Wide Range Of Electron Energies Research Articles

Fully relativistic distorted wave calculations of electron impact excitation of gallium atom: Cross sections relevant for plasma kinetic modeling

Electron impact excitation cross sections of neutral Ga are calculated using the relativistic distorted wave approximation with the aid of multiconfigurational Dirac-Fock wavefunctions. The cross sections are calculated from the fine structure resolved levels of the 4s24p manifold, i.e., from the ground 2P1/2 and the first excited state 2P3/2, to the fine structure resolved levels of the manifolds 4s24pdf, 4s4p2(4P1/2,4P3/2, and 4P5/2), 4s25spdf, 4s26spdf, 4s27spdf, and 4s28spdf for incident electron energies ranging from threshold to 500 eV. Where available, a comparison of the calculated oscillator strengths, transition probabilities, and cross sections with the previously reported results has been performed. The cross sections for the excitation from 4s24p2P1/2 to 4s24f, 4s4p2(4P1/2,4P3/2, and 4P5/2), 4s25f, 4s26f, 4s27df, and 4s28spdf and all the excitation cross sections from 4s24p2P3/2 are calculated and reported for the first time for a wide range of electron energies. Furthermore, the rate coefficients are also calculated using the present cross sections for an electron temperature varying from 0.5 to 5 eV. An analytic fitting of the calculated rate coefficients is provided to facilitate their inclusion in various plasma kinetic models.

A unified description of atomic physics for electron Fokker–Planck calculations

Most realistic kinetic calculations for tokamak plasmas are now required to incorporate the effect of partially ionized high-Z elements arising either from uncontrolled influxes of metallic impurities, such as tungsten in high input power regimes or from mitigation of runaway electrons generated after possible major disruptions by massive gas injection. The usual electron–ion Fokker–Planck collision operator must therefore be modified, because all plasma atoms are not entirely ionized, as is the case for light elements. This represents a challenge, in order to perform fast but also accurate calculations, regardless of the type of element present in the plasma, but also their local levels of ionization while covering a wide range of electron energies in a consistent way, from a few keV to tens of MeV in plasmas whose electron temperature may itself vary from 10 eV to several keV. In this context, a unified description of the atomic models is proposed, based on a multi-Yukawa representation of the electrostatic potential calibrated against results obtained by advanced quantum calculations. Besides the possibility to improve the description of inner and outer atomic shells in the determination of the atomic form factor, this model allows one to derive analytical formulations for both elastic and inelastic scattering, which can then be easily incorporated in kinetic calculations. The impact of the number of exponentials in the description of the atomic potential is discussed, and a comparison with simple and advanced atomic models is also performed.

Relativistic calculation of electron impact excitation of tin: cross sections of importance in plasma modeling

The present work includes the calculation of electron impact excitation cross sections for the fine structure resolved transitions in Sn I and Sn II, using the relativistic distorted wave approximation with the aid of Dirac-Fock multiconfigurational wavefunctions. For Sn I, the cross sections are calculated for the excitations from the fine structure resolved energy levels of the manifolds, , , , and including the ground state, to the fine structure resolved levels of , , , , and manifolds for a wide range of projectile electron energy from 0 to 500 eV. Further, the cross sections are also calculated for the excitations of Sn II from the fine structure resolved energy levels of the manifolds, , , , and including the ground state, to the fine structure resolved levels of , , , , and manifolds. All the cross sections for Sn I except for the excitation from the ground state to the fine-structure resolved excited states of , and manifolds and all the cross sections for Sn II are reported for the first time. In order to ensure the consistency of the present calculations, a comparison of the oscillator strengths and transition probabilities is carried out with the available calculations and measurements. The present cross sections are also compared with previous calculations and observed to be in reasonable agreement. The calculated cross sections are of great interest in the modeling and diagnostic of various plasmas such as laser induced plasma, fusion plasma, etc. Further, to ease the use of calculated cross sections, the analytic fitting of the cross sections is also provided.

