Electron Kinetic Energy Research Articles

Anti‐Stoke effect induced enhanced photocatalytic hydrogen production

AbstractPhotocatalytic hydrogen production via solar energy is considered one of the most strategic ways to produce renewable energy. However, expensive Pt is normally used as the cocatalyst during photocatalysis, which prevents commercialization. Therefore, extensive research has been performed to seek abundant and low‐cost alternative catalysts. In this work, MoS2 quantum dots (QDs) synthesized by a hydrothermal method are incorporated with graphitic carbon nitride to form a heterostructure for photocatalytic hydrogen evolution. MoS2 QDs/g‐C3N4 heterostructure containing 5% and 10% MoS2 QDs exhibited a high hydrogen production of 140 and 152 µmol h−1g−1, respectively, demonstrating the potential of MoS2 as an effective economic cocatalyst. Detailed investigations indicate that incorporating MoS2 QDs with carbon nitride to form heterostructure reduces the bandgap, suppresses the recombination, and enhances electron kinetic energy resulting from the anti‐Stoke effect, thus leading to better performance for hydrogen evolution.

Moiré potential renormalization and ultra-flat bands induced by quasiparticle-plasmon coupling

Moiré potential profile can form flat electronic bands and manifest correlated states of electrons, where carrier doping is essential for observing those correlations. In this work, we uncover a hidden but remarkable many-electron effect: doped carriers form a two-dimensional plasmon and strongly couple with quasiparticles to renormalize moiré potential and realize ultra-flat bands. Using many-body perturbation theory, we demonstrate this effect in twisted MoS2/WS2 heterobilayer. The moiré potential is significantly enhanced upon carrier doping, and the bandwidth is reduced by order of magnitude, leading to drastic quenching of electronic kinetic energy and stronger correlation. We further predict that the competition between correlated mechanisms can be effectively controlled via doping, giving hope to a quantum transition between Mott and charge-transfer insulating states. Our work reveals that the potential renormalization effect of doping is much more significant in determining and controlling many-electron electronic correlations than sole filling-factor tuning in semiconducting moiré crystals.

Fast electron contribution in electronic stopping for p-He and p-Ar collisions

Our study has shown that in collisions p-He and p-Ar with energies above 50 keV fast electrons carry away as kinetic energy up to 70 % of energy released during particle stopping. Ignoring the fast electron contribution makes impossible bringing independently measured cross sections of elementary processes of ionization, excitation, stripping and charge exchange into compliance with measured cross sections of electronic stopping. Fast electron emission in collisions of energetic ions with solids may induce extra radiation effects (create additional vacancies and defects in solid).Mean kinetic energies of ejected electrons We obtained from the analysis of cross section ratio agree well with directly measured energy spectra of the emitted electrons. The dynamic ionization theory implies the We proportionality to initial velocity v. In the considered cases, such a dependence takes place at the velocities below 2 a.u. At higher velocities, the observed dependence is successfully described in the framework of the Born approximation.

Angle-Resolved Time-of-Flight Electron Spectrometer Designed for Femtosecond Laser-Assisted Electron Scattering and Diffraction

We developed an apparatus for measuring kinetic energy and two-dimensional angular distributions of femtosecond laser-assisted electron scattering (LAES) signals with a high detection efficiency, consisting of a photocathode-type ultrashort pulsed electron gun, a gas injection nozzle, an angle-resolved time-of-flight analyzer, and a time-and-position sensitive electron detector. We also established an analysis method for obtaining the kinetic energy and two-dimensional angular distributions of scattered electrons from raw data of their flight times and the detected positions at the detector recorded using the newly developed apparatus. From the measurement of the LAES processes of Ar atoms in a femtosecond near-infrared intense laser field, we obtained a two-dimensional angular distribution image of the LAES signals and showed that the detection efficiency of the LAES signals was raised by a factor of 40 compared with that achieved before in 2010.

Velocity map imaging with no spherical aberrations.

