Dust in Plasma II. Effects of Secondary Electrons: Ionization and Surface Emission

In this second paper, the effect of secondary electrons on the charge and potential of a dust particle immersed in plasma has been studied. The processes of electron‐induced ionization and those of photo‐electron and secondary electron emission from the particle surface as a function of primary electron temperature have been taken into account. Starting from temperatures as low as 6 eV in an Ar plasma, ionization produces an extra ion flux to the dust surface comparable to that of the ion charge exchange effect. For what concerns the surface emission, results show that a transition from negative to positive dust charge/potential takes place, and that the transition regime is characterized by a non‐monotonic behavior of the electric potential around the particle. In the case of photoelectric emission, the dust charge and potential are monotonic decreasing functions of the electron temperature, while in the case of emission induced by primary electrons a minimum charge/potential is reached before they grow towards positive values. In no case multiple dust charge states have been observed due to the presence of the potential well attached to the particle surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Emission and accretion of electrons by negatively charged dust particles in a plasma

Taking into account the image force on the electron and Debye shielding an expression for the reduction in the work function for a spherical metallic particle in a dusty plasma on account of the negative charge on the surface has been derived. The dependence of the reduction of the work function on the charge on the particle and Debye radius has been tabulated. The effect of this reduction on the thermionic and photoelectric emission and accretion of electrons by the particle has been discussed. As an interesting application of the work, the observed enhancement of the photoelectric emission from individual micron size grains with increasingly negative surface potential has been explained; the grains were selected from samples of dust collected in the Apollo-17 and Luna-24 missions.     

Fermi Arcs and DC Transport in Nanowires of Dirac and Weyl Semimetals

The transport properties and electron states in cylinder nanowires of Dirac and Weyl semimetals are studied paying special attention to the structure and properties of the surface Fermi arcs. The latter make the electric charge and current density distributions in nanowires strongly nonuniform as the majority of the charge density is accumulated at the surface. It is found that a Weyl semimetal wire also supports a magnetization current localized mainly at the surface because of the Fermi arcs contribution. By using the Kubo linear response approach, the direct current (DC) conductivity is calculated and it is found that its spatial profile is nontrivial. By explicitly separating the contributions of the surface and bulk states, it is shown that when the electric chemical potential and/or the radius of the wire is small, the electron transport is determined primarily by the Fermi arcs and the electrical conductivity is much higher at the surface than in the bulk. Due to the rise of the surface-bulk transition rate, the relative contribution of the surface states to the total conductivity gradually diminishes as the chemical potential increases. In addition, the DC conductivity at the surface demonstrates noticeable peaks when the Fermi level crosses energies of the surface states.     

The surface potential of insulating thin films negatively charged by a low-energy focused electron beam

We report on the surface potential characteristics in the equilibrium state of the grounded insulating thin films of several 100 nm thickness negatively charged by a low-energy (<5 keV) focused electron beam, which have been simulated with a newly developed two-dimensional self-consistent model incorporating electron scattering, charge transport and charge trapping. The obtained space charge is positive and negative within and outside the region, respectively, where the electron and hole densities are greater than the trap density. Thus, the surface potential is relatively high around the center, then it decreases to a maximum negative value and finally tends to zero along the radial direction. The position of the maximum value is far beyond the range of e-beam irradiation as a consequence of electron scattering and charge transport. Moreover, a positive electric field can be generated near the surface in both radial and axial directions. The surface potential at center exhibits a maximum negative value in the condition of the ～2 keV energy non-penetrating e-beam in this work, which is supported by some existing experimental data in scanning electron microscopy. Furthermore, the surface potential decreases with the increase in beam current, trap density and film thickness, but with the decrease in electron mobility.     

Particle dynamics simulations of triboelectric charging in granular insulator systems

Particle dynamics simulations are carried out to study triboelectric charging in granular systems composed of a single insulating material. The simulations implement a model in which electrons trapped in localized high energy states can be transferred during collisions to low energy states in the other particle. It is shown that this effect alone can generate electrostatic charging in the system, and cause net electron transfer from larger particles to smaller particles. The magnitude of charging is small for systems of a single particle size but becomes much greater for a system with polydispersal particle sizes, due to the net electron transfer from larger to smaller particles. The negative charge of smaller particles, and positive charge of larger particles has been observed in field studies and laboratory experiments of granular systems.     

