Metastable Ions Research Articles

State‐selective single‐electron capture in 30‐keV N3+–He collisions

The cold‐target recoil‐ion momentum spectroscopy has been employed to study the single‐electron‐capture processes in collisions of N3+ ions with He atoms at an impact energy of 30 keV. From the longitudinal momentum spectrum of recoil ions, the different electronic configuration was identified, and the metastable projectile ions were distinguished. For the ground‐state N3+(2s2 1S) projectile, the single‐electron capture into the N2+(2s22p 2P) state is the dominant reaction channel, whereas for the metastable‐state N3+(2s2p 3P) projectile, the single‐electron capture into the N2+(2s2p2 2D) state is the main reaction channel. The fraction of the metastable‐state N3+(2s2p 3P) of the incident projectile beam is about 46%. The relative differential cross sections for the capture to different orbitals of N3+ ions are obtained and are compared with other experiments at low energy and the predictions of the classical over‐the‐barrier model.

Measurement of wave-particle interaction and metastable lifetime using laser-induced fluorescence

Extensive information, such as ion temperature and wave-particle interactions, can be obtained by the direct measurement of ion distribution functions using laser-induced fluorescence (LIF). This nonintrusive plasma diagnostics provides an important window into the ion motions in phase-space. Previous simulation results suggest that LIF measurements, which are performed on metastable ions produced directly from neutral gas particles and also from ions in other electronic states, place restrictions on the metastable lifetime. In the case where metastable population is produced from direct ionization of neutral atoms, the velocity distribution measured using LIF will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime. In this paper, the metastable lifetime effects are explored experimentally for the first time using wave-particle interaction in an Argon multidipole plasma. We demonstrate that this systematic effect can be corrected using the theory addressed in this paper based on the metastable lifetime and relative fraction of metastables produced from pre-existing ions.

Saturated fluorescence spectroscopy measurement apparatus based on metastable Li+ beam with low energy.

The apparatus for fluorescence spectroscopy measurement is developed to determine the fine structure (FS)/hyperfine structure (HFS) splittings of 1s2p 3PJ(J=0,1,2) states of Li+. The instrument is composed of a low energy Li+ ion source and a saturated fluorescence spectroscopic probing system. A low energy Li+ ion source, containing 1.8(7)% of 1s2s 3S1 metastable ions with an energy of ∼500 eV, is obtained by an electron bombardment process. The ion current can stay more than 250 h with the variation of ∼0.3%, and the divergence of ion beam is ∼0.5 mrad. The symmetric profile of Lamb dip signals of 1s2s 3S1--1s2p 3PJ transitions with linewidths of ∼50 MHz are obtained after subtracting Doppler background from the saturated fluorescence signals. A back-and-forth scan method is adopted to determine the FS/HFS splittings of 1s2p 3PJ states. Under these conditions, as a preliminary test, several splittings of 7Li+ are measured. The statistical uncertainties of the FS/HFS splittings are estimated to be less than 50 kHz, and the results are one order of magnitude better than previous results. The apparatus is feasible to precisely determine the splittings of energy levels of alkali and alkaline earth ions.

Determining Metastable Ion Lifetime and History through Wave-Particle Interaction.

Laser-induced fluorescence (LIF) performed on metastable ions is frequently used to probe the dynamics of ground-state ion motions in many laboratory plasmas. However, these measurements place restrictions on the metastable ion lifetime. Metastable states are produced from direct ionization of neutral atoms as well as ions in other electronic states, of which the former will only faithfully represent processes that act on the ion dynamics in a time shorter than the metastable lifetime. We present here the first experimental study of this type of systematic effect using wave-particle interaction in an argon multidipole plasma. The metastable lifetime and relative fraction of metastables produced from preexisting ions, necessary for correcting the LIF measurement errors, can be determined by fitting the experimental results with the theory we propose.

Negative Ion Formation in Low-Energy Electron Collisions with the Actinide Atoms Th, Pa, U, Np and Pu

