Time-dependent Ionization Research Articles

Nonequilibrium Ionization Plasma during a Large Solar Limb Flare Observed by Hinode/EIS

Abstract This study on plasma heating considers the time-dependent ionization process during a large solar flare on 2017 September 10, observed by Hinode/EUV Imaging Spectrometer (EIS). The observed Fe xxiv/Fe xxiii ratios increase downstream of the reconnection outflow, and they are consistent with the time-dependent ionization effect at a constant electron temperature T e = 25 MK. Moreover, this study also shows that the nonthermal velocity, which can be related to the turbulent velocity, reduces significantly along the downstream of the reconnection outflow, even when considering the time-dependent ionization process.

Chromospheric activity in 55 Cancri: II. Theoretical wave studies versus observations

ABSTRACT In this study, we consider chromospheric heating models for 55 Cancri in conjunction with observations. The theoretical models, previously discussed in Paper I, are self-consistent, non-linear, and time-dependent ab-initio computations encompassing the generation, propagation, and dissipation of waves. Our focus is the consideration of both acoustic waves and longitudinal flux tube waves amounting to two-component chromosphere models. 55 Cancri, a K-type orange dwarf, is a star of low activity, as expected by its age, which also implies a relatively small magnetic filling factor. The Ca ii K fluxes are computed (multi-ray treatment) assuming partial redistribution and time-dependent ionization. The theoretical Ca ii H+K fluxes are subsequently compared with observations. It is found that for stages of lowest chromospheric activity the observed Ca ii fluxes are akin, though not identical, to those obtained by acoustic heating, but agreement can be obtained if low levels of magnetic heating – consistent with the expected photospheric magnetic filling factor – are considered as an additional component; this idea is in alignment with previous proposals conveyed in the literature.

Ultrafast Size Expansion and Turn-On Luminescence of Atomically Precise Silver Clusters by Hydrogen Sulfide.

The formation of high-nuclearity silver(I) clusters remains elusive and their potential applications are still underdeveloped. Herein, we firstly prepared a chain-like thiolated AgI complex {[Ag18 (St Bu)10 (NO3 )8 (CH3 CN)2 (H2 O)2 ] ⋅ [Ag18 (St Bu)10 (NO3 )8 (CH3 CN)6 ]}n (abbreviated as Ag18 ) in which two similar Ag18 clusters are assembled by NO3 - anions. The solution containing Ag18 reacted with hydrogen sulfide with controlled concentration, promptly producing another identifiable and bright red-emitting high-nuclearity silver(I) cluster, Ag62 (S)13 (St Bu)32 (NO3 )4 (abbreviated as Ag62 ). We tracked the transformation using time-dependent electrospray ionization mass spectrometry (ESI-MS), UV/Vis absorption and photoluminescence spectra. Based on this cluster transformation, we further developed an ultra-sensitive turn-on sensor detecting H2 S gas with an ultrafast response time (30 s) at a low detection limit (0.13 ppm). This work opens a new way of understanding the growth of metal clusters and developing their luminescent sensing applications.

A new ionization network and radiation transport module in pluto

ABSTRACT We introduce a new general-purpose time-dependent ionization network (IN) and a radiation transport (RT) module for the magnetohydrodynamic (MHD) code pluto. Our IN is reliable for temperatures ranging from 5 × 103 to 3 × 108 K and includes all ionization states of H, He, C, N, O, Ne, Mg, Si, S, and Fe, making it suitable for studying a variety of astrophysical scenarios. Radiation loss for each ion–electron pair is calculated using cloudy-17 data on the fly. Photoionization and charge exchange are the main processes contributing to chemical heating. The IN is fully coupled to the RT module over a large range of opacities at different frequencies. The RT module employs a method of short characteristics assuming spherical symmetry. The radiation module requires the assumption of spherical symmetry, while the IN is compatible with full 3D. We also include a simple prescription for dust opacity, grain destruction, and the dust contribution to radiation pressure. We present numerical tests to show the reliability and limitations of the new modules. We also present a post-processing tool to calculate projected column densities and emission spectra.

