of DC glows with a liquid anode, it has been shown that a higher working gas flow or nitrogen shielding flow [9] can eliminate pattern formation. It is postulated that the shielding gas or higher flow rates may limit the entrainment of external species like oxygen, thereby preventing pattern formation by limiting the transport of negative ions into the discharge column. Additionally, higher flow rates introduce convective cooling of the discharge, reducing the local vapor density derived from evaporation at the liquid electrode surface.To examine the impact of oxygen alone, we conducted an experiment where the oxygen fraction was artificially reduced. The results showed that oxygen is essential for pattern formation. This raises the question: What is the role of oxygen in pattern formation? Oxygen may be important for its lower ionization potential than nitrogen. In a simulation of simplified helium (50 mol%), nitrogen, and oxygen mixing gas, increasing oxygen mole fraction (from 0 to 50%) raises the ionization frequency by about three orders of magnitude, as calculated by Bolsig+ [12]. Note that increasing the helium fraction boosts ionization more prominently than oxygen but it suppresses the pattern formation in the experiment. In this scenario, oxygen not only elevates the ionization rate but also introduces more complex kinetics that consume energetic electrons [13]. Atomic oxygen contributes to many loss mechanisms, offsetting any increase in ionization frequency [14]. Additionally, oxygen can lead to electron collisional losses through processes like rapid gas heating and dissociative attachment, which are important in the pattern formation mechanism [15]. These electron loss mechanisms could induce local ionization instability, with oxygen negative ions being significant candidates that deplete electrons in atmospheric discharges.Oxygen, an electronegative gas, can form anions with a high electron affinity, attracting electrons. This additional electron loss term due to attachment to electronegative neutrals significantly affects the electron charge flow balance. Negative ions formed from electron attachment can be quickly neutralized by recombination with positive ions. This volumetric process can drive the plasma into an attachment-controlled regime rather than a diffusion-controlled regime, which requires higher electron temperatures and a higher reduced electric field to sustain the discharge [16]. At atmospheric pressure, a noble gas feed glow discharge would exhibit a larger electron density gradient near its periphery, where negative ions form. Near the electrode or dielectric, negative ions can affect the sheath due to their low mobility, enhancing the local electric field radially [17]. The importance of negative ions in weakly ionized molecular gas discharges has been analyzed and found to stimulate an attachment instability wave mode [18]. Interestingly, pattern dynamics have proven sensitive to oxygen [9]. Acidic electrolytes such as H 2 SO 4 , HCl, HF have been found to lower pattern complexity, speculated to be due to electrolysis products like O 2 , Cl 2 , F 2 with high electron affinity restricting pattern formation [10]. Investigating the role of negative oxygen ions in plasma pattern mechanisms is therefore of interest.Negative ion diagnostics include mass spectrometry [19] or the laser-induced photodetachment method [20]. Both methods are well-established in the study of low-pressure plasma. At atmospheric pressure, the thin sheath and less diffusive plasma make mass spectrometry more challenging, though skimmers and differential pumping methods can sample ion species. The laser photodetachment method involves using a tunable laser to illuminate a plasma with photon energy exceeding the negative ion's electron affinity. Free electrons formed in the detachment process are detected as an increase in electron density, typically measured with a Langmuir probe to detect changes in electron saturation current or as a temporal increase in discharge current within the circuit. Sasaki et al recently measured the negative ion species in an atmospheric glow discharge over metallic and liquid cathode [21]. Results suggested that O − and OH − , derived from the dissociative electron attachment of H 2 O 2 , were the dominant negative ions. It is notable that patterns are absent in glow discharges with liquid cathodes. However, the measurements of negative ion species and density have not been conducted for glow discharges with liquid anodes, and their role in pattern dynamics is unclear. Therefore, we aimed to investigate negative ions in glow discharges with liquid anodes.In this work, we performed a laser photodetachment experiment on negative ions in atmospheric glow discharge plasma with a liquid anode, including the identification of the dominant negative ion species. We also examined the impact of