The experiments were performed in a unique coplanar dielectric barrier discharge (DBD)-like geometry, in which a thin layer of deionized water flows over flat metal electrodes separated by insulating plastic blades. The water itself becomes part of the dielectric barrier, and a short-lived plasma is generated by a transient discharge propagating along the water-air interface. A pulsed power supply delivers a positive high-voltage pulse followed by a negative pulse, separated by a delay of a few microseconds. This bipolar excitation scheme allowed the researchers to isolate and compare the discharges produced by each polarity within a single event, using deionized water with a conductivity of about 5 microsiemens per centimeter flowing at a constant rate of 20 milliliters per minute.

The positive-polarity discharge exhibits a distinct two-phase morphology. Fast imaging with sub-nanosecond resolution captured a bundle of double-headed streamers initiating at the insulating blade, propagating bidirectionally toward both the high-voltage and grounded electrode regions. These closely spaced, overlapping structures — a collective streamer front — subsequently transformed, after reaching a certain length, into discrete spark filaments. The double-headed streamer phenomenon is usually observed in sprite-halo events developing at altitudes below 75 kilometers in the upper atmosphere, where they arise from inhomogeneities in electron density initiated by lightning-induced electric fields. Observing them at a water surface in the laboratory provides a rare terrestrial analogue of these atmospheric events.

In contrast, the negative-polarity discharge reveals a fundamentally different morphology, characterized by thin, discrete filaments from the earliest stages that persist and prolong at later times. Crucially, the formative streamer pre-phase observed during the positive pulse is completely absent during the negative pulse. The negative discharge initiates directly as filamentary structures within the same channels formed during the preceding positive discharge. Photomultiplier tube waveforms confirmed this difference: the positive pulse produces a double-peak emission structure reflecting its two-phase development, while the negative pulse yields broader, single-peak waveforms consistent with direct spark evolution.

Analysis of optical emission spectra showed that the negative pulse yields significantly higher rotational temperatures, reaching up to 1300 Kelvin, compared to the preceding positive pulse, where temperatures approached 700 Kelvin in the blade region. The vibrational excitation of the N2(C3Πu) electronic state was also markedly elevated during the negative pulse. The vibrational temperature of the excited state increased from about 3500 ± 300 Kelvin during the positive pulse to 6000 ± 400 Kelvin during the negative pulse in the blade region, corresponding to ground-state vibrational temperatures rising from roughly 2000 Kelvin to above 5000 Kelvin. These measurements indicate that the negative discharge develops in a preheated, vibrationally excited medium left in the wake of the positive discharge.

The plasma electron density, estimated via the Stark-broadened Hα line, reached 2×10^17 per cubic centimeter for both polarities and exhibited a gradual decay over time with minimal spatial or polarity dependence. During the negative pulse, the Hα line dominates from the very beginning, allowing the electron density to be monitored over a relatively long interval of 50 to 250 nanoseconds. Densities around 10^17 per cubic centimeter persisted up to 200 nanoseconds, consistent with the extended atomic line emission captured in the spatiotemporal color maps derived from photomultiplier data.

The researchers attribute the polarity-dependent behavior to a combination of memory effects established by the preceding positive pulse. Thermal memory is most directly supported by the measurements: gas heating from roughly 300 Kelvin to 700 Kelvin during the positive pulse decreases the neutral gas density, enhancing the reduced electric field experienced by the subsequent negative pulse. Monte Carlo calculations using the LoKI-MC code showed that this thermally induced increase in the reduced electric field can shift discharge conditions from a regime where electron multiplication is suppressed into a regime enabling rapid electron multiplication in the preionized channel. The calculations also revealed that water vapor evaporating from the heated surface modifies the effective ionization coefficient, enhancing electron attachment at low reduced fields and shifting the balance toward net ionization at higher fields.

Preionization effects provide another memory mechanism. Although free electrons generated by the positive pulse decay rapidly through attachment to oxygen molecules within about 13 nanoseconds, residual negative ions can persist on microsecond time scales through slow ion-ion recombination. Once the electric field associated with the negative pulse is established, a substantial fraction of the residual negative-ion population can be rapidly converted into free electrons through field-induced detachment, with characteristic detachment times of approximately 10 to 40 nanoseconds at representative reduced fields. The resulting seed-electron density is comparable to values typically reported for streamer discharges, potentially substituting for the initial streamer phase and explaining its absence during the negative pulse.

