Electron Impact Ionization Processes Research Articles

Ab initio study of energy characteristics of small polyatomic molecules in threshold electron-impact dissociative ionization processes

SynopsisA joint theoretical study of electron-impact dissociative ionization (DI) processes of small polyatomic molecules is presented. Different organic (CH4 and C2H6) and inorganic (SF6 and Sn, n = 2-8) target molecules were examined. The investigated dissociative ionization processes take place at threshold energies, so our main focus is to study the appearance energies (E ap) of different ion fragments as well as to analyse the corresponding DI reaction pathways.

Temporal electron collisions and adjunct ionization mechanisms in the positive column of a flowing N2 DC discharge

We studied a flowing nitrogen DC discharge by Langmuir double probes and optical emission spectroscopy (OES). We measured the reduced electric field (E/N) and electron density (ne) with the double probes. The gas (Tg) and N2(X1Σ+g) vibrational temperature (θ) were measured by OES from the nitrogen first and second positive systems transitions. We constructed a kinetic numerical model accounting for N2(X1Σ+g, 0 ≤ v ≤ 45), N2(A3Σ+u), N2(B3Πg), N2(a1Πg), N2(a′1Σ−u), N2(w1Δu), N2(a″1Σ+g), N2(W3Δu), N2(E3Σ+g), N2(C3Πu), N2+(X2Σ+g), N2+(B2Σ+u), N+, N3+, N4+, N(4S), N(2D), and N(2P) states. The species densities are calculated as functions of the discharge gas residence time in pure nitrogen for our experimental conditions. The kinetic model assumes as input parameters the experimental discharge gas pressure (p), tube radius (R), electron density (ne), gas temperature (Tg), reduced electric field (E/N). In this Letter, the kinetic model is applied to calculations of the fractional contribution of each ionization mechanism to the total ionization generated in the discharge. We compute 24 different ionization mechanisms to the five ions species. We have observed that ionization is generated by different physical-chemical reactions depending on the discharge gas residence time. It changes from dominant electron impact ionization process in the earlier times of the discharge to adjunct ionization mechanisms depending on the pooling of N2(a′1Σ−u) and N2(A3Σ+u) metastable molecules and associative ionization of N2(a′1Σ−u), N(2D) and N(2P) metastable atoms and molecules. Moreover, we have observed the occurrence of approximate charge neutrality for our studied experimental condition: discharge current of 30 mA, gas flow rate of 1 Slm−1, and gas pressure of 230 Pa. Also, the kinetic model fits quite well the measured N2(X1Σ+g) vibrational temperature at the end of the positive column.

Electron-impact single ionization ofFe3+from the ground and metastable states

Single ionization by electron impact of the ${\mathrm{Fe}}^{3+}$ ion is investigated using the Dirac-Fock-Slater approximation for the levels of the ground and first-excited $3{d}^{4}4s$ configuration. Contributions from direct ionization (DI) and excitation-autoionization (EA) processes are taken into account. The scaled distorted-wave (DW) cross sections for DI and excitation by electron impact are used to analyze the experimental data. The study demonstrates the importance of correlation effects for the EA process. Good agreement with experimental data reveals that currently available DW cross sections and, therefore, the corresponding Maxwellian rate coefficients for electron-impact ionization process in ${\mathrm{Fe}}^{3+}$ are overestimated and their usage may lead to wrong predictions for charge state distribution in plasma.

Prominent role of indirect processes in electron-impact ionization of Xe24+ ions

SynopsisThe cross section for electron-impact single ionization of Xe24+ has been studied both experimentally and theoretically. The cross section exhibits significant features from resonant ionization processes such as resonant-excitation double autoionization. These are well described by the theoretical calculations if the latter take into account excitations to atomic shells with principal quantum numbers of up to n = 39.

