Stepwise Ionization Research Articles

A stepwise surface ionization method for ion mobility spectrometry.

The detection of organic nitrogen compounds in exhaled breath is expected to provide an early warning of diseases such as kidney disease. Detecting these trace disease markers in exhaled breath with complex composition and high moisture content is a challenge. Surface ionization (SI) shows a highly selective ionization of organic nitrogen compounds, and it is a good candidate for breath analysis combined with ion mobility spectrometry (IMS). A stepwise SI method of low-temperature adsorption/high-temperature ionization was proposed, and trimethylamine (TMA) was detected when combined with an ion mobility spectrometer. TMA at different concentrations and humidity levels and spiked in human breath was detected to evaluate the method's properties. TMA with concentrations from 2 to 200 ppb was detected. The peak intensity of the TMA characteristic ions was linearly related to the "e" exponent of the concentration with a curve fit of 0.996. A standard deviation of less than 0.306% was obtained with 10 replicate analyses of 10ppb TMA. The signal intensity difference between dry and wet (relative humidity > 93%) TMA samples is only 2.7%, and the recovery rate of the sample was 106.819%. SI-IMS based on the stepwise SI method has the advantages of low ionization temperature, high detection sensitivity, strong resistance to humidity interference, and good repeatability. It is a promising method for detecting organic nitrogen compounds in exhaled breath.

Self-consistent calculation of the optical emission spectrum of an argon capacitively coupled plasma based on the coupling of particle simulation with a collisional-radiative model

We report the development of a computational framework for the calculation of the optical emission spectrum of a low-pressure argon capacitively coupled plasma (CCP), which is based on the coupling of a particle-in-cell/Monte Carlo collision simulation code with a diffusion-reaction-radiation code for Ar I excited levels. In this framework, the particle simulation provides the rates of the direct and stepwise electron-impact excitation and electron-impact de-excitation for 30 excited levels, as well as the rates of electron-impact direct and stepwise ionization. These rates are used in the solutions of the diffusion equations of the excited species in the second code, along with the radiative rates for a high number of Ar-I transitions. The calculations also consider pooling ionization, quenching reactions, and radial diffusion losses. The electron energy distribution function and the population densities of the 30 excited atomic levels are computed self-consistently. The calculations then provide the emission intensities that reproduce reasonably well the experimentally measured optical emission spectrum of a symmetric CCP source operated at 13.56 MHz with 300 V peak-to-peak voltage, in the 2–100 Pa pressure range. The accuracy of the approach appears to be limited by the one-dimensional nature of the model, the treatment of the radiation trapping through the use of escape factors, and the effects of radiative cascades from higher excited levels not taken into account in the model.

Experimental study on the full cycle evolution of high-intensity atmospheric dc arc discharge from breakdown to extinguishment.

In this study, the spatiotemporal evolution of full cycle of high-intensity dc argon arc discharge at atmospheric pressure is investigated by using a transferred arc device, which is easy to be directly observed in the experiment. Combining the voltage and current waveforms with high-speed images, the full cycle evolution process of high-intensity atmospheric dc arc can be divided into five different stages: breakdown pulse stage, cathode heating stage, current climbing stage, stable arc discharge stage, and finally arc extinguishing stage. The characteristics of each different stage are analyzed in detail through the electrical properties, high-speed pictures, and spectroscopic measurements. The results show that the strong luminescence region develops from the vicinity of cathode and anode to the middle in the breakdown pulse stage, which is explained from the spatiotemporal evolution of distributions of excited argon atom and ions. The development velocity of emission intensity of argon ions is mainly determined by the dominant stepwise ionization process. Then the cathode heating stage appears with many bright and nonuniformly distributed light spots on the cathode surface, and the electron emission mechanism of cathode gradually changes to the thermionic emission as the surface temperature rises. With the increase of arc current, the discharge channel significantly expands, then becomes stable due to the increment of the Lorentz force. The characteristics of arc extinguishing stage are clarified in terms of the decay of charged particles density.

