Ionization Mechanism Research Articles

Dynamics of laser-induced formation of color centers in a lithium fluoride crystal

The dynamics of the laser-induced formation of aggregate color centers under the action of the first (950 nm) and second (475 nm) harmonics of a femtosecond Ti:sapphire laser has been studied. During the irradiation of the samples with laser radiation, the photoluminescence spectra of the induced centers were repeatedly measured. It was found that intensity of the spectral luminescence band of F3+ centers exceeds intensity of the band of F2 centers in samples with color centers induced by the first harmonic. At the same time, the opposite ratio of the intensities of these bands was observed in the luminescence spectra of color centers produced by second harmonic radiation for the same energy of laser pulses. It has been established that the observed effect is caused by different degree of heating of the crystalline medium in the regions of passage of laser filaments. Heating accelerates the migration of kinetic particles that carry out the aggregation process. The ratio of the concentrations of F2 and F3+ centers depends on the ratio of the duration of irradiation and the lifetime of kinetic particles, which significantly depends on temperature. A multiphoton-tunneling mode of interband ionization of a crystalline substance was implemented in the experiment. It is shown that the identity of the absorption spectra of color centers induced by the first and second harmonics of femtosecond laser radiation is a new distinguishing characteristic of experimental implementation of the tunneling ionization mechanism in experiment.

Coulomb corrected nonadiabatic instantaneous ionization rate and the electron trajectory in an elliptically polarized laser field

This paper presents an analytical model that investigates the impact of Coulomb effect on tunneling ionization events in an elliptically polarized laser field. The model focuses on ionization rate and momenta, at a specific moment, based on recent experimental findings and strong-field approximation. Results show that incorporating Coulomb correction improves photoionization rate across various intensities and provides valuable insights into ionization mechanism, particularly for argon atoms at 410 and 800 nm wavelengths. This has significant implications for attosecond physics and ultrafast laser technology.

On the Practical Limitations for the Generation of Gunn Oscillations in Highly Doped GaN Diodes

Planar Gunn diodes based on doped GaN active layers with different geometries have been fabricated and characterized. Gunn oscillations have not been observed due to the catastrophic breakdown of the diodes for applied voltages around 20–25 V, much below the bias theoretically needed for the onset of Gunn oscillations. The breakdown of the diodes has been analyzed by pulsed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{I}$</tex-math> </inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{V}$</tex-math> </inline-formula> measurements at low temperatures, and it has been observed to be almost independent of the geometry of the channels, thus allowing to discard self-heating effects as the origin of the device burning. The other possible mechanism for the device failure is an impact-ionization avalanche due to the high electric fields present at the anode corner of the isolating trenches. However, Monte Carlo simulations using the typical value of the intervalley energy separation of GaN, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon_{\text{1}-\text{2}}$</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$=$</tex-math> </inline-formula> 2.2 eV, show that impact ionization mechanisms are not significant for the voltages for which the experimental failure is observed. But recent experiments showed that <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon_{\text{1}-\text{2}}$</tex-math> </inline-formula> is lower, around 0.9 eV. This lower intervalley separation leads to a much lower threshold voltage for the Gunn oscillations, not far from the experimental breakdown. Therefore, we attribute the device’s failure to an avalanche process just when Gunn domains start to form, since they increase the population of electrons at the high electric field region, thus strongly enhancing impact ionization mechanisms that lead to the diode failure.

Remarkably Enhanced Charge Density of Inorganic Material Via Regulating Contact Barrier Difference and Charge Trapping for Triboelectric Nanogenerator