Here we investigate ground and metastable negative ion formation in low-energy electron collisions with the actinide atoms Th, Pa, U, Np and Pu through the elastic total cross sections (TCSs) calculations. For these atoms, the presence of two or more open d- and f- subshell electrons presents a formidable computational task for conventional theoretical methods, making it difficult to interpret the calculated results. Our robust Regge pole methodology which embeds the crucial electron correlations and the vital core-polarization interaction is used for the calculations. These are the major physical effects mostly responsible for stable negative ion formation in low-energy electron scattering from complex heavy systems. We find that the TCSs are characterized generally by Ramsauer-Townsend minima, shape resonances and dramatically sharp resonances manifesting ground and metastable negative ion formation during the collisions. The extracted from the ground states TCSs anionic binding energies (BEs) are found to be 3.09eV, 2.98eV, 3.03eV, 3.06eV and 3.25eV for Th, Pa, U, Np and Pu, respectively. Interestingly, an additional polarization-induced metastable TCS with anionic BE value of 1.22eV is generated in Pu due to the size effect. We also found that our excited states anionic BEs for several of these atoms compare well with the existing theoretical electron affinities including those calculated using the relativistic configuration-interaction method. We conclude that the existing theoretical calculations tend to identify incorrectly the BEs of the resultant excited anionic states with the electron affinities of the investigated actinide atoms; this demonstrates the great need for experimental verification and unambiguous determination of their electron affinities.

Resonance Electron Attachment to Glucose and Fructose Molecules

Low-energy negative ion mass spectra of glucose and fructose suggest the deep fragmentation of short-lived molecular ions. An analysis of metastable ions in the mass spectra shows that some decomposition reactions proceed through intermediate [M–H2O]–, etc. ions. Along with the general similarity of the ionization processes in the isomers, there is a significant difference, which is in the intensive formation of m/z 71 ions from glucose.

Surface characteristics of a dental implant modified by low energy oxygen ion implantation

The good biocompatibility of Titanium-based medical device is related to the thin oxide layer (titanium dioxide, TiO2) formed on the surface. In this study, Plasma Immersion Ion Implantation (PIII) method and Optical Emission Spectrometer (OES) were used to produce oxidized surface in order to study the influence of plasma composition on surface composition. The result of OES showed that higher electron temperature, more metastable argon atom and oxygen ion content facilitate the formation of TiO2 top surface. The treated implant surfaces possessed higher content Ti2O3, lower contact angle. When the atomic percentage of Ti2O3 is similar to the TiO2, the contact angle is close to zero, which could attribute to the photocatalytic activity of self-doped TiO2−x. It generates more hydroxyl groups under visible light, causing a decrease in contact angle. But release of titanium ions was found in some solutions. It is believed that the TiO2−x produced by low energy oxygen ion implant has super hydrophilic properties, which indicates the potential for improved implant osseointegration in vivo.

Electron–ion Recombination Rate Coefficients of Be-like 40Ca16+

Abstract Electron–ion recombination rate coefficients for beryllium-like calcium ions in the center of mass energy from 0 to 51.88 eV have been measured by means of the electron–ion merged-beam technique at the main cooler storage ring at the Institute of Modern Physics in Lanzhou, China. The measurement energy range covers the dielectronic recombination (DR) resonances associated with the core excitations and the trielectronic recombination (TR) resonances associated with the core excitations. In addition, the AUTOSTRUCTURE code was used to calculate the recombination rate coefficients for comparison with the experimental results. Resonant recombination originating from parent ions in the long-lived metastable state ions has been identified in the recombination spectrum below 1.25 eV. A good agreement is achieved between the experimental recombination spectrum and the result of the AUTOSTRUCTURE calculations when fractions of 95% ground-state ions and 5% metastable ions are assumed in the calculation. It is found that the calculated TR resonance positions agree with the experimental peaks, while the resonance strengths are underestimated by the theoretical calculation. Temperature dependent plasma rate coefficients for DR and TR in the temperature range of 103–108 K were derived from the measured electron–ion recombination rate coefficients and compared with the available theoretical results from the literature. In the temperature range of photoionized plasmas, the presently calculated rate coefficients and the recent results of Gu & Colgan et al. are up to 30% lower than the experimentally derived ones, and the older atomic data are even up to 50% lower than the present experimental result. This is because strong resonances situated below electron–ion collision energies of 50 meV were underestimated by the theoretical calculation, which also has a severe influence on the rate coefficients in low-temperature plasmas. In the temperature range of collisionally ionized plasmas, agreement within 25% was found between the experimental result and the present calculation as well as the calculation by Colgan et al. The present result constitutes a set of benchmark data for use in astrophysical modeling.

The role of molecular ions in cryogenic dusty plasmas of dc discharge in neon

The diffusion-drift model of positive column of glow discharge in neon have been used to study the effect of molecular ions in the interaction of plasma with dust particles of micron size. Simulations have been carried out at a pressure of 20 – 100 Pa and gas temperature of 77 K. The electric field of the discharge, the concentrations of discharge components, including metastable atoms and molecular ions, and the charge of dust particles have been simulated.