Hybrid simulation of the plasma characteristics in a dual‐frequency dual‐antenna inductively coupled plasma discharge

AbstractDual‐frequency (DF) dual‐antenna inductively coupled plasma (ICP) sources have been proposed as one of the methods to produce large‐area uniform plasma. In this study, a hybrid model, consisting of a fluid module and an electron Monte Carlo (eMC) module, is used to further investigate the modulation of the plasma characteristics (i.e., induced electric field, electron energy distribution, electron heating mechanism, electron temperature, and plasma density) by the low‐frequency (LF) and high‐frequency (HF) currents in a DF argon discharge. When the inner LF current increases from 12 to 22 A at a fixed outer HF current of 10 A, the induced electric field is strengthened near the LF coil and weakened near the HF coil. The fraction of high‐energy electrons at different spatial positions increases with the increase of LF current, which is caused by the sufficient collision heating in the skin layer of LF source, leading to an increase in electron temperature. When the outer HF current rises from 7 to 13 A with the inner LF current fixed at 17 A, the induced electric field is enhanced near the HF coil and weakened near the LF coil. The fraction of low‐energy electrons at different positions increases with the increase of HF current since the energetic electrons from the skin layer of HF source are cooled due to the ionization, excitation, and stepwise ionization collisions between electrons and neutral particles, as well as the weakened induced electric field near the LF coil. As a result, the electron temperature drops with the increase of HF current. Moreover, when the LF and HF currents increase, the plasma density increases, and the distribution of plasma density is also significantly modulated.

Investigation of the significance of excited species in wave-heated dense non-ideal microplasmas using particle-in-cell Monte Carlo-collision modeling

A computational model for the interaction of a high-pressure microplasma with an electromagnetic wave is presented. A one-dimensional particle-in-cell Monte Carlo collision model is used to investigate the plasma non-ideality effects in a second-stage laser-heated xenon plasma with a comprehensive chemistry mechanism, including excited species. A xenon microplasma at a temperature of 300 K and pressure of 10 bar becomes non-ideal after the laser heating stage with lower ionization and electronic excitation barriers, which makes the presence of excited species important to the plasma generation process. For these conditions, two-step ionization via excited species becomes a major contributor to plasma ionization. We find that full ionization of the plasma is obtained in ∼2 ps when excited species are included in the chemistry model, as opposed to ∼2.5 ps for a chemistry mechanism that does not consider their effect. With excited species, the ionization mode transitions from direct ionization via electron-neutral collisions to direct and stepwise ionization as the plasma generation progresses.

Formation mechanisms of striations in a filamentary dielectric barrier discharge in atmospheric-pressure argon

Formation mechanisms of striations along the discharge channel of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure are investigated by means of a time-dependent, spatially two-dimensional fluid-Poisson model. The model is applied to a one-sided DBD arrangement with a 1.5 mm gap using a sinusoidal high voltage at the powered metal electrode. The discharge conditions are chosen to mimic experimental conditions for which striations have been observed. It is found that the striations form in both half-periods during the transient glow phase, which follows the streamer breakdown phase. The modelling results show that the distinct striated structures feature local spatial maxima and minima in charged and excited particle densities, which are more pronounced during the positive polarity. Their formation is explained by a repetitive stepwise ionisation of metastable argon atoms and ionisation of excimers, causing a disturbance of the spatial distribution of charge carriers along the discharge channel. The results emphasise the importance of excited states and stepwise ionisation processes on the formation of repetitive ionisation waves, eventually leading to striations along the discharge channel.

Ionization waves in low-current dc discharges in noble gases obtained with a hybrid kinetic-fluid model.

A hybrid kinetic-fluid model is used to study ionization waves (striations) in a low-current plasma column of dc discharges in noble gases. Coupled solutions of a kinetic equation for electrons, a drift-diffusion equation for ions, and a Poisson equation for the electric field are obtained to clarify the nature of plasma stratification in the positive column. A simplified two-level excitation-ionization model is used for the conditions when the nonlinear effects due to stepwise ionization, gas heating, and Coulomb interactions among electrons are negligible. It is confirmed that the nonlocal effects are responsible for the formation of moving striations in dc discharges at low plasma densities and low values of pR (the product of gas pressure and tube radius). The calculated properties of self-excited waves of S-, P-, and R types in neon and S type in argon agree with available experimental data. The reason for helium plasma stability to stratification is clarified. It is shown that sustaining stratified plasma is more efficient than striation-free plasma when the ionization rate is a nonlinear function of the electric field. However, the nonlinear dependence of the ionization rate on the electric field is not required for plasma stratification. Striations of S-, P-, and R types in neon exist with minimal or no ionization enhancement. Effects of the column length and plasma density on the wave properties are demonstrated.