negative ion depletion on self-organized pattern dynamics. Discharge source for self-organized pattern and the importance of oxygenSelf-organization can be observed in a multi-phase atmospheric pressure DC discharge when a liquid is used as the anode. The plasma setup consists of a gas-phase DC glow discharge with a pin-to-water configuration, as illustrated in figure 1. The plasma is powered by a high-voltage DC power supply (Gamma High Voltage Research RR3-333 R), which can output up to 3 kV (<±0.01%) and 333 mA (<±0.005%). Since the DC discharge voltage remains mostly independent of current after breakdown, a ballast resistor (15 kΩ) is used to accommodate the additional voltage drop when the plasma current increases. A high voltage differential probe (Tektronix P6015A, maximum error <±3%) measures the voltage at the cathode, while the current is recorded by measuring the voltage across a shunt resistor (45 Ω). The discharge operates at 700-1000 V, and the current is maintained within 100 mA. All resistors are non-inductive to minimize electrical resonance. Helium gas at a flow rate of 100 sccm is fed through an orifice at the bottom of the brass cathode, and an 8 mm discharge gap is maintained. Tap water (conductivity: 1.05 mS cm −1 ) is used as the liquid anode in this work. To compensate for water evaporation, the liquid is cycled by a peristaltic pump and over-flooded in the container to sustain a constant water level.Among many variables that affect pattern dynamics, the oxygen volume fraction appears to be particularly important. This was initially demonstrated by applying a shielding nitrogen gas around the plasma column, which limited the entrainment of oxygen from ambient air, preventing any pattern formation [9]. However, using shielding gas also introduces additional convective cooling, shear flow instabilities, and changes in water vapor concentration near the plasmaliquid interface. Thermal dynamics and the presence of water vapor are both potential factors influencing the mechanism of pattern, thus the shielding gas method obscures the specific role of oxygen. Consequently, we conducted experiments in a gas-sealed box that could be flushed with dry nitrogen gas to reduce the oxygen volume fraction. The plasma is highly localized and sufficiently distant from the nitrogen feed gas, thereby avoiding additional convective cooling, shear instabilities, and impacts on water vapor.Figure 2 compares the change in pattern shape, recorded by a CMOS camera with a 20 ms exposure time, at specific discharge currents and oxygen fractions. The complexity and size of the patterns both decrease with reducing oxygen fraction in the ambient gas, eventually degenerating into a featureless and diffusive 'disk'. Spatially-resolved emission spectroscopy revealed that the gas temperature remains high in the absence of oxygen, as the vibrational-translational relaxation of nitrogen also plays a significant role in gas heating reactions [22]. A similar level of volumetric heating should lead to similar water vapor density, and both conditions fail to support the pattern. These observations suggest the critical importance of oxygen and motivated a detailed investigation of the role of oxygen in the pattern mechanism. Experimental setup and methodsFigure 3 shows the schematic of the laser photodetachment experiment. A 3rd Harmonic Nd:YAG pulsed laser (Continuum Surelite III, wavelength of 355 nm, repetition frequency 10 Hz, photon energy 3.49 eV) with a maximum 100 mJ beam energy at a 4 ns pulse width is used to perform the detachment. Table 1 lists the major negative ions presumed to be present in a plasma running in humid air. O − 2 is produced by three-body electron attachment [26,27], while abundant singlet oxygen can participate in dissociative attachment with energetic electrons, yielding O − [28]. OH − is produced from the dissociative attachment of water molecules [29]. The photon energy at 355 nm is higher than the binding energy of attached electrons for all listed negative ions, thus enabling nonselective photodetachment. Other possible negative ions such as H − , O − 3 are negligible in the experiment. The theoretical ratio of the detached negative ions versus all negative ions α can be calculated by:α = ∆n − n − = 1 − exp ( − σλE hcS )(1)∆I I ∼ = ∆n − n e = n − n e × ∆n − n − = A × α (2)where n − represents the negative ion density, σ is the photodetachment cross-section, E, S are the laser pulse energy and beam area, respectively; h, c are Planck constant and speed of light. The increase in the current ratio ∆I/I scales with the liberated electrons from photodetachment ∆n − /n e . This assumes that the discharge current increase is primarily due to these electrons. The detachment current ratio can be represented by the product of the negative ion detachment ratio α and the local negative ion density relative to that