Highly charged tungsten ions generated by nanosecond pulsed laser and influence of magnetic field on ion charge state

Ions emitted from the laser generated tungsten plume were investigated with the help of ion collector (IC) and electrostatic ion energy analyzer (IEA) operating in time-of-flight configuration. Plume was generated by irradiating a plane W target with 6ns pulsed Nd:YAG (λ=1064nm) laser. Laser fluence at the target was varied in the range of 3.0–19.4J/cm2. IC measurements showed that the amplitude of tungsten ions pulse increases with the laser fluence. IEA spectra indicate that the number of available Wn+ ions charge states increases from 1 to 6 when laser fluence at the target was varied in the range of 3.0–19.4J/cm2. With the application of 0.23 T axial magnetic field at the target surface, the number of available Wn+ ions charge states increases from 1 to 10 in the similar range of the laser fluence. In addition, molecular oxygen ions O2n+ were also observed when magnetic field is applied at higher values of the laser fluence. The ion charge state enhancement is presumably due to the magnetic trapping of electrons in front of the target surface, which intensifies electron impact ionization process.

Ion charge state and energy enhancement by axial magnetic field applied during laser produced plasma expansion

AbstractThe effects of axial magnetic field on the properties of the ions ejected from Nd:YAG laser (wavelength = 1064 nm, pulse duration = 6 ns) produced expanding Cu plasma were investigated. A plane Cu target, without and with 0.23 T axial magnetic field at its surface, was irradiated in the fluence range of 2–24 J/cm2. The ions emitted along the target surface normal were analyzed with the help of ion collector and time-of-flight electrostatic ion energy analyzer. The integrated ion yield, highest ion charge state, average ion energy, and energy of individual ion charge states were found to increase by application of the magnetic field. The initial parameters of the non-equilibrium plasma such as average ion charge, equivalent potential, electron temperature, electron density, Debye length, and transient electric field were estimated from the experimental results obtained without and with application of the magnetic field. The increase of ion yield and ion charge state by application of magnetic field are most probably due to the trapping of electrons in front of the target surface, which boosts up the electron impact ionization process. The ion energy increment due to the magnetic field is discussed in the frame work of electrostatic model for ion acceleration in laser plasma.

Characteristics of emission from laser-induced plasma of metallic compounds in gaseous condition: The effects of gas pressure and laser pulse energy

ABSTRACTLow-pressure laser-induced breakdown spectroscopy (LIBS) was employed to measure mercury, strontium, and cesium under different conditions. Mercury was measured in detail to discuss the effects of pressure, laser pulse energy, delay time, and buffer gas. The continuum emission from plasma reduced dramatically to enhance signal-to-background ratio due to the change of influence of electron impact ionization process when reducing pressure and decreasing laser pulse energy to some extent. The comparison of mercuric chloride and mercury, as well as strontium and cesium measurements, demonstrates the enhancement of detection ability for trace species measurement using low-pressure LIBS.

EUV induced low temperature SF6-based plasma

In this work spectral investigations of low temperature F-rich photoionized plasmas were performed. The photoionized plasmas were created by irradiation of SF6 gas with intense EUV (extreme ultraviolet) radiation pulses. Two laser plasma EUV sources of different parameters used in the experiments were based on 0.8 J /4ns and 10 J/ 10 ns Nd:YAG lasers respectively. Both sources operated at 10 Hz repetition rate. The EUV radiation was focused using a dedicated reflective collector onto the gas stream, injected into a vacuum chamber synchronously with the EUV pulses. Irradiation of the SF6 gas resulted in dissociative ionization of the molecules, leading to creation of SFn+ ions and fluorine atoms. Further photo- or electron impact ionization and excitation processes allow for formation of photoionized plasmas emitting radiation in the wide spectral range. Emission spectra were measured in the EUV and optical ranges. The EUV spectra contained multiple spectral lines, originating from F II, F III and S II ions. The UV/VIS spectra were composed of spectral lines corresponding to radiative transitions in F II, F I and S II species. A computer simulation of the F II spectrum was performed using a collisional-radiative PrismSPECT code. Parameters of the photoionized plasmas were estimated by fitting the spectrum obtained from the simulations to the experimental one. Apart from that, the electron temperature was estimated employing Boltzmann plots based on the UV/VIS spectrum.