Сцинтилляторы для прецизионных y-спектрометров

The article analyzes the possibilities of using modern scintillators that provide a relative energy resolution along the y-line of 137Cs ER662keV ≤ 5%. Precision gamma spectrometers are relevant for use in technological processes of the nuclear industry, at nuclear power plants for monitoring the activity of air, waste water and adjacent areas, etc.
 Now 36 scintillation crystals are known that meet the above mentioned requirement. Of these, only 8 are produced by industry. Seven crystals are candidates for development in industrial production. The rest are hardly suitable for practical use due to the low light yield of LY (Cs2NaGdCl6:Ce, Tl2LiYCl6:Ce and TlCaCl3, for example). Eu-activated crystals have a high light output (LY = 60÷100 ph/MeV) and a low light output nonlinearity (NLY = 7÷10%) in a wide range of electron energies, which is extremely important for the spectrometry of “soft” y-radiation (ER662keV ≤ 10%). Due to the high self-absorption of light, these scintillators lose their advantages in the energy range Ey = 200÷3000 keV.
 The article shows that among the scintillators suitable for the simultaneous detection of gamma and neutron radiation, crystals containing 6Li are more preferable.

Electron emission from bromouracil and uracil induced by protons and radiosensitization

Absolute double differential cross sections (DDCS) of electrons emitted from uracil and 5-bromouracil (BrU) in collisions with protons of energy 200 keV have been measured for various forward and backward emission angles over wide range of electron energies. The measured DDCS are compared with the continuum distorted wave-eikonal initial state (CDW-EIS) calculations. The optimized structure of the BrU was estimated along with the population analysis of all the occupied orbitals using a self-consistent field density. A comparison between the measured DDCS data for the two molecules show that the cross section of low energy electrons emitted from BrU is substantially larger than that for uracil. The BrU-to-uracil DDCS ratios obtained from the present measurements indicate an enhancement of the electron emission by a factor which is as large as 2.0 to 2.5. These electrons being the major agent for damaging the DNA/RNA of the malignant tissues, the present results are expected to provide an important input for the radiosensitization effect in hadron therapy. It is noteworthy to mention that the CDW-EIS calculations for Coulomb ionization cannot predict such enhancement. A large angular asymmetry is observed for uracil with a broad structure, which is absent in case of BrU.

Inclusion of Electron Interactions by Rate Equations in Chemical Models

The concept of treating subranges of the electron energy spectrum as species in chemical models is investigated. This is intended to facilitate simple modification of chemical models by incorporating the electron interactions as additional rate equations. It is anticipated that this embedding of fine details of the energy dependence of the electron interactions into rate equations will yield an improvement in computational efficiency compared to other methods. It will be applicable in situations where the electron density is low enough that the electron interactions with chemical species are significant compared to electron–electron interactions. A target application is the simulation of electron processes in the D-region of the Earth’s atmosphere, but it is anticipated that the method would be useful in other areas, including enhancement of Monte Carlo simulation of electron–liquid interactions and simulations of chemical reactions and radical generation induced by electrons and positrons in biomolecular systems. The aim here is to investigate the accuracy and practicality of the method. In particular, energy must be conserved, while the number of subranges should be small to reduce computation time and their distribution should be logarithmic in order to represent processes over a wide range of electron energies. The method is applied here to the interaction by inelastic and superelastic collisions of electrons with a gas of molecules with only one excited vibrational level. While this is unphysical, it allows the method to be validated by checking for accuracy, energy conservation, maintenance of equilibrium and evolution of a Maxwellian electron spectrum.

Measuring the photoelectron emission delay in the molecular frame

How long does it take to emit an electron from an atom? This question has intrigued scientists for decades. As such emission times are in the attosecond regime, the advent of attosecond metrology using ultrashort and intense lasers has re-triggered strong interest on the topic from an experimental standpoint. Here, we present an approach to measure such emission delays, which does not require attosecond light pulses, and works without the presence of superimposed infrared laser fields. We instead extract the emission delay from the interference pattern generated as the emitted photoelectron is diffracted by the parent ion’s potential. Targeting core electrons in CO, we measured a 2d map of photoelectron emission delays in the molecular frame over a wide range of electron energies. The emission times depend drastically on the photoelectrons’ emission directions in the molecular frame and exhibit characteristic changes along the shape resonance of the molecule.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane.