Velocity map imaging (VMI) is a powerful technique to deduce the kinetic energy of ions or electrons that are produced from a large volume in space with good resolution. The size of the acceptance volume is determined by the spherical aberrations of the ion optical system. Here we present an analytical derivation for velocity map imaging with no spherical aberrations. We will discuss a particular example for the implementation of the technique that allows using the reaction microscope recently installed in the cryogenic storage ring (CSR) in a VMI mode. SIMION simulations confirm that a beam of electrons produced almost over the entire volume of the source region, with a width of 8 cm, can be focused to a spot of 0.1 mm on the detector. The use of the same formalism for position imaging, as well as in a mixed mode where position imaging is in one axis and velocity map imaging is in a different axis, is also discussed.

Dissolution mechanism of cellulose in a benzyltriethylammonium/urea deep eutectic solvent (DES): DFT-quantum modeling, molecular dynamics and experimental investigation.

In this paper, a benzyltriethylammonium/urea DES was investigated as a new green and eco-friendly medium for the progress of organic chemical reactions, particularly the dissolution and the functionalization of cellulose. In this regard, the viscosity-average molecular weight of cellulose (M̄w) during the dissolution/regeneration process was investigated, showing no significant degradation of the polymer chains. Moreover, X-ray diffraction patterns indicated that the cellulose dissolution process in the BTEAB/urea DES decreased the crystallinity index from 87% to 75%, and there was no effect on type I cellulose polymorphism. However, a drastic impact of the cosolvents (water and DMSO) on the melting point of the DES was observed. Besides, to understand the evolution of cellulose-DES interactions, the formation mechanism of the system was studied in terms of H-bond density and radial distribution function (RDF) using molecular dynamics modeling. Furthermore, density functional theory (DFT) was used to evaluate the topological characteristics of the polymeric system such as potential energy density (PED), laplacian electron density (LED), energy density, and kinetic energy density (KED) at bond critical points (BCPs) between the cellulose and the DES. The quantum theory of atoms in molecules (AIM), Bader's quantum theory (BQT), and reduced density gradient (RDG) scatter plots have been exploited to estimate and locate non-covalent interactions (NCIs). The results revealed that the dissolution process is attributed to the physical interactions, mainly the strong H-bond interactions.

Behaviour of tungsten oxide on phosphor-tellurite glasses for photon proton and alpha particles shielding

This research, photons shielding properties such as mass attenuation coefficients (mm), effective atomic number (Zeff), effective electron density (Nel) and kinetic energy per unit mass (kerma) relative to air for WO3 based on TeO2–P2O5–WO3 glass system have been simulated by WinXCom software program at energies of 10–3–105 MeV. Also, buildup factors (BFs) have been estimated at widely energy ranging 15 keV – 15 MeV for penetration depths (PD) until 40 mean free path (mfp). The results of glass system in formula (70–x) TeO2–30P2O5–xWO3 at x = 10, 20, 30, 40, and 50 mol% exhibited that the partial replacement of TeO2 by WO3 was adjusted photons attenuation behaviors for get better. In addition, mass sopping power (MSP) and projected range (PR) were evaluated using SRIM software program for proton (H+1) and alpha particles (He+2) at kinetic energy ranging 10 keV – 10 MeV. The results may be inferred that glass sample with high WO3 content was superb for photons, proton and alpha particles attenuation. The results of this research may be useful in enhancing optimization and potential to use as a transparent material to against photon, proton and alpha particles.

Inelastic mean-free path and mean escape depth of 10-140 eV electrons in SiO2 nanoparticles determined by Si 2p photoelectron yields.

We report on photoelectron spectra of SiO2 nanoparticles (d = 157 ± 6 nm) above the Si 2p threshold in the photon energy range 118-248 eV with electron kinetic energy 10-140 eV and analyze the photoelectron yield as a function of photon energy. Comparison of the experimental results with Monte-Carlo simulations on electron transport allows us to quantify the inelastic mean-free path and mean escape depth of photoelectrons in the nanoparticle samples. The influence of the nanoparticle geometry and electron elastic scattering on photoelectron yields is highlighted. The results show that the previously proposed direct proportionality of the photoelectron signal to the inelastic mean-free path or the mean escape depth does not hold for photoelectron kinetic energies below 30 eV due to the strong influence of electron elastic scattering. The present results deviate for photoelectron kinetic energies below 30 eV from the previously proposed direct proportionality of the photoelectron signal to the inelastic mean-free path or the mean escape depth, which is the result of a strong influence of electron elastic scattering. The presented inelastic mean-free paths and mean escape depths appear to be useful for the quantitative interpretation of photoemission experiments on nanoparticles and for modeling of the experimental results.