Non-equilibrium vibrational kinetics add a further layer of memory. Vibrationally excited nitrogen molecules can influence electron kinetics through superelastic collisions that transfer internal molecular energy to electrons, enhancing the high-energy tail of the electron energy distribution function. Ionization from vibrationally excited states also requires less additional electron energy than ionization from the vibrational ground state. Since vibrational relaxation occurs on much longer time scales than electron density decay, a significant fraction of the vibrational excitation generated during the positive pulse persists until the arrival of the negative pulse. Long-lived excited states, atomic radicals, and water-related species may further modify the plasma chemistry during the subsequent pulse.

The findings provide important insights into the role of pre-existing plasma environments in the dynamics of surface-discharge evolution at gas-liquid interfaces under double-pulse excitation. The synergy of positive and negative pulses allows the formation of transient spark filaments with a much higher concentration of atomic species directly at the water-air interface compared to what would be expected from a classical surface DBD. The negative pulse produces considerably higher concentrations of atomic hydrogen, oxygen, and nitrogen, as demonstrated by the spatiotemporal mapping of atomic line intensities. This has obvious implications for guiding future optimization of double-pulse discharge configurations to increase the production of atomic radicals, with potential applications ranging from plasma-activated water for agriculture to nitrogen fixation. The team is now working to complement their top-down diagnostics with lateral projections to reveal the characteristic thickness and vertical structure of the plasma layer on the water surface during various discharge phases.

Subject of Research: Nanosecond pulsed plasma discharges at the water-air interface and polarity-dependent memory effects

Article Title: Streamer-spark discharge at the water-air interface: spatio-temporal tracking of non-equilibrium kinetics and memory effects

Article References: Arora, G., Fujera, J., Hoffer, P., Prukner, V., Bílek, P., Bonaventura, Z., & Šimek, M. (2026). Streamer-spark discharge at the water-air interface: spatio-temporal tracking of non-equilibrium kinetics and memory effects. Frontiers in Physics, 14, Article 1924513. https://doi.org/10.3389/fphy.2026.1924513

Image Credits: AI Generated

DOI: 10.3389/fphy.2026.1924513

Keywords: plasma physics, streamer discharge, spark discharge, water-air interface, dielectric barrier discharge, optical emission spectroscopy, memory effect, double-headed streamer, nanosecond pulses, electron density, vibrational excitation, plasma-activated water

The D region, spanning roughly 60 to 100 kilometers in altitude, is a peculiar place. It is a weakly ionized plasma, meaning that for every free electron there are between a hundred million and a trillion neutral molecules. In such an environment, electrons collide far more often with neutral particles than with each other, and the electron-neutral collision frequency exceeds even the frequency at which electrons spiral around Earth’s magnetic field lines. This makes the D region a realm where individual collisions, not collective plasma behavior, dictate what happens to each electron. When a high-power high-frequency radio wave sweeps through this region, electrons absorb energy from the wave’s oscillating electric field through a process known as Ohmic heating. Elastic collisions scatter the electrons in random directions, converting the ordered energy of the wave into thermal motion, while less frequent inelastic collisions drain energy away by exciting the neutral molecules around them.

It is those inelastic collisions that give the electron distribution its strange and beautiful shape. Myrvang and Gustavsson’s numerical model, a modern re-implementation of a kinetic solver first derived by physicist Peter Stubbe in 1981, tracks how electrons gain energy from the radio wave and then lose it in discrete chunks as they excite vibrational, rotational, and electronic states in molecular nitrogen, molecular oxygen, and atomic oxygen. The most dramatic effect comes from molecular nitrogen, whose vibrational states have large cross sections for electron impact in the energy range between roughly 2 and 3.5 electron volts. When enough electrons reach those energies, they slam into nitrogen molecules and lose about 0.29 electron volts per collision, exciting the molecule’s vibrational modes. The result, visible in the model output, is a sharp cut-off in the electron distribution at approximately 2 electron volts, an effective cliff that truncates the population of energetic electrons far more abruptly than any Maxwellian would allow.

