Energetic Distribution Research Articles

Statistical Study of Energetic Electron Butterfly Pitch Angle Distributions During Magnetic Dip Events

AbstractThe magnetic dips can create the electron butterfly pitch angle distribution (PAD) in the inner magnetosphere. However, its universality is still unconfirmed. We statistically investigate the global distribution of the occurrence rate of the magnetic dip‐related butterfly PAD for 466 keV and 2.1 MeV electrons using the measurements of the twin Van Allen Probes spanning from September 2012 to November 2018. The statistical results show that the magnetic dip‐related butterfly PADs are mainly confined to duskside to midnight within 4.5 < L < 6, and the peak occurrence rate (30% for 466 KeV and 70% for 2.1 MeV) occurs at the duskside equator around L = 5. The parametric study results suggest that the butterfly PADs preferably occur for high energy electrons with a negative initial radial flux gradient and a nearly isotropic initial PAD, which is consistent with the statistic results. This work confirms the vital importance of the magnetic dips on the evolution of the electron PADs.

Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

Summary Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.

The mechanisms of gallium-catalysed skeletal rearrangement of 1,6-enynes – Insights from quantum mechanical computations

The transition metal-catalysed skeletal reorganization of 1,6-enynes can lead to three types of products – a type I product occurring via the cleavage of the alkene C–C bonds and the migration of the terminal alkene carbon to the terminus of the alkyne; a type II product arising from cleavage of both the double and the triple bonds followed by insertion of the terminal alkene carbon into the alkyne C–C triple bond; and a type III product which is obtained when there is a cleavage of the olefinic bond followed by formation of two new bonds from each carbon to each of the acetylenic carbons. The course of these reactions is highly dependent on the metal catalyst used and type of substitution at the alkene and alkyne moieties of the enyne. In this mechanistic study of the re-organization of 1,6-enynes catalysed by GaCl3, performed at the DFT M06/6-311G(d,p) level of theory, the parent reaction selectively leads to the formation of the type I product through the formation of the open cyclopropane ring. The presence of substituents at the acetylenic moiety governs the preferred position of the metal along the alkyne bond within the pi-complex: with electron-withdrawing groups (EWGs), the metal prefers the terminal carbon while electron-donating groups (EDGs) lead to the metal preferring the internal carbon. EWGs at the alkyne moiety efficiently favour the formation of the type I product. Substituents at the olefin moiety alter the mechanism of the reaction which may favour the selective formation of the type I or III product depending on the type of substituent. EWGs at the olefinic moiety favour formation of the type III product when the alkyne moiety is unsubstituted whiles EDGs forms the type I product selectively. Solvent and temperature have no substantial effects on the energetic trends and product distribution. Hence, gas-phase calculations are deemed adequate for the problem at hand.

Organic Solar Cells Based on Non-Fullerene Small-Molecule Acceptors: Impact of Substituent Position

Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end-groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end-groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end-groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton dissociation and nonradiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.

Critical energetic particle distribution in phase space for the Alfvén eigenmode burst with global resonance overlap

Comprehensive computer simulations of the Alfvén eigenmode burst, which is the synchronized sudden growth of multiple Alfvén eigenmodes (AEs) interacting with energetic particles, were conducted with continuous neutral beam injection, collisions, and particle losses. It is found that the energetic-particle distribution in phase space reaches a ‘critical distribution’ with a stairway structure where a resonance overlap triggers the Alfvén eigenmode burst. Before the burst, the gradual growth of the AEs associated with the beam injection broadens the resonant regions in phase space forming the distribution into a stairway shape. When the distribution reaches the ‘critical distribution,’ a resonance overlap triggers multiple resonance overlaps leading to the synchronized growth of AEs and the collapse of the distribution. For another run with the beam deposition power reduced to one-half, the energetic-particle distribution function just before the Alfvén eigenmode burst is close to that for the original beam power. This result indicates that the critical distribution for the Alfvén eigenmode burst is present.

Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor ITIC-Based Organic Bulk Heterojunction Photovoltaic System

Nonfullerene organic molecules that usually act as acceptors have exhibited prominent optoelectronic properties in organic bulk heterojunction (BHJ) solar cells. Although the highly efficient exciton dissociation can happen even with a relatively smaller driving force, the underlying energy loss including exciton dissociation and electron–hole recombination in the nonfullerene acceptor (NFA) system remains unclear. In principle, the energetic disorder characteristic is a common feature in organic semiconductors as well as their organic blends due to the molecular random distribution and the donor–acceptor interpenetration network. Owing to their completely different molecular structures in contrast to the bulky-ball shaped fullerene-derivative counterpart such as PCBM, a comprehensive understanding for the trap-associated energetic disorder characteristic and distribution are merits to further eliminate the open-circuit voltage (Voc) loss. In this work, a highly efficient planar organic BHJ solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al was fabricated. Both the polymeric donor PBDB-T and NFA ITIC form an interpenetrating network. With an assist from the nondestructive impedance spectroscopic technique, surface and bulk trap states can be fully revealed. More importantly, both of them respectively contain deep and shallow traps with different energetic locations in an energy gap. The results help to explain the appearance of double and triple capacitance–voltage (C–V) bands. Furthermore, with pump–probe laser spectroscopic measurements, the trap states may locate far below the photoabsorption band-edge. This work serves an insightful view on the trap states in the NFA-based organic BHJ system, and it is valuable for minimizing the energy loss.

Self-consistent Modeling of Gamma-ray Spectra from Solar Flares with the Monte Carlo Simulation Package FLUKA

We use the Monte Carlo particle physics code FLUKA (Fluktuierende Kaskade) to calculate $\gamma$-ray spectra expected from solar flare energetic ion distributions. The FLUKA code includes robust physics-based models for electromagnetic, hadronic and nuclear interactions, sufficiently detailed for it to be a useful tool for calculating nuclear de-excitation, positron annihilation and neutron capture line fluxes and shapes, as well as $\approx \; {\rm GeV}$ continuum radiation from pion decay products. We show nuclear de-excitation $\gamma$-ray line model spectra from a range of assumed primary accelerated ion distributions and find them to be in good agreement with those found using the code of Murphy et al. (2009). We also show full $\gamma$-ray model spectra which exhibit all the typical structures of $\gamma$-ray spectra observed in solar flares. From these model spectra we build templates which are incorporated into the software package Objective Spectral Executive (OSPEX) and used to fit the combined Fermi Gamma-ray Burst Monitor (GBM)/Large Area Telescope (LAT) spectrum of the 2010 June 12 solar flare, providing a statistically acceptable result. To the best of our knowledge, the fit carried out with the FLUKA templates for the full $\gamma$-ray spectrum can be regarded as the first attempt to use a single code to implement a self-consistent treatment of the several spectral components in the photon energy range from $\approx 100$s ${\rm keV}$ to $\approx 100$s ${\rm MeV}$.

All-Polymer Solar Cells: Impact of the Length of the Branched Alkyl Side Chains on the Polymer Acceptors on the Interchain Packing and Electronic Properties in Amorphous Blends

All-polymer solar cells are attracting increasing attention because polymer acceptors present specific advantages, especially over fullerene derivatives. The length of the branched alkyl side chains on the polymer acceptors has been reported to have different impacts in the crystalline versus amorphous domains of donor/acceptor blends. Given that amorphous domains are difficult to characterize experimentally, here, molecular dynamics simulations are combined with density functional theory calculations to examine at the molecular scale the role that the side-chain length plays on the interchain packing and electronic properties. As representative examples, blends of the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PCE10) donor with the poly(thieno[3,4-c]1-dione-alt-3,4-difluorothiophene) (PTPD[2F]T) acceptor are discussed; two different branched side chains on the thieno[3,4-c]pyrrole-4,6-dione (TPD) moieties of the acceptor polymer are considered, that is, 2-hexyldecyl and 2-decyltetradecyl. Increasing the side-chain length is found to decrease the PTPD[2F]T backbone planarity and importantly to bring a higher extent of interactions between the side chains and their own TPD moieties, which plays a critical role in determining the interchain packings. The nature of these packings are then correlated with the following: (i) the electron-transfer rates between neighboring PTPD[2F]T chains; (ii) the energetic distribution of the interfacial charge-transfer states; and (iii) the nonradiative recombination processes from the charge-transfer states to the ground state and the associated voltage losses. Overall, our findings point to a higher electron mobility and a lower nonradiative voltage loss in the PCE10/PTPD[2F]T blends where the polymer acceptor has shorter side chains.