Aerosol mass spectrometry: particle–vaporizer interactions and their consequences for the measurements

Abstract. The Aerodyne aerosol mass spectrometer (AMS) is a frequently used instrument for on-line measurement of the ambient sub-micron aerosol composition. With the help of calibrations and a number of assumptions on the flash vaporization and electron impact ionization processes, this instrument provides robust quantitative information on various non-refractory ambient aerosol components. However, when measuring close to certain anthropogenic or marine sources of semi-refractory aerosols, several of these assumptions may not be met and measurement results might easily be incorrectly interpreted if not carefully analyzed for unique ions, isotope patterns, and potential slow vaporization associated with semi-refractory species. Here we discuss various aspects of the interaction of aerosol particles with the AMS tungsten vaporizer and the consequences for the measurement results: semi-refractory components – i.e., components that vaporize but do not flash-vaporize at the vaporizer and ionizer temperatures, like metal halides (e.g., chlorides, bromides or iodides of Al, Ba, Cd, Cu, Fe, Hg, K, Na, Pb, Sr, Zn) – can be measured semi-quantitatively despite their relatively slow vaporization from the vaporizer. Even though non-refractory components (e.g., NH4NO3 or (NH4)2SO4) vaporize quickly, under certain conditions their differences in vaporization kinetics can result in undesired biases in ion collection efficiency in thresholded measurements. Chemical reactions with oxygen from the aerosol flow can have an influence on the mass spectra for certain components (e.g., organic species). Finally, chemical reactions of the aerosol with the vaporizer surface can result in additional signals in the mass spectra (e.g., WO2Cl2-related signals from particulate Cl) and in conditioning or contamination of the vaporizer, with potential memory effects influencing the mass spectra of subsequent measurements. Laboratory experiments that investigate these particle–vaporizer interactions are presented and are discussed together with field results, showing that measurements of typical continental or urban aerosols are not significantly affected, while measurements of semi-refractory aerosol in the laboratory, close to anthropogenic sources or in marine environments, can be biased by these effects.

Modeling of Subnanosecond Discharge in Hydrocarbon Liquid

Results of numerical simulation, in the framework of a 2-D model, of streamer dynamics in a hydrocarbon liquid (cyclohexane) in strong electric fields are presented. The obtained data show that fast propagation (with velocities exceeding $10^{8}$ cm $\text{s}^{\mathrm {-1}})$ of streamers due to electron impact ionization process in liquid phase is possible in the regions of strong electric fields. The calculated streamer parameters are compared with the available experimental data and with analytical estimates.

Low-energy (E0 = 65 eV) electron-impact ionization of neon: Internormalized triple-differentical cross sections in 3D kinematics

SynopsisWe present a combined experimental and theoretical study on the low-energy (E0 = 65 eV) electron- impact ionization of neon. The experimental data are compared to predictions from a hybrid second-order distorted-wave Born plus R-matrix approach (DWB2-RM), the distorted-wave Born approximation with inclusion of post-collision interaction (DWBA-PCI), a three-body distorted-wave approach (3DW), and a B-spline R-matrix (BSR) with pseudostates approach. Excellent agreement is found between experiment and the 3DW and BSR theories. The importance of PCI effects is clearly visible in this low-energy electron-impact ionization process.

Influence of excitations to high-nlshells for the ionization process in theW26+ion

It is shown that excitations from the ground configuration to high-$nl$ shells of the ${\text{W}}^{26+}$ ion have a large contribution to the electron-impact ionization process. Large-scale calculations for the excitations to over 900 configurations are performed using the Dirac-Fock-Slater approximation. The investigation includes subconfiguration-average and detailed level-to-level approaches. Our findings illustrate how important it is to study convergence of cross sections for the excitations to the high-$nl$ shells. The obtained results show that the excitations from the $4s$, $4p$, and $4d$ shells to the shells with the orbital quantum number $l=4$ dominate in the indirect process of the ionization. The excitation-autoionization cross sections make about 40% of the total ionization cross sections. Comparison between our data and previous configuration-average distorted-wave calculations for Maxwellian rate coefficients reveals large discrepancies for the direct and indirect parts of the ionization process.