OH+ Formation in the Low-temperature O+(4S) + H2 Reaction

Abstract Formation of OH+ in collisions of ground-state O+(4S) ions with normal H2 has been studied using a variable temperature 22-pole RF ion trap. From 300 to 30 K the measured reaction rate coefficient is temperature-independent, with a small decrease toward 15 K. The recent wave packet calculation predicts a slightly steeper temperature dependence. The rate coefficients at 300 and 15 K are almost the same, (1.4 ± 0.3) × 10−9 cm3 s−1 and (1.3 ± 0.3) × 10−9 cm3 s−1, respectively. The influence of traces of the two metastable ions, O+(2D) and O+(2P), has been examined by monitoring the H+ products of their reactions with H2, as well as by chemically probing them with N2 reactant gas.

Solvation suppression of ion recombination in gas discharge afterglow.

An effect which suppresses recombination in ion plasmas is considered both theoretically and experimentally. Experimental results are presented for the ion recombination rate in fluorine plasma, which are obtained from data for the gas discharge afterglow. To interpret them, a suppression factor is considered: ion solvation in weakly ionized plasma. It is shown that the recombination process has a two-stage character with the formation of intermediate metastable ion pairs. The pairs consist of negative and positive ion-molecular clusters. A theoretical explanation is given for the slowing down of the ion recombination with the increase of the Coulomb coupling compared to the ion recombination rate calculated in the ideal plasma approximation. The approximate similarity of the recombination rate of the ion temperature and concentration and reasons for the slight deviation from the similarity are elucidated.

On limitations of laser-induced fluorescence diagnostics for xenon ion velocity distribution function measurements in Hall thrusters

Hall thruster operation is characterized by strong breathing oscillations of the discharge current, the plasma density, the temperature, and the electric field. Probe- and laser-induced fluorescence (LIF) diagnostics were used to measure temporal variations of plasma parameters and the xenon ion velocity distribution function (IVDF) in the near-field plasma plume in regimes with moderate (<18%) external modulations of applied DC discharge voltage at the frequency of the breathing mode. It was shown that the LIF signal collapses while the ion density at the same location is finite. The proposed explanation for this surprising result is based on a strong dependence of the excitation cross-section of metastables on the electron temperature. For large amplitudes of oscillations, the electron temperature at the minimum enters the region of very low cross-section (for the excitation of the xenon ions); thus, significantly reducing the production of metastable ions. Because the residence time of ions in the channel is generally shorter than the time scale of breathing oscillations, the density of the excited ions outside the thruster is low and they cannot be detected. In the range of temperature of oscillations, the ionization cross-section of xenon atoms remains sufficiently large to sustain the discharge. This finding suggests that the commonly used LIF diagnostic of xenon IVDF can be subject to large uncertainties in the regimes with significant oscillations of the electron temperature, or other plasma parameters.

A Lagrangian model for laser-induced fluorescence and its application to measurements of plasma ion temperature and electrostatic waves

Extensive information can be obtained on wave-particle interactions and wave fields by a direct measurement of perturbed ion distribution functions using laser-induced fluorescence (LIF). For practical purposes, LIF is frequently performed on metastable states that are produced from neutral gas particles and ions in other electronic states. If the laser intensity is increased to obtain a better LIF signal, then optical pumping can produce systematic effects depending on the collision rates which control metastable population and lifetime. We numerically simulate the ion velocity distribution measurement and wave-detection process using a Lagrangian model for the LIF signal for the case where metastables are produced directly from neutrals. This case requires more strict precautions and is important for discharges with energetic primary electrons and a high density of neutrals. Some of the results also apply to metastables produced from pre-existing ions. The simulations show that optical pumping broadening affects the ion velocity distribution function f0(v) and its first-order perturbation f1(v,t) when the laser intensity is increased above a certain level. The results also suggest that ion temperature measurements are only accurate when the metastable ions can live longer than the ion-ion collision mean free time. For the purposes of wave detection, the wave period has to be significantly shorter than the lifetime of metastable ions for a direct interpretation. It is more generally true that metastable ions may be viewed as test-particles. As long as an appropriate model is available, LIF can be extended to a range of environments.

Role of metastable-adsorbed charges in the stability degradation of carbon-based electrodes for capacitive deionization

The dynamic movement of metastable ions at the surface of aged anodes is illustrated to elucidate the inversion effect.

Fragmentation studies on metastable diethylaniline derivatives using mass-analyzed ion kinetic energy spectrometry.