Coupling plasma physics and chemistry in the PIC model of electric propulsion: Application to an air-breathing, low-power Hall thruster

This work represents a first attempt to include the complex variety of electron-molecule processes in a full kinetic particle-in-cell/test particle Monte Carlo model for the plasma and neutral gas phase in a Hall thruster. Particular emphasis has been placed on Earth’s atmosphere species for the air-breathing concept. The coupling between the plasma and the gas phase is self-consistently captured by assuming the cold gas approximation and considering gas-wall and gas recycling from the walls due to ion neutralization. The results showed that, with air molecular propellants, all the most relevant thruster performance figures degraded relative to the nominal case using Xe propellant. The main reasons can be ascribed to a reduced ionization cross-section, a larger gas ionization mean free path due to lighter mass air species, and additional electron collisional power losses. While vibrational excitations power losses are negligible, dissociation and electronic excitations compete with the ionization channel. In addition, for molecular oxygen, the large dissociation leads to even faster atoms, further reducing their transit time inside the discharge channel. Future studies are needed to investigate the role of non-equilibrium vibrational kinetics and metastable states for stepwise ionization.

The principle of minimal power

This article is devoted to the memory of Yuri P Raizer, who passed away in 2021. He left a noticeable trace in gas discharge physics. The principle of minimal power (the state that requires minimal power is most probable) is thoroughly used in his books. Although the fundamental laws of physics do not imply this ad hoc principle, a detailed analysis of underlying phenomena can often reveal why nature prefers this path. Raizer illustrated this principle for plasma stratification, formation of electrode spots, discharge constriction, the shape of an arc channel, etc. We argue that the nonlinearity of equations describing gas discharges can often justify the realization of a plasma state maintained at minimal electric power. This nonlinearity appears because small groups of energetic electrons often control the ionization processes. The number of these electrons depends strongly on the ratio of the electric field to gas density, E/N. Under certain conditions, the ionization rate can also depend nonlinearly on electron density due to stepwise ionization and Coulomb collisions. We use the principle of minimal power to illustrate some of Raizer’s contributions to gas discharge physics from a single point of view. We demonstrate that nonlinearity of ionization processes in gas discharges can substantiate this principle for plasma stratification. However, striations of s, p, and r types in neon could exist with minimal or no ionization enhancement. This reminds us of Raizer’s warning that applying the minimal power principle could lead to erroneous predictions, and a proper theory is required in each case to justify its use. ‘The phenomenon of striations satisfies the principle of minimal power’ – Yuri Raizer

Three in one: Rational engineering of multifunctional MIL-101-based ionic catalysts for carbon dioxide-epoxide cycloaddition

CO2-epoxide cycloaddition represents one of most attractive approaches for chemical fixation and utilization of CO2, but highly efficient and fully heterogeneous green catalysis is still challenging. Ionic metal-organic frameworks (IMOFs) are nice porous matrices heterogenizing the essential free nucleophilic anions and meanwhile providing Lewis acids, but the catalytic efficiency has been limited for low anion content and lack of auxiliary catalytic sites. In this work, we report stepwise optimization of IMOF catalysts by integrating multiple catalytic sites into MIL-101. Various monocationic IMOFs with quaternary ammonium and bromide anions were prepared by facile alkyne-azide click chemistry. Different numbers of hydroxyl groups were introduced in this ionization step to optimize the activity. To introduce more anions, dicationic IMOFs were created by a second ionization step, N-quaternization of triazolyl to triazolium. By the stepwise ionization and functionalization, we demonstrated the synergic catalytic effects among different sites, the impacts of hydroxyl/bromide content, and the size effect of the cationic group. The optimal IMOFs are superior to previous ionic heterogeneous catalysts in absence of cocatalysts. In particular, the dicationic catalyst can quantitatively convert epichlorohydrin to the carbonate within 1 h at 1 MPa CO2 and 80 °C, the apparent turnover frequency being up to 478 h−1. The highly active and selective, fully heterogeneous and recyclable catalyst has the appeal for practical applications.