Formation of carbon propeller-like molecules from starphenes under electron irradiation.

Formation of carbon propeller-like molecules (CPLMs) from starphenes on a graphene substrate under electron irradiation with about 100% yield is observed in molecular dynamics simulations using the REBO-1990EVC_CH potential and CompuTEM algorithm. A CPLM consists of three carbon atomic chains connected to the central hexagon and is formed as a result of the spontaneous breaking of bonds between zigzag atomic rows in starphene arms after hydrogen removal by electron impacts. In the absence of the substrate, the CPLM yield is slightly decreased due to sticking between forming chains, while the formation time is increased threefold. The increase of the kinetic electron energy from 45 to 80 keV has no effect on the CPLM formation. Density functional theory (DFT) calculations performed show the stability of CPLMs with respect to the formation of new bonds between carbon atoms in the chains. DFT calculations using the accurate hybrid B3LYP functional provide an insight into the electronic structure of these new molecules.

Measurements of the ionization efficiency of protons in methane

The amount of energy released by a nuclear recoil ionizing the atoms of the active volume of detection appears “quenched” compared to an electron of the same kinetic energy. This different behavior in ionization between electrons and nuclei is described by the Ionization Quenching Factor (IQF) and it plays a crucial role in direct dark matter searches. For low kinetic energies (below 50~textrm{keV}), IQF measurements deviate significantly from common models used for theoretical predictions and simulations. We report measurements of the IQF for proton, an appropriate target for searches of Dark Matter candidates with a mass of approximately 1~textrm{GeV}, with kinetic energies in between 2~textrm{keV} and 13~textrm{keV} in 100~textrm{mbar} of methane. We used the Comimac facility in order to produce the motion of nuclei and electrons of controlled kinetic energy in the active volume, and a NEWS-G SPC to measure the deposited energy. The Comimac electrons are used as a reference to calibrate the detector with 7 energy points. A detailed study of systematic effects led to the final results well fitted by textrm{IQF}~(E_K)= E_K^alpha ~/~(beta + E_K^alpha ) with alpha = 0.70pm 0.08 and beta = 1.32pm 0.17. In agreement with some previous works in other gas mixtures, we measured less ionization energy than predicted from SRIM simulations, the difference reaching 33% at 2~textrm{keV}.

Plasma wave electron acceleration in IFEL mechanism with quadruple wiggler and ion channel guiding

In this paper, the electron acceleration by plasma wave in IFEL mechanism in the presence of a quadruple magnetic field and ion channel guiding effects has been investigated both analytically and numerically. The results illustrate that the increasing of the quadruple wiggler amplitude and wavelength up to an optimum value has considerable effects on the electron acceleration and its relativistic dynamics in laser ion channel. It was found that in the presence the optimum values of quadruple wiggler amplitude and wavelength, the electron energy gain increases more than 90% than that obtained in the absence of the wiggler. Moreover, the numerical simulations reveal that if the electrons are thrown with higher initial kinetic energy into the wiggler field, they can longer remain in the acceleration phase of the plasma wave and gain more energy from the wave. Therefore, it turns out that, in a laser-plasma accelerator, the electron acceleration and its relativistic dynamics can be tuned by the wiggler field strength, wiggler wavelength, and initial kinetic energy of electrons. The favorable results of studies in this field can be promising for the new generation of the laser-plasma accelerators based on the wiggler structures.

Estimating the Maximum Magnetic Energy of Plasma Current Converted into Runaway Electron Kinetic Energy during Tokamak Disruptions

Estimating the Maximum Magnetic Energy of Plasma Current Converted into Runaway Electron Kinetic Energy during Tokamak Disruptions

Improvements to collisional ionization models for particle-in-cell codes

A novel electron-impact ionization algorithm has been designed for use in particle-in-cell codes. This improved model uses a combination of modified-Bell and relativistic-binary-encounter-Bethe cross sections for greater accuracy and samples the secondary electron kinetic energies from a differential cross section. The algorithm also conserves the number of real-particle ionization events for arbitrary weighted macro-particles, while ionization schemes in existing algorithms are shown to break this condition. Further limitations of existing models have been explored, and the improved model is described within this framework. Benchmarks have been provided to demonstrate the accuracy of this new model.