Self‐consistent multifluid MHD simulations of Europa's exospheric interaction with Jupiter's magnetosphere

AbstractThe Jovian moon, Europa, hosts a thin neutral gas atmosphere, which is tightly coupled to Jupiter's magnetosphere. Magnetospheric ions impacting the surface sputter off neutral atoms, which, upon ionization, carry currents that modify the magnetic field around the moon. The magnetic field in the plasma is also affected by Europa's induced magnetic field. In this paper we investigate the environment of Europa using our multifluid MHD model and focus on the effects introduced by both the magnetospheric and the pickup ion populations. The model self‐consistently derives the electron temperature that governs the electron impact ionization process, which is the major source of ionization in this environment. The resulting magnetic field is compared to measurements performed by the Galileo magnetometer, the bulk properties of the modeled thermal plasma population is compared to the Galileo Plasma Subsystem observations, and the modeled surface precipitation fluxes are compared to Galileo Ultraviolet Spectrometer observations. The model shows good agreement with the measured magnetic field and reproduces the basic features of the plasma interaction observed at the moon for both the E4 and the E26 flybys of the Galileo spacecraft. The simulation also produces perturbations asymmetric about the flow direction that account for observed asymmetries.

Influence of renormalization shielding on the electron-impact ionization process in dense partially ionized plasmas

The renormalization shielding effects on the electron-impact ionization of hydrogen atom are investigated in dense partially ionized plasmas. The effective projectile-target interaction Hamiltonian and the semiclassical trajectory method are employed to obtain the transition amplitude as well as the ionization probability as functions of the impact parameter, the collision energy, and the renormalization parameter. It is found that the renormalization shielding effect suppresses the transition amplitude for the electron-impact ionization process in dense partially ionized plasmas. It is also found that the renormalization effect suppresses the differential ionization cross section in the peak impact parameter region. In addition, it is found that the influence of renormalization shielding on the ionization cross section decreases with an increase of the relative collision energy. The variations of the renormalization shielding effects on the electron-impact ionization cross section are also discussed.

Effects of small thundercloud electrostatic fields on the ionospheric density profile

AbstractSmall electrostatic fields of values not strong enough to activate electron impact ionization, attachment, and detachment processes can be established in the upper atmosphere by underlying thunderstorms. This paper investigates their effects on the ionospheric densities by using a simplified ion chemistry model. The modeling results indicate that under the steady state condition, the nighttime electron density profile can be reduced by up to ∼40% or enhanced by a factor of up to ∼6 because of the variation of the three‐body electron attachment rate constant with the electric field.

Prediction of Nonequilibrium Air Plasma Radiation Behind a Shock Wave

The absolute radiation measurements obtained in the electric arc-driven shock-tube facility at NASA Ames Research Center were analyzed to test the collisional-radiative model developed at École Centrale Paris. Two conditions representative of Earth reentry at 10.54 and were investigated in the vacuum ultraviolet and infrared spectral ranges. For each of the conditions, the corresponding charge-coupled device images were analyzed. The electron number density was inferred from Stark-broadened nitrogen and lines. Comparisons with the predicted electron number density profiles enabled us to validate the ionization rate constant model implemented in the flowfield solver and to accurately locate the shock front. For both freestream conditions and all the spectral ranges, the predictions of the initial intensity rises were improved when the total spatial smearing (due to the shock motion, the optics, and the camera) was taken into account. The nonequilibrium intensities observed in the vacuum ultraviolet and infrared spectral ranges were underpredicted by the collisional-radiative model when only electron-impact excitation and ionization processes were taken into account. Then, the effect of heavy-particle impact processes was studied by applying various multitemperature dissociation rate constant and vibration–dissociation coupling models as well as heavy-particle impact excitation models. The nonequilibrium peak intensities observed in the vacuum ultraviolet and infrared spectral ranges were shown to be controlled by heavy-particle impact excitation processes.