Interpreting observations of ion cyclotron emission from large helical device plasmas with beam-injected ion populations

Ion cyclotron emission (ICE) is detected from all large toroidal magnetically confined fusion (MCF) plasmas. It is a form of spontaneous suprathermal radiation, whose spectral peak frequencies correspond to sequential cyclotron harmonics of energetic ion species, evaluated at the emission location. In ICE phenomenology, an important parameter is the value of the ratio of energetic ion velocity to the local Alfvén speed . Here we focus on ICE measurements from heliotron-stellarator hydrogen plasmas, heated by energetic proton neutral beam injection (NBI) in the large helical device, for which takes values both larger (super-Alfvénic) and smaller (sub-Alfvénic) than unity. The collective relaxation of the NBI proton population, together with the thermal plasma, is studied using a particle-in-cell (PIC) code. This evolves the Maxwell–Lorentz system of equations for hundreds of millions of kinetic gyro-orbit-resolved ions and fluid electrons, self-consistently with the electric and magnetic fields. For LHD-relevant parameter sets, the spatiotemporal Fourier transforms of the fields yield, in the nonlinear saturated regime, good computational proxies for the observed ICE spectra in both the super-Alfvénic and sub-Alfvénic regimes for NBI protons. At early times in the PIC treatment, the computed growth rates correspond to analytical linear growth rates of the magnetoacoustic cyclotron instability (MCI), which was previously identified to underlie ICE from tokamak plasmas. The spatially localised PIC treatment does not include toroidal magnetic field geometry, nor background gradients in plasma parameters. Its success in simulating ICE spectra from both tokamak and, here, heliotron-stellarator plasmas suggests that the plasma parameters and ion energetic distribution at the emission location largely determine the ICE phenomenology. This is important for the future exploitation of ICE as a diagnostic for energetic ion populations in MCF plasmas. The capability to span the super-Alfvénic and sub-Alfvénic energetic ion regimes is a generic challenge in interpreting MCF plasma physics, and it is encouraging that this first principles computational treatment of ICE has now achieved this.

Total Ionizing Dose Effects in FDSOI Compact Model for IC Design

The total ionizing dose (TID) is a well-known reliability issue for integrated circuits (ICs). It consists in changes of MOSFETs dc characteristics following the irradiation of devices in a given radiative environment, such as space environment. TID effects have been investigated from an experimental and theoretical standpoint, and the compact modeling of TID effects has been included into BULK transistor compact models using PSP formalism in the case of uniform and nonuniform energetic distributions of interface traps. However, TID effects are still not included in fully depleted SOI (FDSOI) standard models. In this paper, TID effects modeling is proposed and included in Leti-UTSOI compact model for FDSOI MOSFETs. This model takes both uniform and nonuniform energetic distributions of both gate oxide/silicon and silicon/buried oxide (BOX) interface traps into account, as well as the contribution of oxide/BOX-trapped charges. Validation of the model has been done through TCAD simulations, and we used this model to extract TID parameters from the experimental data.

Mapping the Nucleation of H2 Bubbles on Polycrystalline Pt via Scanning Electrochemical Cell Microscopy.

Electrochemically generated bubbles are gaining increasing attention for their detrimental effects on the efficiency of electrocatalysis involving gaseous products, including hydrogen evolution reaction (HER). As a model system, bubble nucleation is also of fundamental interest. Herein, we report a single-entity approach to map the nucleation of H2 bubbles on polycrystalline Pt via scanning electrochemical cell microscopy (SECCM). Individual H2 bubbles (∼500 nm radius) are generated via HER in the attoliter electrochemical cell of an SECCM instrument. Nucleation and growth of a H2 bubble results in a characteristic voltammetric peak current, which is related to the concentration and activation energy required for nucleation. By mapping the local voltammograms at various locations on the polycrystalline Pt, we found a heterogeneous distribution of energetics of nucleation for H2 bubbles on the Pt surface. However, such heterogeneity is not correlated with crystal grains or grain boundaries.