Chromospheric activity in 55 Cancri – I. Results from theoretical wave studies

ABSTRACT We present theoretical models of chromospheric heating for 55 Cancri, an orange dwarf of relatively low activity. Self-consistent, non-linear, and time-dependent ab initio numerical computations are pursued encompassing the generation, propagation, and dissipation of waves. We consider longitudinal waves operating among arrays of flux tubes as well as acoustic waves pertaining to non-magnetic stellar regions. Additionally, flux enhancements for the longitudinal waves are also taken into account as supplied by transverse tube waves. The Ca ii K fluxes are computed (multi-ray treatment) assuming partial redistribution as well as time-dependent ionization (TDI). The self-consistent treatment of TDI (especially for hydrogen) greatly impacts the atmospheric temperatures and electron densities (especially behind the shocks); it also affects the emergent Ca ii fluxes. Particularly, we focus on the influence of magnetic heating on the stellar atmospheric structure and the emergent Ca ii emission, as well as the impact of non-linearities. Our study shows that a higher photospheric magnetic filling factor entails a larger Ca ii emission; however, an increased initial wave energy flux (e.g. associated with mode coupling) is of little difference. Comparisons of our theoretical results with observations will be conveyed in forthcoming Paper II.

Many-electron effects of strong-field ionization described in an exact one-electron theory

If one-electron observables of a many-electron system are of interest, a many-electron dynamics can be represented exactly by a one-electron dynamics with effective potentials. The formalism for this reduction is provided by the Exact Electron Factorization (EEF). We study the time-dependent features of the EEF effective potentials for a model of an atom ionized by an ultrastrong and ultrashort laser pulse, with the aim of understanding what is needed to develop computationally feasible approximations. It is found that the simplest approximation, the so-called time-independent conditional amplitude (TICA) approximation, is complementary to single-active electron (SAE) approaches as it reproduced the exact dynamics well for high photon frequencies of the laser field or large Keldysh parameter. For relatively low frequencies of the laser field or for smaller Keldysh parameters, we find that excited state dynamics in the core region of the atom leads to a time-dependent ionization barrier in the EEF potential. The time-dependence of the barrier needs to be described accurately to correctly model many-electron effects, and we conclude that a multi-state extension of the TICA approximation is a possible route how this can be achieved. In general, our study sheds a different light on one-electron pictures of strong-field ionization and shows that many-electron effects for such processes may be included by solving a one-electron Schr\"odinger equation, provided the core dynamics can be modeled successfully.

A NICER View of a Highly Absorbed Flare in GRS 1915+105

Abstract After 26 yr in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the “obscured state,” is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with xstar, ultimately attributing the absorption to a radially stratified absorber of density ∼1012–1013 cm−3 at ∼few ×1011 cm from the black hole. We argue that a vertically extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.

Imprint of transient electron localization in using circularly-polarized laser pulse* *Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB1801904), the National Natural Science Foundation of China (Grant No. 12075036), and the Open Research Fund of National Mobile Communications Research Laboratory, Southeast University (Grant No. 2019D14).

Photoelectron momentum distribution of hydrogen molecular ion in a circularly polarized laser pulse is calculated by solving the three-dimensional time-dependent Schrödinger equation (3D-TDSE). At the intermediate internuclear distance, an unusual multi-peak structure is observed in the angular distribution, which is proved to be a signature of the transient localization of the electron upon alternating nucleus. By tracing the time-dependent ionization rate and bound state populations, we provide a clear evidence that the transient electron localization still exists in circularly polarized pulse and the corresponding multiple ionization bursts are directly mapped onto observable angular distributions. In addition, we introduce an intuitive strong-field approximation model which incorporates laser-induced subcycle internal electron dynamics to isolate the effect of the Coulomb potential of the parent ions. In this way, the timing of each ionization burst can be directly read out from the angular distributions. Our results suggest that the ionization time serves as a sensitive tool encoding intramolecular electron dynamics and can be measured using attoclock technique.