of free electrons A = n − /n e , as shown in equation ( 2). The local density ratio A depends on both the density difference and the plasma volume illuminated by the laser. The current ratios measured at different laser energies are fitted with the theoretical detachment ratio multiplied by a constant representing the local density ratio A. When compared to curves of known species, A normalizes ∆I/I into ∆n − /n − , revealing relative differences in the detachment cross-section. Note that the detachment ratio α depends on the laser fluence (energy per area) at a constant photon energy (wavelength) and will saturate at sufficiently high laser fluence. When higher laser energy is unavailable, a focusing lens is placed between the plasma column and laser output port to increase the laser fluence at the targeted volume.A laser viewing card was used to measure the effective beam area. The laser beam energy injection is adjusted by a delay generator and measured by a laser power meter. A small portion of the laser beam is diverted by a beamsplitter toward a photodiode to correlate the laser pulse with the current perturbation using an oscilloscope. A beam dump is set at the back of the plasma to terminate the laser beam.The detachment current signal occurs on a nanosecond timescale corresponding to the laser pulse, and such current changes are detectable only at the anode, where free electrons drift. A non-inductive shunt resistor is placed between the liquid anode and ground, with its voltage measured directly by an oscilloscope. Circuit impedance can significantly affect signal dynamics when components are mismatched. We found that a 45 Ω shunt resistor critically dampens the signal, providing a characteristic time closest to its physical relaxation time. An intensified CCD camera (PIMAX 4) is synchronized with the pulsed laser to observe pattern dynamics when negative ions are destroyed by photodetachment. A 300 ns gate width is used to capture sufficient emission from the pattern to record any changes. Results and discussion Discharge current response to photodetachmentThe temporal dynamics of discharge current at different helium gas flow rates and oxygen volume fractions upon laser photodetachment are shown in figure 4. Note that the ∆I/I is plotted to unify the response across various conditions. The laser energy is 97 mJ, and the beam, with a diameter of 8.6 mm, is directed at the geometric center of the plasma. The beam covers the majority of the plasma column to detach as many negative ions as possible. Operating the plasma at a higher discharge power (current) increases the absolute detachment current without significantly affecting ∆I/I. A minor decrease in ∆I/I is observed only when the plasma's positive column expands beyond the laser beam diameter. The black curve represents the laser pulse, which starts 8 ns earlier than the detachment current signal due to the position difference between the photodiode and the plasma column. In principle, photodetachment occurs almost immediately after the laser illuminates the plasma. The current pulse has a rise time of ∼20 ns and then decays exponentially with a time constant on the order of 100 ns. This fast decay may imply that free electrons are reattaching to electronegative gas at a fast rate, predominantly through three-body processes when the partial pressure of oxygen is high [30]. Importantly, circuit impedance leads to transient responses, where the shunt's resistance alters the decay time. We found that a 45 Ω shunt achieves a critically damped response, meaning the measured decay is closest to the relaxation of detached electrons.Remarkably, higher helium gas rates extend the decay time significantly. In figure 4, the decay time constant increases from 108 ns to 330 ns at helium flow rates of 100 sccm and 400 sccm, respectively. The attachment of electrons to oxygen molecules is a three-body reaction where the efficiencies of nitrogen and helium as third bodies are much lower than pure oxygen [30]. A higher helium flow decreases the air concentration and slows down the reattachment rate. Furthermore, the attachment coefficient peaks at 0.1 eV and decreases with higher electron temperatures. Increasing the helium concentration increases the average electron energy in the plasma due to helium's high ionization potential, thereby slowing down the attachment rate. The lower detachment current ratio also suggests reduced negative ion density due to the decreased presence of oxygen. When plasma operates in a 2 vol% oxygen ambient air, the detachment current ratio (shown in the orange curve) drops substantially, indicating that the majority of negative ions in the plasma are produced from oxygen. Interestingly, the attachment rate is higher despite the expectation that a low oxygen concentration would decrease the reattachment coefficient. This is because the average