Some aspects of the plasma potential in 3D simulation of ECRIS operation

A 3D particle-in-cell plus Monte Carlo collision (PIC/MCC) code is developed for the simulation of electron cyclotron resonance ion source (ECRIS). The self-consistent interaction between the plasma and the potential field is taken into account, as well as Coulomb collisions, stepwise ionization, and charge exchange collisions between particles. In addition, a precalculation module based on a single-particle approach is introduced to speed up simulations. The stable distributions of the high-energy electrons are obtained and then sent to the subsequent simulation of ECRIS operation as the well-confined warm and hot electrons. An implicit electrostatic PIC model in this simulation self-consistently describes the evolution of the ECR plasma. The results are obtained for the plasma potential in a steady state, including the global amplitude and distribution profiles. The potential distribution of the ECR plasma is characterized by magnetic fields. These results, together with those for the charge density, are analyzed and discussed from the perspective of plasma diffusion.

Insights from Collisional-Radiative Models of Neutral and Singly Ionized Xenon in Hall Thrusters

Collisional-radiative (CR) models for neutral and singly ionized xenon have been constructed for applications to Hall thruster diagnostics. The models use primarily theoretical electron-impact excitation cross sections calculated using the Dirac B-spline R-matrix method. This paper describes the models’ construction and their validation against heritage CR models and previously published Hall thruster data, and then outlines several applications. The baseline 0D steady-state models are used to investigate electron density dependence in neutral xenon emission line intensity ratios commonly used to measure electron temperature, and also to demonstrate that stepwise ionization through excited states is an important process in Hall thrusters. The models are extended to evaluate impacts of neutral and ion transport on 0D CR model applicability, intrinsic bandwidth limits for time-dependent line intensity measurements, and errors introduced when the plasma is assumed to be optically thin.

Formation and destruction of striation plasmas in helium glow discharge at medium pressures

The striation plasmas are usually generated within a positive column of glow discharge, where rich and complex physical interactions are involved, especially, in the medium or high pressures. Along these lines, our work aims to thoroughly investigate the formation and destruction of helium striation plasmas at kPa level pressures. The characteristics of the helium striation plasmas, and especially the optical emission properties are explored. The emission lines of 706.52 nm and 391.44 nm related to the energetic electrons and the high-energy metastable helium atoms respectively, were focused on in this work. The formation of striation plasmas in a helium glow discharge, is mainly associated with the instability originating from the stepwise ionization of high-energy metastable state atoms, Maxwellization of the electron distribution functions and gas heating. Additionally, the destruction effect of helium striation plasmas is of great significance when a small amount of nitrogen or oxygen is mixed into the discharge plasmas. The reduction of the mean electron energy and the consumption of the high-energy metastable helium atoms are considered as the underlying reasons for the destruction of striation plasmas.

Importance of higher‐level excited species in argon remote plasma sources: Numerical modeling with consideration of detailed chemical reaction pathways

AbstractStable plasma ignition of remote plasma sources (RPSs) for various plasma processes is essential to ensure the stability of the discharge, steady operating conditions, and high plasma density. In an RPS in which the gas pressure exceeds 1 Torr, the reaction paths leading to the high‐level excited state play an essential role in the power balance and energy cascade at high applied input power. In this study, 97 chemical reactions in a detailed reaction set involving argon discharge are investigated in the steady‐state to understand the extent to which the dominant reactions are affected by variations in the process parameters. We observed that the stepwise ionization process of the argon 4p excited state significantly contributes to sustaining high‐density plasmas in the near 1 Torr regime.

Importance of stepwise ionization from the metastable state in electron cyclotron resonance ion thrusters

In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.