Diagnostics of many-particle electronic states: Non-stationary currents and residual charge dynamics

We propose the method for identifying many particle electronic states in the system of coupled quantum dots (impurities) with Coulomb correlations. We demonstrate that different electronic states can be distinguished by the complex analysis of localized charge dynamics and non-stationary characteristics. We show that localized charge time evolution strongly depends on the properties of initial state and analyze different time scales in charge kinetics for initially prepared singlet and triplet states. We reveal the conditions for existence of charge trapping effects governed by the selection rules for electron transitions between the states with different occupation numbers.     

Effective charge of a macroparticle in a non‐isothermal plasma within the Poisson–Boltzmann model

The electrostatic potential distribution around a charged, spherical, finite‐size macroparticle in a non‐isothermal plasma‐like medium is studied numerically within the Poisson–Boltzmann model. It is assumed that plasma consists of electrons and one species of singly charged ions. The effective charge of a macroparticle is calculated and its dependence on the electron to ion temperature ratio as well as on the particle radius and bare charge is considered. Numerical results for the effective charge in an isothermal plasma are compared with known analytical expressions.     

The positive charging effect of dielectric films irradiated by a focused electron beam

Space charge and surface potential profiles are investigated with numerical simulation for dielectric films of SiO₂ positively charged by a focused electron beam. By combining the Monte Carlo method and the finite difference method, the simulation is preformed with a newly developed comprehensive two-dimensional model including electron scattering, charge transport and trapping. Results show that the space charge is distributed positively, like a semi-ellipsoid, within a high-density region of electrons and holes, but negatively outside the region due to electron diffusion along the radial and beam incident directions. Simultaneously, peak positions of the positive and negative space charge densities shift outwards or downwards with electron beam irradiation. The surface potential, along the radial direction, has a nearly flat-top around the center, abruptly decreases to negative values outside the high-density region and finally increases to zero gradually. Influences of electron beam and film parameters on the surface potential profile in the equilibrium state are also shown and analyzed. Furthermore, the variation of secondary electron signal of a large-scale integration sample positively charged in scanning electron microscopic observation is simulated and validated by experiment.     

Quantization of electronic states on metal surfaces

An electron in front of a metal surface experiences an attractive force due to the induced image charge. Band gaps in the band structure can prevent a penetration into the metal along certain directions. The Coulomb-like potential supports bound states in front of the surface which correspond to a hydrogen atom in one dimension. These image states can be measured with high resolution by two-photon photoemission. The adsorption of metals modifies the states. If the electrons can penetrate into the metal, quantum-well states can develop corresponding to standing waves in the overlayer. Image states on small islands show the quantization effects due to the lateral localization. The spectroscopy of image states by two-photon photoemission permits the investigation of growth and morphology of deposited metal layers, a well as the illustration of fundamental quantum-mechanical effects.     

尘埃颗粒与等离子体鞘层相互作用的数值模拟

通过求解一维稳态的尘埃等离子体鞘层模型,得到了等离子体鞘层势、正离子密度、电子密度和尘埃颗粒密度随一维横向的分布,Bohm判据及鞘层边界无量纲Bohm速度随尘埃密度的变化曲线,尘埃颗粒的带电量和尘埃密度的关系,尘埃颗粒的温度对尘埃颗粒自身在鞘层中分布的影响。结果表明,随着尘埃密度的增加,鞘层的厚度在减小,鞘层内的电子密度在下降,而且尘埃颗粒的带电量也在逐渐减少;随着尘埃温度的增加,鞘层的厚度减小,电子密度下降,而且鞘层附近的尘埃颗粒在逐渐增多。     

Double layers of H2 adsorption on an AlN sheet induced by electric field

In this paper, we propose an original functional method that makes it easy to determine the effect of any deviation in the shape of a nano-object from the well-studied shape (e.g., spherical) on the quantum characteristics of charge localized inside the nano-object. The maximum dimension of the object is determined by the magnitude of influence of quantum-size effects on quantum states of charge, and is limited by 100?nm. This method is ideologically similar to the perturbation theory, but the perturbation of the surface shape, rather than the potential, is used. Unlike the well-known variational methods of theoretical physics, this method is based on the assumption that the physical quantity is a functional of surface shape. Using the method developed, we present the quantum-size state of charges for two different complex shapes of nano-objects. The results from analyzing the quantum-size states of charge in the nano-objects with a deformed spherical shape indicated that the shape perturbations have a larger effect on the probability density of locating a particle inside the nano-object than on the surface energy spectrum and quantum density of the states.     