AbstractCurrently, the research on improving charge density of charge‐excitation triboelectric nanogenerators (CE‐TENG) mainly focuses on surface modification and thinning of organic polymer materials. However, to increase durability and output, the exploration of high permittivity and high‐thickness inorganic materials is important for extensive applications. Meanwhile, the physical mechanism of air ionization caused by air breakdown during secondary self‐charge excitation (SSCE) needs to be explored. Herein, this study proposes a new strategy for CE‐TENG based on inorganic materials by de‐trapping reverse charge and increasing the contact barrier difference to obtain high charge density and achieve secondary self‐charge excitation. SSCE of three types of films with different leakage currents are evaluated according to their trapping and de‐trapping reverse charges. It is found that the trap distribution of dielectric films with larger leakage currents are mostly shallow‐level traps, whose de‐trapping ability becomes stronger after reverse charge injection. By regulating the contact barrier difference with thin organic polymer on thick inorganic high permittivity material, this study has achieved a high charge density of 1310 µC m−2 with a fast start‐up time of SSCE, which is a new record for the charge density of inorganic materials. This study promotes TENG of inorganic materials toward high‐energy output applications.

Outflow densities and ionization mechanisms in the NLRs of the prototypical Seyfert galaxies NGC 1068 and NGC 4151

ABSTRACT Despite being thought to play an important role in galaxy evolution, the true impact of outflows driven by active galactic nuclei (AGNs) on their host galaxies is unclear. In part, this may be because electron densities of outflowing gas are often underestimated: recent studies that use alternative diagnostics have measured much higher densities than those from commonly used techniques and consequently find modest outflow masses and kinetic powers. Furthermore, outflow ionization mechanisms – which are often used to probe acceleration mechanisms – are also uncertain. To address these issues, we have analysed archival HST/STIS spectra of the inner regions (r &amp;lt; 160 pc) of the nearby prototypical Seyfert galaxies NGC 1068 and NGC 4151, which show evidence of warm-ionized outflows driven by the central AGN. We derive high electron densities (103.6 &amp;lt; ne &amp;lt; 104.8 cm−3) using the transauroral [O ii] and [S ii] emission line ratios for the first time with spatially resolved observations. Moreover, we find evidence that the gas along the radio axis in NGC 1068 has a significant AGN-photoionized matter-bounded component, and there is evidence for shock-ionization and/or radiation-bounded AGN-photoionization along the radio axis in NGC 4151. We also note that the outflow extents are similar to those of the radio structures, consistent with acceleration by jet-induced shocks. Taken together, our investigation demonstrates the diversity of physical and ionization conditions in the narrow-line regions of Seyfert galaxies, and hence reinforces the need for robust diagnostics of outflowing gas densities and ionization mechanisms.

Role of excitation in the electron rotational asymmetry in circularly polarized laser fields

We comparatively study electron rotational asymmetries in photoionization of excited hydrogen atoms and photodetachment of the fluorine anions in single circularly polarized laser fields by numerically solving the three-dimensional time-dependent Schrödinger equation. We first show that single-photon ionization (or detachment) is more favorable as the electron initially corotates with respect to the direction of the laser field. More interestingly, we systematically demonstrate that the electron rotational asymmetry of the excited hydrogen atom is completely contrary to that of the fluorine anion in the multiphoton ionization (or detachment) regime, which can be attributed to the role played by excited bound states by analyzing the total excitation probability in the excited hydrogen atom. Furthermore, we also confirm that the electron rotational asymmetry of and degenerate orbitals in the excited hydrogen atom changes when the ionization mechanism switches from three-photon to over-the-barrier.

Improved breakdown voltage mechanism in AlGaN/GaN HEMT for RF/Microwave applications: Design and physical insights of dual field plate

This work investigated and proposed the source–gate dual field plate (SG-FP) AlGaN/GaN HEMT for different gate-to-drain drift distances with a fixed gate length of 0.25 μm using ATLAS-TCAD. The increment in the GaN buffer thickness and gate-to-drain drift distance with the variation in the field plate length improves the device performance and breakdown voltages by maintaining the impact ionization mechanism. The breakdown voltage behavior, physical insights of electric field contour, impact generation rate, and potential profile intricacies of source field plate (S-FP), gate field plate (G-FP), and dual field plate (SG-FP) are illustrated. The proposed source–gate dual field plate (SG-FP) with the SiN passivation interfacial layer, SiO2, thicker GaN buffer, and extended gate-to-source drift region is best suited for optimal performance in high breakdown voltage, with optimum cut-off frequency and low gate-leakage current. The proposed dual field plate AlGaN/GaN device with the source field plate (LS-FP = 4 μm), gate field plate (LG-FP = 2 μm), the AlGaN buffer thickness of 3.697 μm, and gate-to-drain drift distance of 8 μm show the highest breakdown voltage of 562 V with the optimum frequency of 8.861 GHz. Finally, the AC/DC characteristics of the optimized dual field plate (SG-FP) are demonstrated. We observed that the source–gate dual field plate (SG-FP) device shows a 6.70% and 4.09% improvement in breakdown voltage compared to individual source field plate (S-FP) and gate field plate (G-FP) designs, respectively. This work shows that the proposed dual field plate device is used in power switching, high-power, and RF/Microwave applications due to its superior breakdown voltage performance, reduction in parasitic capacitances, and optimal cut-off frequency.