The technical achievement of the new work lies in how meticulously it handles this energy degradation. Where earlier solvers treated electrons as jumping between discrete velocity points, losing exactly the excitation energy in a single step, the Norwegian team used continuous, energy-dependent cross sections for every inelastic process and carefully partitioned degrading electrons across the finite velocity bins of their numerical grid. They also corrected subtle errors in the original formulation, noting that some of the factors in Stubbe’s published equations carried incorrect units. Starting from a Maxwellian guess, their iterative scheme converges, with a damping factor applied for stability, within about sixty iterations under typical ionospheric conditions. Neutral densities come from standard atmospheric models, the neutral temperature is set to 200 kelvin, and the electron density is assumed to be ten billion per cubic meter, values representative of the quiet D region.

To make sure the kinetic solver was trustworthy, the authors built an entirely independent check: a Monte Carlo simulation that follows one million individual electrons as they are accelerated by the oscillating electric field of the heating wave between collisions. Collision times are drawn from exponential random distributions, scattering is assumed isotropic, a good approximation for electrons below about 5 electron volts, and each collision is classified as elastic or inelastic according to the best available laboratory cross sections. The simulation captures the full time evolution of the electron population, from a cold 300-kelvin Maxwellian start to a steady state in which the energy pumped in by the radio wave exactly balances the energy lost to the neutral gas. That steady state arrives remarkably quickly at lower altitudes, in about three milliseconds at 80 kilometers, but takes as long as ten seconds at 120 kilometers, where collisions are rarer and the electrons take longer to settle.

The agreement between the two methods is convincing at most altitudes. At 80 and 100 kilometers, the Boltzmann solver and the Monte Carlo simulation produce energy distributions that match well above 0.1 electron volts, with second-moment temperatures of roughly 1750 and 1230 kelvin respectively in the kinetic calculation. Below 0.1 electron volts the two methods diverge somewhat, a discrepancy the authors attribute to the approximate analytical expressions used for rotational excitation and elastic collisions in the Boltzmann approach, approximations that matter most where the neutral density is highest. At 120 kilometers a more serious problem emerges: the electron-neutral collision frequency drops below the electron gyro-frequency, violating the solver’s assumption that electrons are unmagnetized. There the Boltzmann method overestimates the temperature significantly, predicting 350 kelvin where the Monte Carlo simulation, which fully accounts for the magnetic field, finds only 219 kelvin. The lesson is clear: the kinetic solver is a superb tool for the D region, but it cannot simply be extended upward into the magnetized E and F regions without modification.

Perhaps the most provocative result concerns temperature itself. The authors show that even when a temperature can be defined for the heated electrons, by taking the second moment of the distribution as a measure of total thermal energy, that single number conceals more than it reveals. At 90 kilometers with an electric field of 2.0 volts per meter, the second-moment temperature is a scorching 5124 kelvin, yet the actual distribution bears little resemblance to a Maxwellian at that temperature. An alternative measure, an energy-dependent effective temperature derived from the local slope of the distribution, swings wildly across the energy spectrum. It peaks near 10,000 kelvin around 1.1 electron volts, where electrons degraded from higher energies pile up in a relatively flat region of phase space, then plunges in the 1.8 to 2.7 electron volt range where nitrogen vibrational excitation drains the population, and finally settles toward the neutral gas temperature at the lowest energies. Undulations between 0.1 and 1.2 electron volts trace the resonance peaks in the vibrational cross sections of molecular oxygen.

These findings matter far beyond the niche community of ionospheric heating experiments. Maxwellian assumptions are baked into standard calculations of electron cooling rates, radio wave absorption, refractive indices, and D region chemistry. If the true distribution deviates sharply from Maxwellian whenever electrons are heated above roughly 500 to 600 kelvin, then every one of those derived quantities may need revisiting. The shape of the non-Maxwellian distribution is not fixed either: it changes with altitude and with the strength of the heating field, growing more distorted as more energy is injected. At 70 kilometers, where frequent collisions keep electrons from gaining much energy between impacts, the distribution stays relatively close to Maxwellian even at strong fields, with temperatures rising only from about 400 to 1600 kelvin as the electric field increases from 0.75 to 4.0 volts per meter. Higher up, at 90 to 100 kilometers, the same field range drives temperatures from a few hundred kelvin to between 5000 and 6000 kelvin, and the deviations become dramatic.