Nanosecond plasma enhanced H2/O2/N2 premixed flat flames

The effect of nanosecond pulsed plasma discharges on a laminar, lean (ϕ=0.5) premixed H2/O2/N2 flame is studied at low pressure (25Torr) using a novel plasma–flame facility, non-intrusive laser diagnostics, and high-fidelity numerical simulations. Spatially-resolved, quantitative OH mole fraction and temperature measurements are performed with and without a burst of 200 discharge pulses using laser-induced fluorescence. Measured temperatures increase by ∼20% in both the pre-heat and post-flame zones with the use of the plasma discharge. In addition, OH mole fractions increase by as much as 500% in the preheat zone and by 40% in the post-flame gases. Simulations are conducted with a one-dimensional, multi-scale, pulsed-discharge model with detailed plasma-combustion kinetics to develop additional insight into the complex plasma and flame interactions. Good agreement between measured and predicted OH and temperature profiles provides confidence in the model framework. The reduced electric field, E/N, varies inversely with number density during each pulse. A significant fraction of the input energy is consumed during electron impact ionization processes in the high E/N (700–1000Td) regions downstream of the flame. Lower E/N values (100–700Td) in the lower-temperature preheat zone promote efficient generation of radicals and excited species via electron impact processes, as well as by collisional quenching of excited states. The plasma action results in a significant increase in O and H atom density, with the peak values increasing by a factor of 6 and 4 respectively. In presence of the discharge, species and temperature profiles shift upstream (closer to the burner) by approximately 0.2cm. Simulations show that electron impact dissociation and excitation processes in the plasma have a major impact on the observed temperature and species profile displacement, such that Joule heating alone does not account for the observed effects of the plasma.

Electron-impact direct double ionization as a sequence of processes

It is shown that the electron-impact direct double ionization (DDI) process can be presented as a sequence of two- and three-step processes. Contribution from ionization-ionization, ionization-excitation-ionization, and excitation-ionization-ionization processes is taken into account. A good agreement with experimental measurements is obtained for double ionization cross sections of ${\text{O}}^{1+}$, ${\text{O}}^{2+}$, ${\text{O}}^{3+}$, ${\text{C}}^{1+}$, and ${\text{Ar}}^{2+}$ ions. We show that distribution of the energy of scattered and ejected electrons, which participate in the next step of ionization, strongly affects DDI cross sections.

Electron-impact excitation and single- and multiple-ionization cross sections of heavy ions: Sn13+as an example

The cross sections of electron impact with Sn${}^{13+}$ resulting in the production of Sn${}^{13+}$-Sn${}^{16+}$ are investigated theoretically by using the fine-structure-level distorted-wave approximation from the threshold to 4000 eV. The electron-impact excitation, ionization, and resonant excitation processes are included in the cross sections. Contributions from different processes are determined by independent processes and isolated resonance approximations. The decay pathways and branching ratios for single and sequentially multiple autoionization for the final autoionized states are determined by detailed calculations. The cross section forming Sn${}^{13+}$ is the largest and next are those for Sn${}^{14+}$ and Sn${}^{15+}$. The double-ionization cross section forming Sn${}^{15+}$ is smaller than that forming Sn${}^{14+}$, yet the relative magnitude is larger at higher incident electron energy. A comparison with a recent measurement for the single-ionization cross section [A. Borovik, Jr. et al., J. Phys. B 46, 175201 (2013)] showed that accurate determination of the branching ratios for single and double autoionization and including contributions of the resonant excitation double autoionization are crucial to obtain accurate results.

Direct and indirect processes in electron-impact ionization of 1s2 1S0 ground-state and 1s2s 3S1 metastable He-like N5+ ions

Absolute cross sections for electron-impact ionization of two-electron N5+ ions in the 1s2 1S0 ground state and in the 1s2s 3S1 metastable state have been measured. Spectroscopic-quality electron-energy scans were obtained revealing the details of indirect ionization mechanisms which contribute to net single ionization of the metastable, K-shell excited parent N5+(1s2s) ion.