electron energy decreases at lower oxygen mole fractions than nitrogen's [31], which boosts the three-body attachment coefficient.Note that there is also a slower-falling part of the curve, the origin of which is unknown. It is possible that additional electrons modify the plasma, potentially representing a hysteresis effect. Importantly, the current signal is not smooth but contains a series of peaks, a phenomenon observed in other research [32,33]. External circuitry might precipitate this underdamped resonance due to its impedance [34]. Hypothetically, these peaks could also represent neutral oscillations or relaxation oscillations that damp over time as photodetachment electrons reattach. For instance, a burst of electrons can generate ion-acoustic oscillations [33]. Another possible source is the reflected laser rays shining on the plasma column later than the main beam, generating secondary peaks, as discussed in [35]. Curtains and masks were placed around the plasma to reduce this effect as much as possible. Photodetachment current ratioAccording to equation ( 1), the percentage of detached negative ions should increase and then saturate if the laser pulse fluence is high enough. This percentage corresponds to the change of detachment current ∆I/I, where the largest ∆I/I is achieved when the detachment ratio ∆n − /n − saturates to unity. Figure 5 shows the change in the normalized detachment ratio as a function of pulse energy. Unfortunately, the detachment ratio only reaches 62% at the highest pulse energy and fails to saturate. The pulsed laser is expected to provide enough fluence for saturation if O − is the major negative species, as shown by [21]. Because the laser beam area and photon energy are known, the overall detachment cross-section can be deduced by fitting the data with equation ( 2), where the local density ratio A and effective photodetachment cross-section are set as unknown parameters. An atmospheric pressure glow discharge has electron density of 10 13 cm −3 [36], thus a density ratio A = n − /n e 0.104 means a ∼10 12 cm −3 negative ion density present in the plasma column. It was also reported by [37] that the negative ion density should be two orders of magnitude smaller than the electron density. The fitted effective cross-section lies between that of O − 2 increases with the chamber pressure since the charge transfer between O − and O 2 (a 1 ∆ g ) becomes more significant at higher pressure (>10 mTorr) [38]. Another study also pointed out the importance of this reaction in conditions of larger reduced electric fields and small current densities (<10 mA cm −2 ), making O − 2 dominant even at low pressures [39].In this work, we believe the bulk plasma column has a low electric field (<30 Td), which does not favor the dissociative attachment of water and oxygen; thus, three-body attachment is dominant, producing O − 2 [40]. The low electric field also diminishes the contribution from the dissociative attachment of water molecules, which can be a more significant electron loss mechanism compared to oxygen [41].Although the laser pulse energy was limited, the detachment ratio correlates with the laser fluence, which is energy per area. Therefore, more negative ions per unit volume can be detached if the laser beam is focused into a smaller beam. On the other hand, a smaller beam would not cover the whole plasma, and the detachment current would only represent the negative ions detached from the plasma volume within the beam coverage. Therefore, a smaller local density ratio is expected. Figure 6 shows the detachment ratio at a beam diameter of 3.7 mm. As anticipated, the data is closer to saturation, but the detachment current abnormally increases beyond a certain beam energy, deviating from the theoretical trend. This anomaly was also found in similar research and is attributed to the laser ablation of the probe surface contacting the beam [35,42,43]. Indeed, a very strong current signal can be found when the laser beam contacts the cathode surface. A highly focused beam is particularly problematic as it can drive electron density and thus discharge current changes via photoionization electrons. Except for the anomaly, the data at smaller pulse energies follow the trend, and the fitted effective cross-section is very close to that of the unfocused beam.Figure 7 shows the detachment ratio as a function of laser fluence for four different beam sizes. By plotting the data as a function of laser energy per area, different beam sizes can be compared with the theoretical curve together, with detailed fitting results found in table 2. The smaller the beam area, the higher the laser fluence, and the higher the chance of reaching photodetachment saturation. Although the anomaly persists at higher beam energies, the fitted effective photodetachment cross-sections across different beam sizes are narrowly