UV Photoelectron Spectroscopy of Aqueous Solutions.

ConspectusKnowledge of the electronic structure of an aqueous solution is a prerequisite to understanding its chemical and biological reactivity and its response to light. One of the most direct ways of determining electronic structure is to use photoelectron spectroscopy to measure electron binding energies. Initially, photoelectron spectroscopy was restricted to the gas or solid phases due to the requirement for high vacuum to minimize inelastic scattering of the emitted electrons. The introduction of liquid-jets and their combination with intense X-ray sources at synchrotrons in the late 1990s expanded the scope of photoelectron spectroscopy to include liquids. Liquid-jet photoelectron spectroscopy is now an active research field involving a growing number of research groups. A limitation of X-ray photoelectron spectroscopy of aqueous solutions is the requirement to use solutes with reasonably high concentrations in order to obtain photoelectron spectra with adequate signal-to-noise after subtracting the spectrum of water. This has excluded most studies of organic molecules, which tend to be only weakly soluble. A solution to this problem is to use resonance-enhanced photoelectron spectroscopy with ultraviolet (UV) light pulses (hν ≲ 6 eV). However, the development of UV liquid-jet photoelectron spectroscopy has been hampered by a lack of quantitative understanding of inelastic scattering of low kinetic energy electrons (≲5 eV) and the impact on spectral lineshapes and positions.In this Account, we describe the key steps involved in the measurement of UV photoelectron spectra of aqueous solutions: photoionization/detachment, electron transport of low kinetic energy electrons through the conduction band, transmission through the water-vacuum interface, and transport through the spectrometer. We also explain the steps we take to record accurate UV photoelectron spectra of liquids with excellent signal-to-noise. We then describe how we have combined Monte Carlo simulations of electron scattering and spectral inversion with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution to develop an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. The huge potential of our experimental and spectral retrieval methods is illustrated using three examples. The first is a measurement of the vertical detachment energy of the green fluorescent protein chromophore, a sparingly soluble organic anion whose electronic structure underpins its fluorescence and photooxidation properties. The second is a measurement of the vertical ionization energy of liquid water, which has been the subject of discussion since the first X-ray photoelectron spectroscopy measurement in 1997. The third is a UV photoelectron spectroscopy study of the vertical ionization energy of aqueous phenol which demonstrates the possibility of retrieving true photoelectron spectra from measurements with contributions from components with different concentration profiles.

Energetic Origins of Force Constants: Adding a New Dimension to the Hessian Matrix via Interacting Quantum Atoms.

The Interacting Quantum Atoms (IQA) energy decomposition scheme divides the total energy of a molecule into intra- and interatomic contributions. While the former relates to the kinetic and potential energies of electrons inside a unique individual atomic basin, the latter contains the Coulomb and exchange-correlation potentials between electrons from two atomic basins. Considering that the molecular energy is a sum of IQA contributions, the Hessian matrix can also be written as a sum of "IQA Hessian" matrices, whose elements are second derivatives of IQA terms. Herein, we present a mathematical formalism for the IQA decomposition of force constants revealing their energetic origins. The method consists of adding a new dimension to the Hessian matrix, which becomes 3N × 3N × N2, with N being the number of atoms in the molecule and N2 the number of IQA terms. Since there is no analytical method that produces the IQA second derivatives, the three-dimensional IQA Hessian is numerically calculated. When studying molecular vibrations, force constants, providing information about the nature of chemical bond and related to infrared frequencies, can be obtained by Wilson's FG method, which involves detailed manipulations of the Hessian matrix. In this paper, the methodology is reported and validated for a set of 30 molecules and more than 200 force constants and their interactions. Energetic origins of force constants are presented for diatomics and small molecules containing carbon-carbon, oxygen-oxygen, and carbon-oxygen bonds with different bond orders. It is found that bond stability and stiffness can have strikingly different energetic origins.

Multiplicative potentials for kinetic energy and exact exchange.

Harriman showed that within finite basis sets of one-electron functions that form linearly independent products (LIP), differential and integral operators can be represented exactly and unambiguously by multiplicative (local) potentials. Although almost no standard basis sets of quantum chemistry form LIPs in a numerical sense, occupied self-consistent field (SCF) orbitals routinely do so. Using minimal LIP basis sets of occupied SCF orbitals, we construct multiplicative potentials for electronic kinetic energy and exact exchange that reproduce the Hartree-Fock and Kohn-Sham Hamiltonian matrices and electron densities for atoms and molecules. The results highlight fundamental differences between local and nonlocal operators and suggest a practical possibility of developing exact kinetic energy functionals within finite basis sets by using effective local potentials.