Low-energy (0-15eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ- are detected in a wide electron energy range (0-3eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

Electron scattering cross-sections for particle transport modeling in a weakly ionized air plasma

Data on processes of electron scattering on ions and neutral atoms are required in fundamental studies and in applied research in such fields as astro- and laser physics, low density plasma simulations, kinetic modeling etc. Experimental and computational data on elastic and inelastic electron scattering in a wide range of electron energies is available mostly for the electron interaction with neutral atoms, but are very limited for the scattering on ions, notably for elastic processes. In present work the calculational approaches for the cross-section computation of electron elastic and inelastic scattering on neutral atoms and ions are considered. The atomic and ion properties obtained in quantum-statistical Hartree-Fock-Slater model are used in the direct computation of electron elastic scattering and ionization cross-sections by a partial waves method, semiclassical and distorted-wave approximations. Calculated cross-sections for elastic scattering on nitrogen and oxygen atoms and ions, and electron ionisation cross-sections are compared with the available experimental data and widely used approximations and propose consistent results. Considering applicability of Hartree-Fock-Slater model in wide scope of temperatures and densities, such approach to the cross-section calculation can be used in a broad range of energies and ion charges.

Vacuum ultraviolet coherent undulator radiation from attosecond electron bunches

Attosecond duration relativistic electron bunches travelling through an undulator can generate brilliant coherent radiation in the visible to vacuum ultraviolet spectral range. We present comprehensive numerical simulations to study the properties of coherent emission for a wide range of electron energies and bunch durations, including space-charge effects. These demonstrate that electron bunches with r.m.s. duration of 50 as, nominal charge of 0.1 pC and energy range of 100–250 MeV produce 10^9 coherent photons per pulse in the 100–600 nm wavelength range. We show that this can be enhanced substantially by self-compressing negatively chirped 100 pC bunches in the undulator to produce 10^{14} coherent photons with pulse duration of 0.5–3 fs.

Electron Dynamics and Thomson Scattering for Ultra-Intense Lasers: Elliptically Polarized and OAM Beams

We investigated the classical nonlinear Thomson scattering (TS), from a single relativistic electron, generated by either: (a) an incoming plane wave monochromatic laser radiation and general elliptical polarization or (b) incoming radiations with intrinsic orbital angular momentum (OAM). Both (a) and (b) propagate along the z direction, with wave vector k0, frequency ω0, and initial phase φ0≠0 and have any intensity. Item (a) enables obtaining general electron TS Doppler frequencies and other quantities, for fusion plasmas. We explored the possibility of approximating nonlinear TS with OAM beams (Item (b)) by means of nonlinear TS with plane wave beams (Item (a)). For Item (a), a general explicit solution of the Lorentz relativistic equation and the subsequent TS are given in terms of ζ=ω0t−k0z (t denoting time). In particular, it includes the cases for linear and circular polarizations and φ0≠0 for fusion plasmas, thereby extending previous studies for φ0=0. The explicit solutions give rise to very efficient computations of electron TS Doppler frequencies, periods of trajectories, and drift velocities, and the comparisons with ab initio numerical solutions (for Item (a)) yield an excellent match. The approximate approach, using explicit solutions for Item (a), towards TS OAM (employing ab initio numerical computations for Item (b)), extending previously reported ones) yields a quite satisfactory agreement over time spans including several optical cycles, for a wide range of laser intensities, polarizations, and electron energies. The role of φ0≠0 was analyzed. A simple quantitative criterion to predict whether the agreement between the two approaches (a) and (b) would be observed over a given time span is discussed.

Плотность энергетического спектра электрона в поле изображения и запирающем электрическом поле

In the semiclassical approximation, the density of the electron energy spectrum near the metal surface is described, when electron is bound by the image field and the blocking electrostatic field. In the system under consideration, the confinement mechanism is realized, and the energy spectrum for the motion of an electron in the direction perpendicular to the metal surface is completely discrete. The density of states of the energy spectrum is expressed in terms of elliptic integrals, the argument of which is a sigmoidal function. When the field is turned off, it becomes the Heaviside step function. A dimensionless energy parameter is introduced, which determines the intervals with qualitatively different changes in the width of the classically accessible region of motion. For large positive values of the energy parameter, the spectrum density asymptotically tends to the density in the triangular potential with the addition of the Coulomb logarithmic correction, and for negative values of the energy parameter, the spectrum density tends to dependence for a one-dimensional Coulomb potential. Approximate expressions are obtained for the spectrum density in terms of elementary functions in a wide range of electron energies and electric field strength.