dispersed. A smaller beam also limits the plasma volume undergoing photodetachment, resulting in a smaller local density ratio. The effective cross-sections from the four different beam sizes all align closely with O − 2 , strongly indicating that O − 2 dominates the negative ions in the glow discharge at atmospheric pressure. However, the exact mechanism requires a kinetic model for full understanding. Note that data tends to deviate from the theoretical curve at smaller laser energies when using a smaller laser beam. Although the precise reason for this deviation is unclear, a tightly focused beam should be avoided in future photodetachment experiments. Impact photodetachment on the self-organized patternIdentifying the dominant negative ion, O − 2 , again underscores the importance of oxygen gas in atmospheric glow discharge. However, the pattern shows no obvious change during or after photodetachment, as depicted in figure 8. The original images are in 16-bit grayscale and have been converted to color images for better contrast. Due to intense laser ray reflections toward the camera, the earliest image was taken at 23 ns after the laser pulse, and no prominent deviation is observed. Given that the rise time of the photodetachment current is 20 ns (see figure 4), at least 62% of negative ions should have been detached at this moment. The right image of figure 8 shows the pattern image at 1 µs after the laser pulse, and no observable change exists. Various delays ranging from nanoseconds to a few hundred microseconds were examined, yet no impact on pattern morphology from photodetachment was found.Based on these results, plasma self-organization does not appear to be correlated with the negative ion mechanism under the experimental conditions investigated here. Recently, a pulsed DC glow discharge experiment by Srivastava et al suggested that pattern development occurs on a timescale of ∼100 µs and reaches a steady state on a millisecond timescale [11]. This timescale is three to four orders of magnitude longer than kinetic processes but is more comparable to gas convection, gas-phase diffusive transport, evaporation, and gas heating. Srivastava et al also showed that water vapor saturates at anode layer ∼500 µs, which is similar to the millisecond establishment time for patterns [11]. Through measurements of electron density and temperature in the same plasma source, they concluded that the evaporation of water reduces the electron density through attachment and increases electron temperature via dissociative electron-ion recombination. This raises the voltage drop at the anode layer and supports pattern formation [44].When considering the importance of negative ions and the fact that the pattern requires ∼100 µs to develop, it becomes apparent that photodetachment using a short pulse width (4 ns) laser running at 10 Hz is not performed frequently enough. The detached negative ion would reform on a 100 ns timescale, thereby sustaining the pattern again. Utilizing a highfrequency, long pulse width for photodetachment is impractical due to the laser's performance limitations. In addition to the temporal constraints, a Gaussian laser beam profile may not effectively photodetach at the liquid surface, where the plasma pattern at the anode glow could be influenced only locally by negative ions. Negative ions are also known to exist around the periphery of the plasma column, where ambient oxygen attaches to diffusing electrons. Consequently, these regions might require further probing. To address this issue, the laser beam was focused into a line shape and targeted at the plasma pattern, yet no significant change in the pattern was observed. This result also suffers from the temporal limitations of the laser.Thus, using laser photodetachment is not ideal for investigating the role of negative ions in pattern formation. Instead, operating the plasma with ambient gases of varying electronegativities, such as SF 6 , Cl 2 , NF 3 could produce patterns at different volume fraction thresholds, thereby testing the hypothesis. In a recent study, Miyazaki et al showed that adding the other electronegative gases SF 6 and CO 2 as shielding gas does not form a pattern [45]. This suggests that oxygen does not support patterns through its electronegativity, and the underlying mechanism could be due to thermal instability caused by gas heating.Both nitrogen and oxygen molecules induce considerable gas heating through vibrational excitation and vibrationaltranslational relaxation. Additionally, oxygen admixture induces extra heating from its electron impact dissociation and quenching processes of electronically excited nitrogen [15,31]. In an atmospheric DBD experimental study, the power fraction consumed by fast gas heating processes rises from 43% to 80% after pure nitrogen is mixed with oxygen in a 1:1 ratio [46]. DC plasma over a