Hydrodynamic Simulations and Time-dependent Photoionization Modeling of Starburst-driven Superwinds

AbstractThermal energies deposited by OB stellar clusters in starburst galaxies lead to the formation of galactic superwinds. Multi-wavelength observations of starburst-driven superwinds pointed at complex thermal and ionization structures which cannot adequately be explained by simple adiabatic assumptions. In this study, we perform hydrodynamic simulations of a fluid model coupled to radiative cooling functions, and generate time-dependent non-equilibrium photoionization models to predict physical conditions and ionization structures of superwinds using the maihem atomic and cooling package built on the program flash. Time-dependent ionization states and physical conditions produced by our simulations are used to calculate the emission lines of superwinds for various parameters, which allow us to explore implications of non-equilibrium ionization for starburst regions with potential radiative cooling.

Optimizing the deposition rate and ionized flux fraction by tuning the pulse length in high power impulse magnetron sputtering

High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapour deposition technique. While HiPIMS provides a high flux of metal ions to the substrate, the disadvantage is a reduced deposition rate compared to direct current magnetron sputtering (dcMS) at equal average power. This is mainly due to the high target back-attraction probability of the metal ions with typical values in the range 70%–90% during the pulse. In this work, we investigate how to reduce this effect by quantifying the contribution of the metal ion flux after each HiPIMS pulse, a period also known as afterglow. Without a negative potential on the target at this stage of the HiPIMS process, the back-attracting electric field disappears allowing remaining ions to escape the ionization region. In order to analyze the fate of the film-forming ions, we extend the time-dependent ionization region model (IRM) by adding consideration of an afterglow. This approach allows to distinguish between fluxes from the ionization region during the pulse and during the afterglow. We show that by shortening the pulse length of a titanium HiPIMS discharge, the contribution to the outward flux of film-forming species from the afterglow increases significantly. The IRM predicts a gain in deposition rate of 46% and 47% for two discharges with different peak discharge currents, when using 40 μs compared to 100 μs-long pulses at the same average power. This is without compromising the ionized flux fraction that remains constant for the range of pulse lengths investigated here.

Size Evolution Dynamics of Gold Nanoclusters at an Atom-Precision Level: Ligand Exchange, Growth Mechanism, Electrochemical, and Photophysical Properties

Interpretation of size evolution is an essential part of nanocluster transformation processes for unraveling the mechanism at an atom-precision level. Here we report the transformation of a non-superatomic Au23 to a superatomic Au36 nanocluster via Au28 cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption ionization mass spectrometry data revealed that the conversion proceeds through ligand exchange followed by the size focusing method, ultimately leading to size growth. We also validated this transformation through time-dependent ultraviolet-visible data. Density functional theory calculations predicted that the kernel of the Au28 cluster evolved through a linear combination of molecular orbitals of the fragment of 2e- units (Au42+ and Au3+) from the kernel of the Au23 cluster. Periodic growth of gold cores through continuous growth of Au4 tetrahedral unit leads to the formation of the Au36 cluster from the Au28 cluster. These results reinforce the plausibility of size evolution through the growth mechanism during the transformation process. Differential pulse voltammetry studies showed that the highest occupied molecular orbital-lowest unoccupied molecular orbital gap inversely varies with the kernel size of these clusters. Photophysical experiments support the molecular-like intersystem crossing rather than core-shell relaxation to these clusters. The trends of photoluminescence lifetime were found to be the reverse of those of the energy gap law. The increment of lifetimes for the larger cluster can be mainly due to the contribution of both hot carriers and band-edge carriers.