This paper is the first of a pair. In the companion study, the authors compute electron cooling rates for vibrational excitation of nitrogen and oxygen and for the excitation of fine structure levels in atomic oxygen, using the non-Maxwellian distributions rather than Maxwellian approximations. Together, the two papers promise to sharpen the tools scientists use to interpret experiments at facilities such as the EISCAT heating facility in Norway, where radio-induced electron heating has been measured in situ since the year 2000. They also serve as a reminder of a deeper truth in physics: temperature is a bookkeeping device, not a description. In the thin, collision-dominated air of the upper atmosphere, the electrons tell a richer story, one that can only be read by solving the kinetic equations in full, one collision at a time.

Subject of Research: Non-Maxwellian electron energy distributions in the ionospheric D region under artificial high-frequency radio wave heating

Article Title: Non-Maxwellian electron distributions in the D region during artificial heating – Part 1: Model development and electron temperature

Article References: Myrvang, M., & Gustavsson, B. (2026). Non-Maxwellian electron distributions in the D region during artificial heating – Part 1: Model development and electron temperature. Annales Geophysicae, 44(2), 921-935. https://doi.org/10.5194/angeo-44-921-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-921-2026

Keywords: ionosphere, D region, artificial heating, electron temperature, non-Maxwellian distribution, Boltzmann equation, Monte Carlo simulation, HF radio waves, vibrational excitation, molecular nitrogen, electron cooling, space physics

The core problem the researchers set out to solve is well known in the electrochemistry community. Copper remains the only metal catalyst that can convert CO2 into multi-carbon products at appreciable rates, because its binding energies sit in a narrow window that permits carbon–carbon coupling. Yet even the best copper-based systems overwhelmingly favor two-carbon products such as ethylene and ethanol, while the formation of C3+ hydrocarbons and oxygenates—propane, butane, propanol, butanol and other chemicals that command higher market value—remains stubbornly inefficient. Decades of catalyst design, from oxide-derived copper to facet-engineered films and tandem catalytic cascades, have delivered incremental gains, but the underlying reaction network on copper constrains which intermediates can form and, ultimately, which products can emerge.

The Yale–Antwerp team took a different approach: instead of redesigning the catalyst, they redesigned the feedstock. In their platform, CO2 or CO gas first passes through a non-thermal plasma, where energetic electrons collide with gas molecules and generate a rich cocktail of vibrationally excited molecules, radicals, dissociation fragments and unusual species such as carbon suboxide, C3O2. This activated gas stream is then delivered directly to a copper gas diffusion electrode, where the electrocatalytic reduction takes place. Crucially, by coupling the plasma to a gas-phase electrode rather than bubbling plasma products through an electrolyte, the design ensures that short-lived, highly reactive species survive long enough to reach the catalyst surface, where their stored chemical energy can be harvested electrochemically.

The results are striking. When CO2 and CO were co-fed through the plasma into the electrochemical cell, productivity of C3+ products increased by a factor of 3.3 and alcohol productivity by 1.5 times relative to electrocatalysis alone. Even more remarkable is the selectivity expansion: plasma activation unlocked the formation of chemicals that are essentially absent from purely electrocatalytic product distributions, including methanol, acetylene, ethane, propane, butane and butanol. In other words, the plasma does not simply accelerate the standard copper chemistry—it opens entirely new reaction channels on the same metal surface.

To understand why, the researchers combined plasma simulations, kinetic modeling and in situ spectroscopy. Plasma simulations of the discharge revealed that a significant fraction of CO2 molecules leaves the plasma vibrationally excited rather than fully dissociated. Vibrational excitation is a form of chemical currency: a vibrationally hot CO2 molecule effectively carries part of the activation energy needed for its own reduction, lowering the energetic barrier for subsequent steps at the electrode. Kinetic simulations comparing plasma-excited and ground-state species showed that these excited molecules, once adsorbed on copper, can follow different reaction trajectories than their thermal counterparts, enriching the population of key surface intermediates that feed C–C coupling and oxygenate formation.