liquid anode also exhibits lower gas temperature with nitrogen shielding gas, which reduces the oxygen concentration [45]. In this recent study, pattern formation was found to be associated with high gas temperature and wider radial spreading of positive columns. The gas temperature increases with discharge current, air gap, and oxygen shielding gas, all of which increase the oxygen concentration in the plasma.The importance of gas heating in pattern formation may be related to thermal instability [47]. An oscillating increase in electron density leads to enhanced ohmic heating and higher gas temperature. This also causes a drop in neutral density and an increase in the reduced electric field (E/N), which further increases ionization and electron density. However, this mechanism was interrupted when the rising gas temperature generated additional vapor, which increased the anode voltage drop and E/N in the positive column, preventing the development of thermal instability and thus stabilizing the plasma [44]. In this case, the anode electric field increases with the gas heating and raises the energy of charged species. This would enhance evaporation and could in turn boost the anode electric field. The nonlinear feedback between the anode electric field and evaporation can shape the dynamics of the plasma pattern.The Leidenfrost effect may be another mechanism that relates thermodynamics to plasma pattern formation. In the conventional Leidenfrost phenomenon, droplets exposed to extreme heat vaporize slowly and exhibit self-organized shapes [48]. The anode gas temperature is measured at 800 K, a condition in which an insulating vapor layer can form [22]. In this context, liquid subjected to intense gas heating forms a dynamic vapor layer, deforms, and oscillates, affecting charge transport and the local electric field. The interactions between liquid motion, the anode electric field, the non-uniform vapor layer, and localized space charge can be coupled, generating a sophisticated feedback mechanism that induces patterns. Verifying this hypothesis requires advanced comprehensive in situ diagnostics, and further discussion is beyond the scope of this work. SummaryA laser photodetachment experiment was conducted to investigate the role of negative ions in the mechanics of pattern. A noticeable increase in discharge current was detected when photodetachment was initiated, and this increase should continue with rising laser energy until saturation is reached. Although the laser energy employed was insufficient to achieve saturation, O − 2 was found as the dominant negative ion species based on the fitted effective photodetachment cross-section. The ratio between the current and the detachment ratio can be utilized to estimate the negative ion density, which is approximately 10 12 cm −3 . A focused laser beam increased the laser fluence, resulting in more intense photodetachment; however, an abnormal current spike was also observed, likely due to the ablation of the exposed electrode. Examination of the pattern dynamics using an ICCD camera revealed no change under the photodetachment conditions. This observation suggests that the self-organization at atmospheric glow discharge with liquid anode is not dependent on the 62% negative ions that are detached by the laser. Therefore, the presence of negative ions may not be crucial for pattern formation. Instead, the mechanism of pattern formation may rely on thermal instability caused by gas heating from oxygen. Future work should focus on identifying the thermal effects of oxygen gas and vapor in the mechanism of pattern formation. Figure 1 . Figure 1. Schematic diagram of glow discharge and liquid phase circulatory setup. Figure 2 . Figure 2. Plasma self-organized patterns at different oxygen volume fractions in the gas phase (8 mm discharge gap, 100 sccm helium feed gas, 20 ms exposure time). Figure 3 . Figure 3. Schematic diagram of laser photodetachment setup. Figure 4 . Figure 4. Photodetachment current temporal change at a 97 mJ laser pulse at different helium flow rate and oxygen volume fraction (discharge current 32 mA, beam diameter 8.6 mm). Figure 5 . Figure 5. Normalized photodetachment ratio at an 8.6 mm diameter laser beam (discharge current 32 mA, laser pulse 97 mJ, density ratio 0.104, effective cross-section 3.25 × 10 −22 m 2 ). 