Спектроскопия потерь энергии отраженных электронов γ-Fe-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

The reflection electron energy losses spectra, obtained in a wide primary electron energy range of 200 - 3000 eV, are investigated. From these experimental spectra, for each primary electron energy, the spectra of the inelastic scattering cross section of electrons are calculated as the dependence of the product of the inelastic electron mean free path and the differential inelastic scattering cross section of electrons on the electron energy loss. The analysis of the fine structure of the reflection electron energy losses was carried out by decomposing the electron energy losses spectra in the region of energy losses of valence electrons into elementary peaks. A relationship is established between each of their elementary peaks with single and multiple energy losses due to the excitation of bulk and surface plasmons and interband transitions of electrons from the valence band to free states above the Fermi level. The analysis of the obtained results was carried out on the basis of experimental and theoretical literature data on the band structure of  Fe2O3.

Electrophoretically Deposited Graphene-Based Composite Coatings for Spacecraft Charging Mitigation

Spacecraft charging has been recognized as an important consideration for spacecraft design, and occurs due to interaction between hazardous space plasmas and spacecraft surface. Spacecraft can experience charging throughout operation due to high flux of incident electrons (ex. during geomagnetic sub-storm). As a result, different materials/components may experience a range of potentials which may lead to plasma-induced arcs, damaging spacecraft components. Graphene are emerging two-dimensional (2D) carbon materials with excellent physical and chemical properties, such as high conductivity, high tensile strength, high transparency, and high carrier mobility. These excellent properties make graphene potentially useful for applications in energy storage, aerospace, coatings and polymer markets and thermal protection. Substrates constituted by graphene harness the thermal and mechanical properties to create novel coatings with superior performance, hence broadening the graphene application spectrum. Of particular interest is the potential capability of graphene-based materials to provide increased protection against spacecraft charging in the space plasma environments. Furthermore, the work function of graphene can be tuned by either metal doping or functionalization, thereby facilitating autonomous or passive electron emission and increasing the electron yields and enhancing the possibility of emission through negative electron affinity. By enabling charge dissipation between the various components at modest potential differences, discharge arcing may be preventedWithin this context, Faraday Technology is developing a graphene-based composite coating with high electron yields over a wide range of incident electron energies. This approach utilizes electrophoretic deposition to deposit graphene-based coatings onto test substrates, which may maintain integrity of spacecraft by autonomous electron emission in relevant environments. Next, an electrochemical process is used to decorate the graphene surface with various low work function metals (alkali, alkaline earth, transition). This approach offered significant improvements in the electron yield (290% increase over the undecorated coating) and substantially extended the range (~9 times) of incident electron energies between crossover points as shown in Figure 1/Table 1.Acknowledgements: The financial support of DOD Air Force Contract No.: FA9453-19-P-0573 is acknowledged. Figure 1

Theoretical derivation and benchmarking of cross sections for low-energy electron transport in gold

Simulation of transport of electrons through matter is used in many applications. Some of them need models that are both efficient in terms of computing time and accurate over a wide range of electron energy. For certain applications such as radiochemistry and radiotherapy enhanced by metallic nanoparticles, it is desirable to consider relatively low-energy electrons. We have implemented a physical model for electron transport down to low energy in solid metallic media that meets both of the aforementioned requirements. The main goal of this paper is to present the theoretical framework of our Monte Carlo simulation, its application to gold metal and an extensive comparison with available data for gold foils irradiated by electron beams, for projectile energies ranging from a few eV to 90 keV. In particular, we calculated secondary electron emission to assess the accuracy of our code at energies below 50 eV. A close agreement with experiment is obtained for a large range of energy, even though the backward emission yields of low-energy electron are systematically underestimated. Nevertheless, the quality and numerical efficiency of the simulation are encouraging for nanoscale applications such as nanodosimetry or radiochemistry in presence of gold nanoparticles.

Spin-dependent electron reflection at W(110)

Spin-dependent reflection of low-energy electrons at the W(110) surface caused by spin–orbit interaction was studied experimentally and theoretically. Comprehensive information for a wide range of electron incidence angles and energies was collected via maps for the reflectivity, the spin-dependent reflection asymmetry, and the figure of merit of the spin separation. The experimental results are compared with calculations of the scattering process using a realistic surface potential barrier. The results are discussed in view of possible applications of W(110) as a scattering target in spin-polarization detectors. Possible working points for use in single- as well as multi-channel spin-polarization-detection devices are identified and discussed.