The spatial distribution of soluble organic matter and their relationship to minerals in the asteroid (162173) Ryugu

We performed in-situ analysis on a ~ 1 mm-sized grain A0080 returned by the Hayabusa2 spacecraft from near-Earth asteroid (162173) Ryugu to investigate the relationship of soluble organic matter (SOM) to minerals. Desorption electrospray ionization-high resolution mass spectrometry (DESI-HRMS) imaging mapped more than 200 CHN, CHO, CHO–Na (sodium adducted), and CHNO soluble organic compounds. A heterogeneous spatial distribution was observed for different compound classes of SOM as well as among alkylated homologues on the sample surface. The A0080 sample showed mineralogy more like an Ivuna-type (CI) carbonaceous chondrite than other meteorites. It contained two different lithologies, which are either rich (lithology 1) or poor (lithology 2) in magnetite, pyrrhotite, and dolomite. CHN compounds were more concentrated in lithology 1 than in lithology 2; on the other hand, CHO, CHO–Na, and CHNO compounds were distributed in both lithologies. Such different spatial distribution of SOM is likely the result of interaction of the SOM with minerals, during precipitation of the SOM via fluid activity, or could be due to difference in transportation efficiencies of SOMs in aqueous fluid. Organic-related ions measured by time-of-flight secondary ion mass spectrometry (ToF–SIMS) did not coincide with the spatial distribution revealed by DESI-HRMS imaging. This result may be because the different ionization mechanism between DESI and SIMS, or indicate that the ToF–SIMS data would be mainly derived from methanol-insoluble organic matter in A0080. In the Orgueil meteorite, such relationship between altered minerals and SOM distributions was not observed by DESI-HRMS analysis and field-emission scanning electron microscopy, which would result from differences of SOM formation processes and sequent alteration process on the parent bodies or even on the Earth. Alkylated homologues of CHN compounds were identified in A0080 by DESI-HRMS imaging as observed in the Murchison meteorite, but not from the Orgueil meteorite. These compounds with a large C number were enriched in Murchison fragments with abundant carbonate grains. In contrast, such relationship was not observed in A0080, implying different formation or growth mechanisms for the alkylated CHN compounds by interaction with fluid and minerals on the Murchison parent body and asteroid Ryugu.Graphical

Electrospray ionization developed by Lidija Gall's group

This historical review is dedicated to Prof. Lidija Gall. She was awarded the Thomson medal (2022) for her contribution to the development of electrospray ionization mass spectrometry in the early 1980s in Leningrad, Soviet Union. We describe the rationale and provide historical references that highlight her way to the discovery of a method of solvated ion extraction at atmospheric pressure (SIEAP), known by the Russian abbreviation ЭРИАД (ERIAD), which is the direct analogue of electrospray ionization developed in the West. Even at the early prototype stage, ERIAD was able to determine the intact masses of a broad scope of polar biomolecules including peptides, small proteins, nucleotides, carbohydrates, and prostanglandins. The ERIAD source was directly coupled to microbore liquid chromatography and was applied to analyze protein digests, to characterize the substrate specificity of proteolytic enzymes, to mapping of mutation sites in physiologically important proteins and also to increase the confidence in protein sequence determinations. In inorganic chemistry, ERIAD was successfully applied to analyze the solutions of rare-earth metals and metal complexes. Altogether, a set of experimental studies and a broad scope of applications contributed to better understanding of the ionization mechanism and the methods needed to acquire and interpret ERIAD spectra. The method was first implemented on a magnetic sector mass spectrometer and then was adopted to a variety of mass analyzers and mobility separators in collaboration with Russian mass spectrometry groups.