It is well known that small substituted aromatic amines lose their substituents after electron ionization in a multistep fragmentation mechanism. In this contribution, the fragmentation reactions of N,N-diethyl-aniline and different methyl-substituted N,N-diethyl-methyl-aniline isomers are investigated with respect to the position of the methyl group attached to the aromatic ring system. Metastable ion decay reveals a complicated fragmentation mechanism leading to an interplay of the aromatic methyl group and the ethyl substituents at the amine function. A small change in the substitution leads to a significant change of the observed fragments indicating a participation of o-methyl groups in the fragmentation mechanism of the diethyl amino side chain. Because the underlying mechanisms are not fully understood, the presented investigations deliver through 13C isotopic labeling a deep insight into the hidden rearrangement and fragmentation mechanisms in these compounds.

On determining the fraction of metastable ions produced by direct ionization

Laser-induced fluorescence (LIF) on metastable ions is a powerful tool in studying wave-particle interactions, velocity-space diffusion and other phenomena in plasmas under the proper conditions. However, measurements which probe particle interactions place restrictions on the metastable lifetime. Metastable states are produced both from direct ionization of neutral gas particles and ions in other electronic states, but the former metastable population will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime. A numerical simulation is performed to study the metastable lifetime effects using a Lagrangian approach for LIF. A theoretical model in determining the fraction of metastable ions produced from direct ionization is reported to provide corrections to the LIF measurements.

Mineralogy and Geochemistry of Biologically-Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand

Detrital gold in Late Pleistocene-Holocene placers has been chemically mobilised and redeposited at the micron scale by biologically-mediated reactions in groundwater. These processes have been occurring in a tectonically active semiarid rain shadow zone of southern New Zealand and are probably typical for this type of environment elsewhere in the world. The chemical system is dominated by sulfur, which has been derived from basement pyrite and marine aerosols in rain. Detrital and authigenic pyrite is common below the water table, and evaporative sulfate minerals are common above the fluctuating water table. Pyrite oxidation was common but any acid generated was neutralised on the large scale (tens of metres) by calcite, and pH remained circumneutral except on the small scale (centimetres) around pyritic material. Metastable thiosulfate ions were a temporary product of pyrite oxidation, enhanced by bacterial mediation, and similar bacterial mediation enhanced sulfate reduction to form authigenic pyrite below the water table. Deposition of mobilised gold resulted from localised variations in redox and/or pH, and this formed overgrowths on detrital gold of microparticulate and nanoparticulate gold that is locally crystalline. The redeposited gold is an incidental byproduct of the bacterially-enhanced sulfur reactions that have occurred near to the fluctuating sulfide-sulfate redox boundary.

Novel mechanism for creating long-lived metastable atomic negative ions

SynopsisA novel mechanism is proposed for creating long-lived metastable atomic negative ions in complex atoms. It exploits the orbital collapse of the 5d orbital in Gd (Z=64) into the 4f orbital of Tb (Z=65). In the region of collapse the properties of the 5d and 4f orbitals are quite sensitive to the changes in the effective potential. Consequently the collapse phenomenon impacts significantly the core-polarization interaction in the relevant atoms. The mechanism is demonstrated in Tb and Dy through the appearance of long-lived Tb− and Dy− anions in the Regge-pole calculated electron elastic cross sections.

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Electron ionisation mass spectra of N-methylimidazolidin-4-one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five-membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.

On singlet metastable states, ion flux and ion energy in single and dual frequency capacitively coupled oxygen discharges

We apply particle-in-cell simulations with Monte Carlo collisions to study the influence of the singlet metastable states on the ion energy distribution in single and dual frequency capacitively coupled oxygen discharges. For this purpose, the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used, in which the discharge model includes the following nine species: electrons, the neutrals O(3P) and O), the negative ions O−, the positive ions O+ and O, and the metastables O(1D), O and O2(b). Earlier, we have explored the effects of adding the species O) and O2(b), and an energy-dependent secondary electron emission yield for oxygen ions and neutrals, to the discharge model. We found that including the two molecular singlet metastable states decreases the ohmic heating and the effective electron temperature in the bulk region (the electronegative core). Here we explore how these metastable states influence dual frequency discharges consisting of a fundamental frequency and the lowest even harmonics. Including or excluding the detachment reactions of the metastables O) and O2(b) can shift the peak electron temperature from the grounded to the powered electrode or vice versa, depending on the phase difference of the two applied frequencies. These metastable states can furthermore significantly influence the peak of the ion energy distribution for O-ions bombarding the powered electrode, and hence the average ion energy upon bombardment of the electrode, and lower the ion flux.