Modeling of Ionization Waves in Atmospheric-Pressure Argon in a Long Gap

Numerical simulation of the development of ionization waves—streamers—in pure argon at atmospheric pressure in a plane-parallel gap has been performed. The velocities and plasma parameters of cathode- and anode-directed ionization waves are obtained, and the streamer dynamics and structure are analyzed. The effects of variation of the background density of charged species are considered. The roles of various ionization mechanisms (the direct and stepwise ionization, and the Penning reaction) during streamer formation and propagation are revealed.

Influence of Metastable Argon Atoms and Dust Particles on Gas Discharge Plasma

The model of a DC glow discharge with metastable argon atoms and dust particles based on the Boltzmann equation for the electron energy distribution function (EEDF), dust particle charging, and balance equation for metastable argon atoms is presented. The processes of direct and stepwise electron impact ionization, metastable-metastable collisions, and recombination of electrons and ions on the dust particle surface and discharge tube wall are taken into account. The results show that the densities of metastable argon atoms and dust particles are in strong correlation, and both sufficiently influence gas discharge parameters.

Plasma-based microwave power limitation in a printed transmission line: a self-consistent model compared with experimental data

Plasma-based microwave power limitation in a suspended microstrip transmission line integrating a micro hollow cathode discharge (MHCD) in its centre is experimentally and numerically studied. Transient and steady state microwave power measurements exhibit a limitation threshold of 28 dBm and time responses of 25 microseconds. Intensified charge-coupled device imaging shows that microwave breakdown occurs at the top of the MHCD. The plasma then extends towards the microwave source within the suspended microstrip transmission line. Besides, a self-consistent model is proposed to simulate the non-linear interaction between microwave and plasma. It gives numerical results in agreement with the measurements, and show that the plasma expansion during the transient response is related to a shift between the ionization source term and the electron density maximum. The propagation speed, under the tested conditions, depends mainly on the stepwise ionization from the excited states.

Ionization waves (striations) in a low-current plasma column revisited with kinetic and fluid models

A one-dimensional particle-in-cell Monte Carlo collisions method has been used to model the development and propagation of ionization waves in neon and argon positive columns. Low-current conditions are considered, that is, conditions where stepwise ionization or Coulomb collisions are negligible (linear ionization rate). This self-consistent model describes the development of self-excited moving striations, reproduces many of the well-known experimental characteristics (wavelength, spatial resonances, potential drop over one striation, and electron “bunching” effect) of the ionization waves called p, r, and s waves in the literature, and sheds light on their physical properties and on the mechanisms responsible for their existence. These are the first fully kinetic self-consistent simulations over a large range of conditions reproducing the development of p, r, and s ionization waves. Although the spatial resonances and the detailed properties of the striations in the nonlinear regime are of kinetic nature, the conditions of existence of the instability can be obtained and understood from a linear stability analysis of a three-moment set of quasi-neutral fluid equations where the electron transport coefficients are expressed as a function of electron temperature and are obtained from solutions of a 0D Boltzmann equation. An essential aspect of the instability leading to the development of these striations is the non-Maxwellian nature of the electron energy distribution function in the uniform electric field prior to the instability onset, resulting in an electron diffusion coefficient in space much larger than the energy diffusion coefficient.

Femtosecond dynamics of stepwise two-photon ionization in solutions as revealed by pump-repump-probe detection with a burst mode of photoexcitation.

Pump-repump-probe spectroscopy with a burst mode of photoexcitation was applied to the direct observation of the photoionization dynamics of perylene in the solution phase. Irradiation of a pump pulse train generated with birefringent crystals effectively accumulated an intermediate S1 state and a repump pulse triggered photoionization in the higher excited state, ensuring sufficiently large signal intensity to probe. Two-photon excitation to the energy level, which is 0.7 eV lower than the ionization potential in the gas phase, results in instantaneous formation of the radical cation of perylene in acetonitrile, unlike aromatic amines in previous reports. In addition, subsequent recombination dynamics between the radical cation and ejected electron was monitored in polar and nonpolar solvents. The ultrafast recombination in nonpolar solvents suggests that the distribution of the distance in the cation-ejected electron pair largely evolves even in acetonitrile in the femtosecond timescale, in which the solvation is not completed and the dielectric constant is still low. The recombination process in acetonitrile was well reproduced with simulations based on the Smoluchowski equation taking account of the transient change in the dielectric constant by solvation.