Nature and origin of γ-gauche effect in sulfoxides: A density functional theory and information-theoretic approach study

Bulky vicinal groups around a single bond usually prefer to be in the opposite (anti) conformation but when extra interactions come into play, more stable could be the gauche conformer, where the neighboring groups embrace a dihedral angle around 60°. This is called the gauche effect. The γ-gauche effect historically features the upfield shift of 13C NMR spectra when a carbon atom and its nonprotonic γ-substituent are in a gauche arrangement. In this work, using sulfoxides as illustrative examples, we show that the nature of the γ-gauche effect is the same as that of the gauche effect, i.e., the gauche conformation is more stable than the anti conformation. To unveil the origin of this effect, we employ two total energy partition schemes in density functional theory and four quantities from the information-theoretic approach. Our results unambiguously show that the γ-gauche effect is of the stereoelectronic origin with a multifaceted nature. Its origin can be understood from a few different perspectives including energetic contributions, dipole moment, charge distribution, hyperconjugation, and information theory. Substantially different from other stereoelectronic effects such as anomeric and generalized anomeric effects, this effect has its unique features as demonstrated from our results in the present work. Having successfully established the link of the γ-gauche with other gauche effects and unveiled its multi-faceted origin, this study should shed new light towards the comprehensive understanding about this important effect of conformational stability in sulfoxides and other systems alike.

Two-color XUV+NIR femtosecond photoionization of neon in the near-threshold region

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low kinetic energy (few eV) sidebands, the photoelectron energy varies as a function of the emission angle. Such behavior has not yet been observed in sideband creation and has not been predicted in commonly used theoretical descriptions such as strong field approximation and soft photon approach. Describing the photoionization with a time-dependent Schrödinger equation allows a qualitative description of the observed effect, as well as the prediction of fine structure in the sideband distribution.

Modeling Energetic Electron Nonlinear Wave‐Particle Interactions With Electromagnetic Ion Cyclotron Waves

AbstractElectromagnetic ion cyclotron (EMIC) waves in duskside plasmasphere and plasmaspheric plume scatter megaelectron volt electrons into the loss cone and are considered a major loss mechanism for the outer radiation belt. Wave‐particle interaction between energetic electrons and EMIC waves has been studied extensively by the quasi‐linear diffusion theory. However, EMIC waves are typically strong enough to trigger nonlinear wave‐particle interaction effects and transport electrons in very different ways from quasi‐linear diffusion. New mathematical method is therefore in demand to study the evolution of energetic electron distribution in response to nonlinear wave‐particle interaction. In this work, we present a Markov chain description of the wave‐particle interaction process, in which the electron distribution is represented by a state vector and is evolved by the Markov matrix. The Markov matrix is a matrix form of the electron response Green's function and could be determined from test particle simulations. Our modeling results suggest that electron loss rate is not significantly affected by phase bunching and phase trapping, but for strong EMIC waves, electron distribution is more saturated near loss cone than quasi‐linear theory prediction, and negative electron phase space density slope develops inside loss cone.

Tunelaje Cuántico en Potenciales Graduales

Uno de los fenómenos paradigmáticos de la mecánica cuántica es sin duda el llamado efecto túnel, el cual se manifiesta como la posibilidad que tienen las partículas en la escala nanométrica de atravesar barreras de potencial. Este fenómeno, a pesar de ser poco intuitivo, es tan real que juega un papel prominente en la tecnología de nuestros tiempos y constituye el mecanismo dominante del transporte electrónico en nuevos conceptos de dispositivos nanoelectrónicos. En este trabajo se ilustra mediante mapas de la densidad electrónica, la distribución espacial y energética de los electrones que se propagan a través de barreras de potencial graduales, visualizando la naturaleza ondulatoria de los electrones y el fenómeno de tunelaje. En particular, se discute el efecto de utilizar barreras graduales en lugar de barreras rectangulares.

Modulation of Locally Generated Equatorial Noise by ULF Wave

AbstractIn this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon.