Depletion effect in high-order harmonic generation with coherent superposition state

Enhancement of the conversion efficiency of high-order harmonics (HHG) generated from an atom in the superposition of two quantum states instead of the ground state is essential because of the recent application in the generation of ultrashort isolated attosecond pulses. In this study, we first confirm the enhancement for a more real system with full three-dimensional geometry and then investigate an interesting effect—the influence of the depletion on the HHG cutoff. For this purpose, we prepare a hydrogen atom initially in the coherent superposition of the ground and the first excited states and calculate the HHG by numerically solving the three-dimensional time-dependent Schrödinger equation (TDSE)—the TDSE method. We find that the plateau of HHG spectra with the coherent superposition state ends earlier than that of the ground state, which can be attributed to the depletion of the excited state. We justify this link by adopting the classical simulation and time-dependent ionization rate. Besides, we show that the conversion efficiency is sensitive to the existence of the excited state. Particularly, for some values of laser field intensity, the excited state population of about 1% increases HHG intensity by more than 5 orders. However, after a critical excited state population, the HHG intensity is stable with continuously increasing initial population of the excited state that is meaningful for experimental setups.

The Large-scale Ionization Cones in the Galaxy

Abstract There is compelling evidence for a highly energetic Seyfert explosion (1056–57 erg) that occurred in the Galactic center a few million years ago. The clearest indications are the X-ray/γ-ray “10 kpc bubbles” identified by the ROSAT and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e., elevated Hα emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular / , Si iv/Si ii) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative “ionization cones” from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D ≳ 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated Hα emission and H ionization fraction (x e ≳ 0.5), enhanced / and Si iv/Si ii ratios, and high and Si iv column densities. Wind-driven “shock cones” are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud–halo interaction) is too weak to explain the Magellanic Stream’s ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f E ≈ 0.1–1). Our time-dependent Seyfert flare models adequately explain the observations and indicate that the Seyfert flare event took place T o = 3.5 ± 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the X-ray/γ-ray bubbles in leptonic jet/wind models (≈2–8 Myr).

Analysis of Transient Performance of Grounding System Considering Frequency-Dependent Soil Parameters and Ionization

Frequency-domain approaches are usually used to study the impact of the frequency dependence of soil electrical parameters on the lightning response of grounding systems. On the other hand, time-domain simulations are required to study nonlinear soil ionization behavior. A time-domain frequency-dependent simulation is needed to incorporate both effects simultaneously. Based on dynamic harmonic domain (DHD) concept and transmission-line method, this paper derives a new approach for simulating the grounding system surge behavior that includes both time-dependent ionization and frequency-dependent effects of the soil. The DHD permits to decouple harmonic contents of each variable state during transients in time-domain simulation. The simulation results are validated against measured and simulated published results from other researchers. The comparison of experimental results and those provided by this model denoted its consistency.

Tracking the Stepwise Formation of the Dysprosium Cluster (Dy10) with Multiple Relaxation Behavior.

High-nuclear lanthanide clusters are generally formed by the rapid accumulation of simple building units. Thus, tracking and observing the stepwise assembly process, which is vital for understanding the assembly mechanism, are extremely difficult. Herein, the decanuclear nanocluster [Dy10(L1)6(μ5-NO3)2(OAc)10(HOAc)2]·8H2O (Dy10, H3L1 = (E)-3-((3-ethoxy-2-hydroxybenzylidene)amino)propane-1,2-diol) was obtained from the reaction of Dy(NO3)3·6H2O, Dy(OAc)3·6H2O, 3-ethoxy-2-hydroxybenzaldehyde (L2), and 3-amino-1,2-propanediol (L3). The reaction process was further tracked by time-dependent high-resolution electrospray ionization mass spectrometry, and seven reaction intermediate fragments were screened. A stepwise assembly mechanism was observed based on these fragments, that is, L → Dy1 → Dy2 → Dy3 → Dy4 → Dy5 → Dy6 → Dy10. This study is the first to discover a stepwise assembly mechanism during the formation of high-nuclear lanthanide clusters (cluster nucleus > 3). Magnetic studies have shown the multiple relaxation behavior of Dy10.