Carbon suboxide emerged as another central player. This unusual C3O2 species, long studied in combustion and plasma physics, is generated in the plasma through reactions of CO with excited CO2. The team’s kinetic simulations and in situ analysis suggest that carbon suboxide arriving at the copper surface can be reduced and hydrogenated along pathways that bypass the conventional CO dimerization route, providing a direct entry point into three-carbon products. The in situ Raman spectroscopy experiments, performed by the Yale group on operating copper electrodes, tracked changes in surface adsorbates when plasma-activated gas was introduced, providing experimental evidence that the plasma species reshuffle the intermediate landscape on the catalyst rather than merely increasing local reactant concentration.

The gas diffusion electrode architecture deserves particular attention. Gas diffusion electrodes have become the workhorse of modern CO2 electrolysis because they bring the gaseous reactant into intimate contact with the catalyst at a three-phase boundary, enabling current densities that far exceed what submerged electrodes can achieve. The researchers deliberately designed the plasma outlet to feed the activated gas stream directly into the gas diffusion layer of the copper electrode, minimizing the residence time between plasma and catalyst. This tight coupling prevents the reactive plasma species from being consumed by recombination reactions or neutralized in the liquid electrolyte—a failure mode that has plagued earlier attempts to combine plasma with electrochemistry in solution-phase cells.

The team systematically varied the operating conditions to disentangle the contributions of different plasma species. Comparing pure CO2 feeds, pure CO feeds and mixed CO2–CO feeds through the plasma showed that the presence of both gases in the discharge enhanced C3+ and alcohol formation beyond what either gas alone could achieve, consistent with the formation of carbon suboxide requiring both CO2 and CO in the plasma zone. Varying the applied electrode potential mapped out how the electrochemical driving force interacts with the chemically activated feed, showing that the plasma effect persists across the potential window relevant to multi-carbon production. Plasma diagnostic experiments on the discharge itself complemented the modeling, anchoring the simulated species distributions in measured reality.

Beyond the mechanistic insights, the work carries significant implications for how renewable electricity might be converted into storable fuels and industrial feedstocks. Electrosynthesis of chemicals from CO2 promises a route to close the carbon cycle using intermittent solar and wind power, but the technology’s economic viability hinges on achieving high rates, high selectivity and high value products simultaneously. By demonstrating that plasma activation can raise production rates of the most valuable C3+ products by more than threefold while adding entirely new product classes, the platform addresses the selectivity bottleneck in a fundamentally new way—as a feedstock activation problem rather than a catalyst design problem. The two technologies are complementary by nature: plasma excitation is fast, catalyst-agnostic and tolerant of dilute feeds, while electrocatalysis offers precise control over electron transfer and product distribution at ambient temperature and pressure.

The researchers are candid that scaling the concept will require attention to energy efficiency, since both plasma generation and electrochemical reduction consume electricity, and the overall energy conversion chain must compete with incumbent fossil-based processes. Stability of the coupled reactor over extended operation, integration of the plasma unit with industrial gas handling, and optimization of discharge type and power coupling all represent engineering challenges ahead. A patent application covering the plasma–electrocatalysis flow cell has been filed by Yale, signaling the team’s intent to translate the concept. Still, the demonstration that vibrationally excited molecules and carbon suboxide can rewrite the reaction network on copper offers the field a genuinely new lever. If the synergy between plasma chemistry and electrocatalysis can be pushed further—through tailored discharge conditions, optimized gas residence times and catalyst surfaces designed to accept plasma species—the electrosynthesis of high-value chemicals and fuels from CO2 could move meaningfully closer to practical reality.

Subject of Research: Plasma-activated electrocatalytic conversion of CO2 and CO to C3+ hydrocarbons and oxygenates on copper

Article Title: Unlocking reaction pathways for CO2 and CO electrocatalytic reduction to C3+ hydrocarbons and oxygenates using plasma activation

Article References: Unlocking reaction pathways for CO2 and CO electrocatalytic reduction to C3+ hydrocarbons and oxygenates using plasma activation. (n.d.). https://doi.org/10.1038/s41929-026-01609-5

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01609-5

Keywords: CO2 reduction, electrocatalysis, non-thermal plasma, copper catalyst, C3+ products, oxygenates, carbon suboxide, vibrational excitation, gas diffusion electrode, plasma-electrocatalysis, renewable fuels, Nature Catalysis