Validation of Field-Dependent Ion-Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions.

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion-solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion-solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20-120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H+(H2O)n, [MeOH + H + n(H2O)]+, [ACE + H + n(H2O)]+, and [PhNH2 + H + n(H2O)]+ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

Effect of chemical bonding on ionization of nanoscale carbon and DNA upon proton irradiation

The separation of positive and negative charges under ion irradiation leads to ionization of the material. This ionization mechanism is closely related to DNA or tissue damage during ion beam therapy, enhancement of plasma oscillations driven by high-energy particles and the ion beam material modification. Here, we explore the effect of chemical bonding on the initial physical stage of ionization of target materials by building material models with different bonding states and using real-time time-depended density functional theory. By building the double layer carbon nanotubes, self-assembly bilayer graphene and DNA double helix, and tracking the evolution of nonadiabatic energy transfer, electronic stopping power, atomic force and charge populations on an attosecond scale, we find that the occupied covalent bond can rapidly conduct the excited electrons and the π-π interaction between molecular layers effectively blocks the backflow of excited electron, resulting in the ionization. In particular, the H-bond transfers a small number of excited electrons and suppresses the charge oscillations during ionization, ultimately leading to the ebb-and-flow ionization mechanism in DNA molecule. These results provide an understanding of the ionization mechanism of materials under ion beam irradiation for ionization control in radiotherapy, ion beam material modification and plasmon-enhanced applications.

Supercritical Fluid Nanospray Mass Spectrometry: II. Effects on Ionization.

Nanospraying supercritical fluids coupled to a mass spectrometer (nSF-MS) using a 90% supercritical fluid CO2 carrier (sCO2) has shown an enhanced desolvation compared to traditional liquid eluents. Capillaries of 25, 50, and 75 μm internal diameter (i.d.) with pulled emitter tips provided high MS detection sensitivity. Presented here is an evaluation of the effect of proton affinity, hydrophobicity, and nanoemitter tip size on the nSF-MS signal. This was done using a set of primary, secondary, tertiary, and quaternary amines with butyl, hexyl, octyl, and decyl chains as analytes. Each amine class was analyzed individually to evaluate hydrophobicity and proton affinity effects on signal intensity. The system has shown a mass sensitive detection on a linear dynamic range of 0.1-100 μM. Results indicate that hydrophobicity has a larger effect on the signal response than proton affinity. Nanospraying a mixture of all amine classes using the 75 μm emitter has shown a quaternary amine signal not suppressed by competing analytes. Competing ionization was observed for primary, secondary, and tertiary amines. The 75 and 50 μm emitters demonstrated increased signal with increasing hydrophobicity. Surprisingly, the 25 μm i.d. emitter yielded a signal decrease as the alkyl chain length increased, contrary to conventional understanding. Nanospraying the evaporative fluid in a sub-500 nm emitter likely resulted in differences in the ionization mechanism. Results suggest that 90% sCO2 with 9.99% methanol and 0.01% formic acid yielded fast desolvation, high ionization efficiency, and low matrix effect, which could benefit complex biological matrix analysis.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation.