2 and O − 2 • (H 2 O), far below those of O − and OH − , suggesting that O − 2 is the dominant negative ions. Normally, in low-pressure plasma with oxygen, negative ions are dominated by O − . However, research suggests that the percentage of O − Figure 6 . Figure 6. Normalized photodetachment ratio at a 3.7 mm diameter laser beam (discharge current 34 mA, local density ratio 0.0434, effective cross-section 3.28 × 10 −22 m 2 ). Figure 7 . Figure 7. Normalized photodetachment ratio as a function of laser fluence with various beam sizes. Figure 8 . Figure 8. Self-organized pattern with laser photodetachment (Laser pulse 97 mJ, beam diameter 8.7 mm, discharge current 52 mA, left: 23 ns after the laser pulse, right: 1 µs after the laser pulse). Table 1 . Table 1 . Electron affinity and photodetachment cross-section of major negative ions. Cross-section at Negative ion Electron affinity (eV) 355 nm (×10 −22 m 2 ) Reference O − 2 O − 0.448 1.44 3.64 9.52 [23] [24] OH − 1.83 9.31 [25] O − 2 • (H 2 O) 2.25 2.58 [23] Table 2 . Table 2 . Effective photodetachment cross-section and negative ion density ratio from various beam size. Beam diameter Local Effective cross-section (mm) density ratio at 355 nm (×10 −22 m 2 ) 8.6 0.104 3.25 5.0 0.0566 3.28 3.7 0.0434 3.28 3.1 0.0215 3.12 [41] Mladenović Željko and Gocić S 2022 Influence of air and water vapor on EEDF, plasma parameters and the main RONS in atmospheric pressure low temperature helium plasmas: global model approach Phys. Plasmas 29 103504 [42] Bradley J W, You S D, Dodd R and Bryant P M 2010 Negative ion density measurements by laser photo-detachment in RF and DC reactive magnetron discharges Laser 1 R1 [43] Sirse N, Oudini N, Bendib A and Ellingboe A R 2016 Measurement of electronegativity at different laser wavelengths: accuracy of langmuir probe assisted laser photo-detachment Plasma Sources Sci. Technol. 25 04LT01 [44] Yue Y and Bruggeman P J 2022 Electron density and electron temperature measurements in an atmospheric pressure plasma interacting with liquid anode Plasma Sources Sci. Technol. 31 124004 [45] Miyazaki T, Sasaki K and Shirai N 2024 Correlation between temperature distribution and changes in self-organized luminous pattern in an atmospheric pressure DC glow discharge with sheath gas flow Plasma Sources Sci. Technol. 33 115007 [46] Nudnova M M, Kindysheva S V, Aleksandrov N L and Yu Starikovskii A 2015 Fast gas heating in N 2 /O 2 mixtures under nanosecond surface dielectric barrier discharge: the effects of gas pressure and composition Phil. Trans. R. Soc A 373 20140330 [47] Yue Y, Kondeti V S S K, Sadeghi N and Bruggeman P J 2022 Plasma dynamics, instabilities and OH generation in a pulsed atmospheric pressure plasma with liquid cathode: a diagnostic study Plasma Sources Sci. Technol. 31 025008 [48] Paul G, Kumar Das P and Manna I 2015 Droplet oscillation and pattern formation during leidenfrost phenomenon Exp. Therm. Fluid Sci. 60 346-53 AcknowledgmentsThis work was supported by National Science Foundation ECosystem for Leading Innovation in Plasma Science and Engineering (ECLIPSE) program Award No. 2206039, Department of Energy, Office of Fusion Energy Sciences Grant DE-SC0024538, and the Princeton Collaborative Research Facility (PCRF https://pcrf.princeton.edu/), which was funded by the U.S. Department of Energy under Contract No. DE-AC02-09CH11466. ORCID iDsZimu Yang  https://orcid.org/0000-0003-4043-8413 Shurik Yatom  https://orcid.org/0000-0001-8716-8487 John Foster  https://orcid.org/0000-0001-8369-1828 Data availability statementAll data that support the findings of this study are included within the article (and any supplementary files). Pattern formation in gas discharge systems with high impedance electrodes, Radehaus, CDirksmeyer, TWillebrand, HPurwins, H-GPhys. Lett. A, 125, (1987). 10.1016/0375-9601(87)90128-9 Observation of self-organized filaments in a dielectric barrier discharge of ar gas, Shirafuji, TKitagawa, TWakai, TTachibana, KAppl. Phys. Lett, 83, (2003). 10.1063/1.1613796 Self-organized anode pattern on surface of liquid or metal anode in atmospheric dc glow discharges, Shirai, NUchida, STochikubo, FIshii, SIEEE Trans. Plasma Sci, 39, (2011). 10.1109/TPS.2011.2158324 , The variation in self-organized anode plasma pattern structure with solution electrolyte type in 1 atm dc glow discharge Plasma Sources Sci, 30, 15007 (2021). The chemical basis of morphogenesis, Turing, Mathison, ABull. Math. Biol, 52, (1990). 10.1016/S0092-8240(05)80008-4 Self-organization in 1 atm dc glows with liquid anodes: Current understanding and potential applications, Foster, J EKovach, YGarcia, Lai, M CPlasma Sources Sci. Technol, 29, 34004 (2020). 10.1088/1361-6595/ab7089 Self-organized pattern formation and plasma diagnosis in atmospheric pressure helium glow discharges with a liquid anode, Qin, XFeng, BWang, RMa, YZhang, QZhong, XPlasma Process. Polym, 21, 2300055 (2024). 10.1002/ppap.202300055 , Rumbach, PLindsay, A EGo, D BPlasma Sources Sci. Technol, 28, 105014 (2019). 10.1088/1361-6595/ab45e4 , Influence of oxygen gas on characteristics of self-organized luminous pattern formation observed in an atmospheric dc glow discharge using a liquid electrode Plasma Sources Sci, 23, 54010 (2014). Self-organized patterns by a dc pin liquid anode discharge in ambient air: Effect of liquid types on formation, Zhang, SDufour, Phys. Plasmas, 25, 73502 (2018). 