Dynamics of Above-Threshold Ionization and Laser-Assisted Electron Scattering inside Helium Nanodroplets.

Laser-assisted electron scattering (LAES) is a fundamental three body interaction process that enables energy transfer between electrons and photons in the presence of matter. Here, we focus on the multiscattering regime of electrons generated by above-threshold ionization (ATI) of In atoms inside a high-density nanostructure, helium nanodroplets (HeN) of ∼40 Å radius. The stochastic nature of the multiscattering regime results in photoelectron spectra independent of laser polarization. Numerical simulations via tunnel-type ionization followed by applying the Kroll-Watson approximation for LAES are in agreement with experimental spectra and yield a mechanistic description of electron generation and the LAES energy modulation processes. We find a negligible influence of the electron start position inside the helium droplet on the simulated electron energy spectrum. Further, our simulations shine light on the interplay of electron time of birth, number of LAES gain/loss events, and final kinetic energy; early ionization leads to the largest number of scattering events and thereby the highest electron kinetic energy.

Improved circuit model for plasmonic resonance of single split-ring resonators

In this work, an improved circuit model is presented for the single split ring resonator (SRR), taking into account non-uniform distribution of conduction current and surface charges. To determine the circuit elements of the SRR, the electron kinetic, electric, and magnetic energy have been derived theoretically. Moreover, in addition to gap and surface capacitances, coupling capacitance is introduced to characterize the electric coupling effect between them. A formula for the fundamental resonance wavelength has been obtained, which shows good agreement with numerical simulations in both optical and microwave frequency ranges. The model will be instructive for engineering resonant properties of the SRRs and provide valuable insight into other complex plasmonic structures.

Diffusion and kinetic energy of electrons accelerated by focused few-cycle azimuthally polarized pulses

Using the model of the focused few-cycle azimuthally polarized ultrashort pulses based on the complex sink-source method, the electron acceleration by the pulses is studied. Under the same peak intensity and beam waist width, the maximum exit kinetic energy of electrons will be increased with the increase of the time domain widths of the pulses. Then, with the further increase of the pulse time domain widths, the maximum exit kinetic energy of the electron will be slowly decreased. The diffusion angle of the electron beam can be as small as 2° and changes little with the carrier envelope phase of the pulse. When the carrier envelope phase is changed, the diffusion angle of the electron beam is reduced by more than 1 order of magnitude with the increase of the time domain widths of the pulses. For the first time, to the best of our knowledge, we found that, by choosing the pulse with the appropriate time domain width, an electron beam with a small diffusion angle and high kinetic energy can be obtained at the same time. When the pulse duration is increased, the radiation spectrum of the acceleration radiation is found to undergo a significant redshift for the first time. These studies can be applied in the fields of high-energy physics experiments, medicine, material detection, and others.

Photoinduced band renormalization effects in the topological nodal-line semimetal ZrSiS

Out-of-equilibrium effects provide an elegant pathway for probing and understanding the underlying physics of topological materials. In particular, controlling electronic band structure properties using ultrafast optical pulses has shown promise for creating exotic states of matter. Of recent interest is band renormalization in Dirac and Weyl semimetals as it leads to direct physical observables through the enhancement of the effective mass or in the shift of resonant energies. Here we provide experimental and theoretical signatures of photoinduced renormalization of the electronic band structure in the topological nodal-line semimetal ZrSiS. Specifically, we show how the change in the transient reflectivity spectra under femtosecond optical excitations is induced by out-of-equilibrium effects that renormalize the kinetic energy of electrons. We associate the observed spectral features with an enhancement of the effective mass and to a redshift of the resonant frequency as a function of pump field strength. Finally, we show that the transient relaxation dynamics of the reflectivity is primarily an electronic effect with a negligible phononic contribution. Our study presents conclusive modifications of electronic properties in ZrSiS using ultrashort pulses and demonstrates the potential of this approach in creating photoinduced phases in topological quantum matter through an all-optical route.