Improved calculations of electron–ion bremsstrahlung Gaunt factors for astrophysical applications

ABSTRACT Electron–ion bremsstrahlung (free–free) emission and absorption occur in many astrophysical plasmas for a wide range of physical conditions. This classical problem has been studied multiple times, and many analytical and numerical approximations exist. However, accurate calculations of the transition from the non-relativistic to the relativistic regime remain sparse. Here we provide a comprehensive study of the free–free Gaunt factors for ions with a low charge (Z ≤ 10). We compute the Gaunt factor using the expressions for the differential cross-section given by Elwert and Haug (EH) and compare to various limiting cases. We develop a new software package, BRpack, for direct numerical applications. This package uses a combination of pre-computed tables and analytical approximations to efficiently cover a wide range of electron and photon energies, providing a representation of the EH Gaunt factor to better than $0.03{{\ \rm per\ cent}}$ precision for Z ≤ 2. Our results are compared to those of previous studies highlighting the improvements achieved here. BRpack should be useful in computations of spectral distortions of the cosmic microwave background, radiative transfer problems during reionization or inside galaxy clusters, and the modelling of galactic free–free foregrounds. The developed computational methods can furthermore be extended to higher energies and ion charge.

A study on the validity of the first Born approximation for high-energy electron scattering with nitrogen molecules

The first Born approximation is one of the most extensively used principles to deal with scattering processes, especially for fast projectiles. In the case of fast electron scattering, an empirical criterion has been proposed and widely accepted that a good match among the fast electron scattering results with higher and higher incident electron energies is enough to demonstrate the validity of the first Born approximation. However, it is found that the mutually agreed experimental generalized oscillator strength of our electron scattering measurement and the cited ones of the and states of nitrogen at a wide range of incident electron energies is obviously smaller than the generalized oscillator strength obtained by our vibronically resolved inelastic x-ray scattering experiment, in which the first Born approximation is always satisfied. Such a phenomenon indicates that the empirical criterion is not strict, and our understanding of electron scattering with multi-electron system is far from complete.

Diagnostics of low-temperature neon plasma through a fine-structure resolved collisional–radiative model

A collisional–radiative (CR) model for low-temperature Ne plasma is developed. Various radiative and collisional processes involving the ground 2p6 and the excited 2p53s, 2p53p, 2p53d, 2p54s and 2p54p states are considered in the plasma. First, we calculate a complete set of required electron-impact excitation cross-sections of Ne, which is an important process in such low-temperature plasma. We used the relativistic distorted wave (RDW) theory and calculated electron excitation cross-sections for the transitions from the ground 2p6 state to the excited 2p53s, 2p53p, 2p53d, 2p54s and 2p54p states, as well as from the excited state 2p53s to the 2p53p and 2p54p excited states of the Ne atom in a wide range of incident electron energies from the threshold to 500 eV. The ground and different excited states of the Ne are represented through the multiconfiguration Dirac–Fock wave functions, which are obtained using the GRASP2K code. To ascertain the reliability of the obtained wave functions, we calculated the oscillator strengths for different dipole allowed transitions, and compared them with the available experimental and theoretical results. Further, the calculated detailed RDW cross-sections are presented and compared with the available experimental and other theoretically calculated values. The cross-sections for 2p53s to 2p54p and 2p53s (J = 1 only) to 2p53p are reported for the first time. The complete set of calculated electron excitation cross-sections of different transitions of Ne along with other processes are used to develop the CR model. The model has been applied to the diagnostics of low-temperature Ne plasma by coupling it to the optical emission and absorption measurements of Boffard et al (2012 J. Phys. D: Appl. Phys. 45 382001, and 2009 Plasma Sources Sci. Technol. 18 035017). The extracted values of electron density, electron temperature and the calculated 1si level populations have been compared with the corresponding measurements available in the pressure range of 5–25 mTorr and are found to be in excellent agreement.

Electron impact elastic scattering from methane and silane molecules

Analytical expressions for the static potentials of the electron scattering from methane (CH4) and silane (SiH4) molecules have been obtained using Gaussian wave functions. Subsequently, we have studied the elastic scattering of electrons from these molecules utilizing their static potentials along with exchange and polarization potentials as well as a purely imaginary absorption potential to represent scattering into inelastic channels. The multi-centre target molecules are represented as STO-6G Gaussian wave functions. We have calculated the differential, integrated and momentum transfer cross sections in a wide range of incident electron energies from 10 to 500 eV. Comparisons of our results for CH4 and SiH4 with the available measurements and theoretical results are presented.