Radiation of a charge moving over the diffraction grating

The states of a charged particle with a finite free path are determined in the field of a resonant electromagnetic wave. The exact resonance conditions, the modulation and beam instability mechanisms, the charge and current densities (Ohm's law) are obtained for the collisionless beam of resonance particles. Quantum theory of radiation is developed for the resonant adiabatic interaction between a particle and a wave taking into account the interaction with a constant magnetic field induced at the grating surface by the charge and nonresonant waves. The radiation power, the spectrum, and the range of generated frequencies are determined. The results obtained can be used in the plasma and solid-state theories and in electronics.     

Three-dimensional tomography of the beryllium fermi surface: surface charge redistribution

The discontinuity in the lattice periodic potential at surfaces often leads to the creation of new electronic surface states. We developed a photoemission based Fermi surface tomography whose surface sensitivity allowed us to quantify the charge redistribution on the Be(0001) surface. The volume enclosed by the bulklike Fermi surface is significantly reduced at the surface, consistent with the charge transfer to the two surface states as estimated from the area within their two-dimensional Fermi contours. This result represents the first quantification of the charge redistribution on a natural surface termination.     

Scenario of charge distributions of f-p shell nuclei

Using the experimental occupation probabilities for the single particle states near the Fermi surface, the charge distribution of⁵⁴Fe,^58–64Ni and^64,66Zn have been calculated on the basis of an optimized one body potential. Excellent agreement with the experimental values have been found both in the case of single particle (-hole) energies and rms charge radii of the nuclei under consideration. Calculated values of Barrett moments and equivalent radii are in good agreement with the experiment. The details of the charge distributions have been evaluated by Fourier-Bessel analysis.The author wishes to thank the Research and Publication Committee of University of Botswana for a research grant. The interesting comments of the respected referee for the betterment of the paper is also acknowledged with thanks.     

高电荷态离子40Arq+与Si表面作用中的电子发射产额

报道了在兰州重离子加速器国家实验室电子回旋共振离子源原子物理实验平台上，用高电荷态⁴⁰Ar^q+(1≤q≤12)离子作用于半导体Si固体表面时的电子发射产额实验测量.实验中，通过改变炮弹离子的电荷态和引出电压选取其不同的势能和动能，系统地研究了入射离子势能沉积和与其在固体中的电子能损对表面电子发射产额的贡献.结果表明，作为引起表面电子发射的两个主要因素，单离子的电子发射产额与炮弹离子在固体表面的势能沉积和电子能损都有近似的正比关系.     

Relative energies of surface and defect states: ab initio calculations for the MgO (001) surface

We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F⁺ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.     

Self-energy of a charge near a metal surface

A quantum mechanical expression for the self-energy of a charged particle near a metal surface has been derived. The expression incorporates the interaction of the charge with the surface and bulk plasmons including their dispersions in the hydrodynamic model. It is found that the inclusion of the plasmons dispersions gives result for the saturated value of the image potential for copper closer to experiment.     

Nonequilibrium accumulation of surface species and triboelectric charging in single component particulate systems

Triboelectric charging occurs in granular systems composed of chemically identical particles even though there is no apparent driving force for charge transfer. We show that such charging can result from nonequilibrium dynamics in which collision-induced electron transfer generates electron accumulation on a particle-size-dependent subset of the system. This idea rationalizes experimental results that suggest that smaller particles charge negatively while the large ones charge positively. This effect occurs generally when there are high energy electrons on a surface that cannot equilibrate to lower energy states on the same surface, but can transfer to lower energy states on other particles during collisions.     

Influence of the promotion of terms on the emission of auger electrons upon collisions of multicharged ions with a metal surface

Upon the collision of multicharged ions with a metal surface there occurs the formation of a dipole, generated from an ion and an induced charge in the metal. Strong rearrangement of electron shells takes place in the dipole field, which leads to the “promotion” of terms generated from states with a principal quantum number n = 2, 3. When the promoted terms cross with the levels of metal atoms, an electron is captured into these states. This effect makes it possible to explain the observed Auger electron spectra.