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

A MUSE view of the multiple interacting system HCG 31

ABSTRACT We present, for the first time, spatially resolved spectroscopy for the entire Hickson Compact Group 31 obtained with the MUSE instrument at the VLT and an in-depth analysis of this compact group. To obtain a complete understanding of the system, we derived radial velocity and dispersion velocity maps, maps of the ionization mechanism of the system, chemical abundances and their distribution over the whole system, star formation rates and ages of the different star-forming regions, and the spatial distribution of the Wolf–Rayet stellar population. We also reconstructed the star formation history of the galaxies HCG 31 A, C, B, and F, measured the emission-line fluxes, and performed a stellar population synthesis. Our main findings are: (i) that there is clearly disturbed kinematics due to the merger event that the system is experiencing; (ii) that the ionization is produced exclusively via star formation except for the nucleus of the galaxy HCG 31 A, where there is a small contribution of shocks; (iii) that there is low oxygen abundance distributed homogeneously through the system; (iv) that there is a prominent population of carbon Wolf–Rayet stars in the central zone of the group; and (v) that there are clear evidences of the tidal origin of the galaxies HCG 31 E, HCG 31 H, and HCG 31 F because they show quite high oxygen abundances for their stellar mass. All these findings are clear evidence that HCG 31 is currently in an early merging phase and manifesting a starburst in its central region.

Extended line emission in the BCG of Abell 2390

ABSTRACT We report CFHT/SITELLE imaging Fourier Transform Spectrograph observations of the brightest cluster galaxy (BCG) of galaxy cluster Abell 2390 at z = 0.228. The BCG displays a prominent cone of emission in H α, H β, [N ii], and [O ii] to the North-West with PA = 42°, 4.4 arcsec in length (15.9 kpc), which is associated with elongated and asymmetric Chandra soft X-ray emission. The H α flux map also contains a ‘hook’ of H α and [N ii] emission resulting in a broadened northern edge to the cone. Using SITELLE/LUCI software, we extract emission line flux, velocity, velocity dispersion, and continuum maps, and utilize them to derive flux ratio maps to determine ionization mechanisms and dynamical information in the BCG’s emission line region. The Baldwin–Phillips–Terlevich diagnostics on the BCG cone indicate a composite ionization origin of photoionization due to star formation and shock. Strong LINER-like emission is seen in the nuclear region which hosts an active galactic nucleus (AGN). As Abell 2390 is a cool-core cluster, we suggest that the cooling flow is falling onto the central BCG and interacting with the central AGN. The AGN produces jets that inflate ‘bubbles’ of plasma in the ICM, as is often observed in local galaxy clusters. Furthermore, combining signs of AGN activities from radio, optical emission line and X-ray data over a large range of physical scale, we find evidence for three possible episodes of AGN activity in different epochs associated with the Abell 2390 BCG.

Novel Liquid Chromatography–Tandem Mass Spectrometry Method for Analysis of 102 Pesticides and Five Mycotoxins Regulated by the State of Colorado in Dried Hemp

A novel liquid chromatography–tandem mass spectrometry (LC–MS/MS) method with a dual electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) source was developed for analyzing 102 pesticides and five mycotoxins that are regulated by the state of Colorado in hemp. The limit of quantitation (LOQ) of the 102 pesticides and five mycotoxins met Colorado state action limit requirements for these analytes in a hemp matrix. For this study, 88 out of 102 pesticides and all five mycotoxins were analyzed using LC–MS/MS with an ESI source, whereas the remaining 14 pesticides were determined using LC–MS/MS with an APCI source. A simple, fast, green, and cheap acetonitrile solvent extraction method was used to extract the pesticides and mycotoxins from the hemp matrix with good extraction efficiency in the range of 80–120%. A hemp matrix is challenging and causes matrix effects such as ion suppression or enhancement. We optimized the LC method and added 30 internal standards to reduce and compensate for these matrix effects to obtain method accuracy in the range of 70–120%. The ionization mechanism of nonpolar pesticides (normally analyzed by gas chromatography–tandem mass spectrometry [GC–MS/MS]) with an APCI ion source was elucidated.