Simulation of time-dependent ionization processes in acetylene

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

Selective catalytic oxidation of aromatic substrates employing mononuclear copper(II) catalyst with H2O2

A novel mononuclear complex [LCuCl2(H2O)]CH2Cl2 (L = 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine) was synthesized and characterized by elemental analysis, ultraviolet visible (UV–Vis) spectrum, and fourier transform infrared spectroscopy (FTIR). The single-crystal X-ray diffraction structure analysis of the complex revealed that the copper atom was octahedrally coordinated. The catalytic potential of this complex was examined by the oxidation reaction of various aromatic substrates, such as styrene, ethyl benzene, α-methyl styrene, and benzyl alcohol. The oxidation reactions were carried out in acetonitrile with H2O2 and catalytic amounts of [LCuCl2(H2O)] at 70 °C. The catalyst exhibited good catalytic activity in the oxidation of benzyl alcohol (97.50% conv. in 1 h; TONs = 341.25), α-methyl styrene (97.72% conv. in 4 h; TONs = 342.02) and methyl styrene (99.00% conv. in 8 h; TONs = 346.50). The major products were 96.07% benzoic acid from benzyl alcohol, 93.54% acetophenone from α-methyl styrene and 86.38% benzaldehyde from styrene, respectively. A plausible catalytic mechanism involving an active cation [LCu2+] and an epoxy intermediate was proposed based the results of time-dependent electrospray ionization mass spectrometry (ESI-MS) of the reaction mixture.

Deciphering synergetic core-shell transformation from [Mo6O22@Ag44] to [Mo8O28@Ag50

The structural transformation of high-nuclearity silver clusters from one to another induced by specific stimuli is of scientific significance in terms of both cluster synthesis and reactivity. Herein, we report two silver-thiolate clusters, [Mo6O22@Ag44] and [Mo8O28@Ag50], which are templated by isopolymolybdates inside and covered by iPrS− and PhCOO− ligands on the surfaces. Amazingly, the [Mo8O28@Ag50] can be transformed from [Mo6O22@Ag44] by adding PhCOOH which increases the degree of condensation of molybdates template from Mo6O228- to Mo8O288-, then enlarging the outer silver shell from Ag44 to Ag50. The evolution of solution species revealed by time-dependent electrospray ionization mass spectrometry (ESI-MS) suggests a breakage-growth-reassembly (BGR) transformation mechanism. These results not only provide a combined assembly strategy (anion-template + induced transformation) for the synthesis of silver-thiolate clusters but also help us to better understand the complex transformation process underpinning the assembly system.

Static and Dynamic Effects of the Incomplete Ionization in Superjunction Devices

The incomplete ionization of impurity atoms affects the free carrier concentration of several wide-bandgap (WBG) semiconductor materials even at room temperature, thus modifying the electrical properties of power devices. In this paper, the influence of the partial ionization of the dopants on the static and dynamic behaviors of WBG semiconductor-based superjunction (SJ) devices has been numerically investigated through extensive 2-D finite-element simulations. While this physical effect has only a minor impact on the static device characteristics, if a reverse bias pulse with a rise time comparable or smaller than the ionization time constant is applied to the structure, a “dynamic ionization” phenomenon can take place. The onset of this time-dependent ionization is the cause of charge unbalance effects in the device structure, due to the temporal dependence of the activated number of dopants. Electrothermal simulations, which have been carried out for a 4H-silicon carbide SJ diode, show a nonuniform temperature distribution during the transients which leads, in turn, to the self-heating of the device. Through an accurate redesign of the cross-sectional view of the device, the drawbacks of the incomplete ionization have been mitigated and the device performance enhanced.

Time-resolved nuclear dynamics in bound and dissociating acetylene.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Vibrating bound acetylene cations or dissociating dications are produced by the first pulse. The second pulse probes the nuclear dynamics by ionization to higher charge states and Coulomb explosion of the molecule. For the bound cations, we observed vibrations in acetylene (HCCH) and its isomer vinylidene (CCHH) along the CC-bond with a periodicity of around 26 fs. For dissociating dication molecules, a clear indication of enhanced ionization is found to occur along the CH- and CC-bonds after 10 fs to 40 fs. The time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.