10.1063/1.5030099 Self-organized patterns at the plasma-liquid anode interface in a helium glow discharge: temporal development and mechanisms, Srivastava, TSimeni, SimeniNayak, MBruggeman, G, P JHagelaar G J M and Pitchford L 2005 Solving the boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models Plasma Sources Sci, 31, 722 (2022). 10.1088/0963-0252/14/4/011 , Park, GLee, H, KimLee, Koo, JPlasma Process. Polym, 5, (2008). 10.1002/ppap.200800019 Cross sections for collisions of electrons and photons with oxygen molecules, Itikawa, YIchimura, AOnda, KSakimoto, KTakayanagi, KHatano, YHayashi, MNishimura, HTsurubuchi, SJ. Phys. Chem. Ref. Data, 18, (1989). 10.1063/1.555841 Fast gas heating in a nitrogen-oxygen discharge plasma: I. kinetic mechanism, Popov, NJ. Phys. D: Appl. Phys, 44, 285201 (2011). 10.1088/0022-3727/44/28/285201 , Fridman, AKennedy, L APlasma Physics and Engineering, , (2004). , Propagation mechanisms of guided streamers in plasma jets: the influence of electronegativity of the surrounding gas Plasma Sources Sci, 24, 35022 (2015). Plasma stability of electric discharges in molecular gases, Haas, RPhys. Rev. A, 8, 1017 (1973). 10.1103/PhysRevA.8.1017 Negative-ion surface production in hydrogen plasmas: determination of the negative-ion energy and angle distribution function using mass spectrometry, Dubois, J P JAchkasov, KKogut, DAhmad, ALayet, JCartry, Simonin, GJ. Appl. Phys, 119, 193301 (2016). 10.1063/1.4948949 Laser induced photo-detachment of O 2 in DC discharge, Legorreta, JPatiño, JYousif, F BPlasma Sci. Technol, 20, 75401 (2018). 10.1088/2058-6272/aab48d Negative ion species in atmospheric-pressure helium DC glow discharge produced in ambient air, Sasaki, KHosoda, RShirai, NPlasma Sources Sci. Technol, 29, 85012 (2020). 10.1088/1361-6595/aba6a3 , Plasma self-organized pattern and the coupling processes at plasma-liquid interface PhD Thesis, , (2024). Photodissociation and photodetachment of molecular negative ions. VI, Lee, L CSmith, G PJ. Chem. Phys, 8600, (1979). 10.1063/1.437690 Absolute total, partial and differential cross sections for photodetachment of o, Génévriez, MDunseath, K MTerao-Dunseath, MHibbert, ADochain, A, Marion, UrbainPhys. Rev. A, 98, 33410 (2018). 10.1103/PhysRevA.98.033410 Photodetachment cross section, electron affinity and structure of the negative hydroxyl ion, Branscomb, LPhys. Rev, 148, 11 (1966). 10.1103/PhysRev.148.11 Measurement of the attachment of slow electrons in oxygen, Ao Chanin, LPhelps, ABiondi, M APhys. Rev. Lett, 2, 344 (1959). 10.1103/PhysRevLett.2.344 Three-body attachment in O 2 using electron beams, Spence, DSchulz, G JPhys. Rev. A, 5, 724 (1972). 10.1103/PhysRevA.5.724 Dissociative electron attachment to singlet oxygen, Jaffke, TMeinke, MHashemi, RChristophorou, LIllenberger, EChem. Phys. Lett, 193, (1992). 10.1016/0009-2614(92)85683-2 Cross sections for electron collisions with water molecules, Itikawa, YMason, NJ. Phys. Chem. Ref. Data, 34, (2005). 10.1063/1.1799251 Measurements of the attachment of low-energy electrons to oxygen molecules, Chanin, LPhelps, MBiondi, A, M APhys. Rev, 128, 219 (1962). 10.1103/PhysRev.128.219 Power transfer to gas heating in pure N 2 and in N 2 -O 2 plasmas, Pintassilgo, CGuerra, VJ. Phys. Chem. C, 120, (2016). 10.1021/acs.jpcc.6b05463 Nonresonant optical tagging and 'monopolar' transport in negative-ion plasmas, Stern, R ADevynck, PBacal, MBerlemont, PHillion, FPhys. Rev. A, 41, 3307 (1990). 10.1103/PhysRevA.41.3307 Plasma perturbation induced by laser photodetachment, Nishiura, MSasao, MWada, MBacal, MPhys. Rev. E, 63, 36408 (2001). 10.1103/PhysRevE.63.036408 Optimum circuit design for the detection of laser photodetachment signals, Bryant, P MBradley, J WPlasma Sources Sci. Technol, 22, 15014 (2012). 10.1088/0963-0252/22/1/015014 , Eclipse laser photodetachment method for avoiding probe surface ablation in negative ion measurement Plasma Sources Sci, 14, 566 (2005). Optical emission spectroscopy investigation of a 1-ATM DC glow discharge with liquid anode and associated self-organization patterns, Kovach, YGarcía, MFoster, J EIEEE Trans. Plasma Sci, 47, (2019). 10.1109/TPS.2019.2918065 Optical emission spectroscopy as a diagnostic for plasmas in liquids: opportunities and pitfalls, Bruggeman, PVerreycken, TGonzalez, MWalsh, J LKong, M GLeys, CSchram, D CJ. Phys. D: Appl. Phys, 43, 124005 (2010). 10.1088/0022-3727/43/12/124005 Electronegativity of low-pressure high-density oxygen discharges, Gudmundsson, JKouznetsov, IPatel, K K AndLieberman, M AJ. Phys. D: Appl. Phys, 34, 1100 (2001). 10.1088/0022-3727/34/7/312 , Experimental and theoretical investigation of oxygen glow discharge structure at low pressures IEEE Trans. Plasma Sci, 27, (1999). Two-and three-body attachment, electron transport and ionisation in water-air mixtures, De Urquijo, JGonzález-Magaña, OJuárez, Basurto, A MJ. Phys. D: Appl. Phys, 57, 125205 (2023). 10.1088/1361-6463/ad164a