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

Context. Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. Aims. We aim to acquire a set of very low-redshift SFGs that exhibit signs of a hard radiation field being present. A subsequent analysis of their emission line properties is intended to shed light on how the conditions prevalent in these objects compare to those predicted to be present in early SFGs that are thought to be LyC emitters (LCEs). Methods. We used archival spectroscopic SDSS DR12 data to select a sample of low-redshift He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an active galactic nucleus, which serves as our only selection criterion. We performed a population spectral synthesis with FADO to reconstruct these galaxies’ star-formation histories (SFHs). Utilising the spectroscopic information at hand, we constrained the predominant ionisation mechanisms in these galaxies and inferred information on interstellar medium (ISM) conditions relevant for the escape of LyC radiation. Results. Our final sample consists of eighteen ionised, metal-poor galaxies (IMPs). These low-mass (6.2 ≤ log(M⋆/M⊙) ≤ 8.8), low-metallicity (7.54 ≤ log(O/H) + 12 ≤ 8.13) dwarf galaxies appear to be predominantly ionised by stellar sources. We find large [OIII] 5007/[OII] 3727 ratios and [SII] 6717,6731/Hα deficiencies, which provide strong indications for these galaxies to be LCEs. At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions ≳10%. The IMPs’ SFHs exhibit strong similarities and almost all galaxies appear to contain an old (&gt; 1 Gyr) stellar component, while also harbouring a young, two-stage (∼10 Myr and &lt; 1 Myr) starburst, which we speculate might be related to LyC escape. Conclusions. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme ISM conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z &lt; 0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.

Cold-target electron-ion-coincidence momentum-spectroscopy study of electron-impact single and double ionization of N2 and O2 molecules

We report experimental studies of electron-impact ionization of nitrogen (${\mathrm{N}}_{2}$) and oxygen (${\mathrm{O}}_{2}$) molecules using a cold target recoil ion momentum spectrometer (COLTRIMS, reaction microscope). The recoil ion is detected in coincidence with one outgoing electron such that the momentum vectors and consequently the kinetic energies for these final-state particles are determined. The ionization cross sections for producing the doubly and singly charged parent ions were measured as a function of the incident electron energy ranging from 50 to 600 eV. For molecular dications, e.g., ${\mathrm{O}}_{2}^{2+}$, the cross sections are obtained by measuring both the ion time of flight and two-dimensional position spectra, which is demonstrated to be an efficient way to suppress the contribution of the dissociation channels with the ${\mathrm{O}}^{+}$ products as both ions have the identical mass-to-charge ratios. The projectile energy-loss spectra correlated to the final-state ions are obtained, which provide direct evidence of the ionization mechanisms of ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ molecules. It is found that the ${\mathrm{O}}_{2}^{2+}$ dication is produced mainly by Auger process after single ionization in $2{\ensuremath{\sigma}}_{g}$ inner-valence shell of ${\mathrm{O}}_{2}$ molecule, while the ${\mathrm{N}}_{2}^{2+}$ dication is generated by the direct removal of two electrons from the outermost $3{\ensuremath{\sigma}}_{g}$ orbital of ${\mathrm{N}}_{2}$ molecule.

Theoretical investigation of electron-impact double ionization of B-like C+ to Ne5+ ions

Electron-impact double ionization (DI) including direct and indirect processes of B-like ${\mathrm{C}}^{+}$ to ${\mathrm{Ne}}^{5+}$ ions is investigated using a two-step approach. The direct process is treated as a primary single ionization (SI) and subsequent knockout mechanism. The indirect process is considered as a primary SI of an inner-shell $1s$ electron followed by a single autoionization. Based on the two-step approach, the DI cross sections are calculated using flexible atomic code with distorted-wave approximation. The potential of the intermediate ion is approximately equivalent to the mean potential constructed by three ions involved for calculating the cross section of the direct process, while using the potential of the ionizing ion in each step of the present two-step approach overestimates results. The present cross sections agree with available theoretical and experimental data.

Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization.

The ionization of uranyl triperoxide monomer, [(UO2)(O2)3]4- (UT), and uranyl peroxide cage cluster, [(UO2)28(O2)42-x(OH)2x]28- (U28), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/MS), use of natural water and D2O as solvent, and use of N2 and SF6 as nebulizer gases, provide insight into the mechanisms of ionization. The U28 nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UOx- (x = 3-8) and UOxHy- (x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UOx- (x = 4-6) and UOxHy- (x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U28 systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U28, (b) reduction-oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UOx- (x = 6-8) were investigated using density functional theory (DFT).