Chemical interactions of heparin in porous polypyrrole, an example of drug–carrier destructive interaction

Plasma polypyrrole doped with iodine (PPy/I) and heparin (Hp) has been used as drugs in different ways in the human body. In this work, Hp was inserted into porous PPy/I to prepare biocompatible mixtures to potentially release Hp into the human body. The study was focused on the absorption and chemical interaction of Hp with PPy/I to find whether their structures suffer modifications in the carrying process. The absorption was performed by cryo-lyophilization immersing Hp in water with a 10:1 polymer/drug mass ratio. Once in contact, Hp went into the pores and partially coated the PPy/I surface. IR analyses of the mixtures on the surface showed that the functional Hp groups predominated over the polymer. The chemical interaction of both components was studied by XPS analyzing the energetic distribution of their atomic 1s or 2p orbitals. The results indicated that Hp interacted chemically with PPy/I losing the highest oxidized C1s, N1s and S2p chemical states in both components, forming new reduced ones with less multiple bonds or with more hydrogenated groups. S2p states were especially modified losing the most reactive states with 6 valences to form new more stable states with 2 valences. O1s orbitals behaved differently losing the most reduced chemical states forming new oxidized ones. These modifications suggest that the chemical reactivity of this PPy/I-Hp binary system was altered reducing the therapeutic capability of both components. This kind of studies should be done in all carrier–drug combinations to evaluate the possible chemical interaction between them.

The high-energy particle package onboard CSES

The China Seismo-Electromagnetic Satellite (CSES) aims to monitor space electromagnetic fields, ionospheric plasma, high-energy charged particles and other features of the global space environment. The high-energy particle package (HEPP), which can effectively detect the energy spectrum, flux and pitch angle distribution of space electrons and protons, and soft X-ray emission from solar flares, is one of the main payloads of CSES. In this study, we designed, developed and calibrated the high-energy particle package and launched it into orbit with CSES. HEPP consists of the high-energy detector (HEPP-H), the low-energy detector (HEPP-L), and the solar X-ray monitor (HEPP-X). The three sub-detectors mainly use silicon detector and crystal calorimeter detection technology. Before launching, we calibrated the three sub-detectors in detail by using radiation source and accelerator beam. All the three sub-detectors have good energy linearity. After launching into orbit, the space energetic particles and X-ray distribution detected by HEPP are consistent with expectations. The performance indices of the detector reach the advanced level of the same kind of detector in the world. HEPP has a wide energy detection range, good energy resolution and high angular resolution ability for electrons and protons. It will play an important role in the study of space particle response and space physics of seismic activity.

Linear properties of global energetic particle induced geodesic acoustic mode with bump-on-tail distribution in tokamak plasmas

A systematic study of the global Energetic particle-induced Geodesic Acoustic Mode (EGAM) has been carried out for bump-on-tail energetic distribution in tokamak plasmas using linear kinetic-fluid hybrid simulation. The stability threshold of EGAM in energetic particle pressure is found to be very low. The eigenmode structure becomes more and more radially extended as energetic particle distribution evolves from bump-on-tail distribution to fully slowing-down distribution. The mode radial scale length increases with the energetic particle orbit width.

Detection and analysis of laser driven proton beams by calibrated Gafchromic HD-V2 and MD-V3 radiochromic films.

The radiochromic film (RCF) is a high-dose, high-dynamic range dosimetry detection medium. A stack of RCFs can be used to detect both spatial and energetic distribution of laser driven ion beams with a large divergence angle and continuous energy spectrum. Two types of RCFs (HD-V2 and MD-V3, from Radiation Products Design, Inc.) have been calibrated using MeV energy protons and carbon ions produced by using a 2 × 6 MV tandem electrostatic accelerator. The proportional relationship is obtained between the optical density and the irradiation dose. For protons, the responses are consistent at all energies with a variation of about 15%. For carbon ions, the responses are energy related, which should be noted for heavy ion detection. Based on the calibration, the broad energy spectrum and charge distribution of laser accelerated proton beam with energy from 3 to 8 MeV and pC charge were detected and reconstructed at the Compact LAser Plasma Accelerator at Peking University.