Density dependence of electron drift velocities in helium and hydrogen at 77.6 K

Electron drift velocities have been measured in helium and hydrogen at 77.6 K and gas density of 6.6×10²¹ cm^?3 (approximately 80 atm). At these high densities the electron drift velocities do not depend only on the ratio of the electric field to gas density (E/N). At constantE/N the electron drift velocity decreases with increasing gas density. In helium a decrease was found down to 6.4% of the value at low density, in hydrogen down to 0.52%. The results are discussed in terms of theories of multiple scattering. Legler's theory fits our data in the lower density range, but at the highest densities predicts too small an effect. The percolation theory by Eggarter and Cohen gives no agreement with the experiment. Up to the highest densities we did not find bubbles; slow negative charge carriers could be identified as oxygen ions.     

Electron temperature determination in LTE and non-LTE plasmas

A method is presented for calculating electron temperatures (T_e) in dense plasmas, which does not assume equivalence with the excited level distribution temperatures (T_ex). The method involves the upper-level Saha ionization equation at the ionization limit, the limiting weighted population density (N_I/g_I) obtained from measured population densities and the experimentally obtained electron density. Electron temperatures calculated for 0.1-bar hydrogen and 1-atm helium and argon arcs are found to be up to twice as large as excited level distribution temperatures. For subatmospheric argon arcs, the calculated T_e are equivalent to the excitation temperature of the middle levels, but are two to three times smaller than the quoted T_ex for the highest levels. Reasons are discussed for the apparent invisibility of true electron temperatures and for differences between them and the excitation temperatures.     

Electron capture in the collision of a proton with a hydrogen atom

Electron capture in the collision of a proton with a hydrogen atom is investigated. The probability of electron capture is calculated from first principles by the direct solving of the three-dimensional nonstationary Schr?dinger equation. The dependence of the probability of electron capture by a proton on the proton??s velocity and impact parameter is obtained and analyzed thanks to highly efficient computations with the use of graphic processing units.     

Polarization ofn=3-terms after electron capture by beam-foil interaction

The electron capture by protons into excitedn=3 states as function of the distance from the surface is calculated in OBK-approximation. The target electrons are treated as free electron gas bounded by a step potential. Electron loss processes are considered by weighting the density matrix with a survival probability for different capture depths. The result is used to determine the Stokes parameters of the emitted light. Comparison is made with polarization measurements of HeI (n=3) states after beam foil excitation.     

Langmuir探针诊断低压氢等离子体电子密度与温度

为研究实验参数对螺旋波诱导的低压氢等离子体状态的影响,用Langmuir探针对等离子体伏安特性曲线进行了原位诊断,采用双曲正切函数的指数变换模型拟合曲线,根据Druyvesteyn方法得到状态参数电子密度、有效电子温度和电子能量几率函数,分析了它们随实验参数的变化规律。结果表明:射频输入功率、气压和约束磁场对等离子体状态有较大影响。随着射频射入功率增大,放电模式发生转变,电子密度跳跃增长;随着气压增大,电子密度先增大后减小,1.5 Pa为最佳电离气压,随约束磁场的增强呈线性增长;有效电子温度随功率和气压的增大而下降,随约束磁场的增强线性降低,电子能量几率函数曲线峰位和高能部分都向低能移动,与有效电子温度变化规律吻合。     

Fluid Model of a Sheath Formed in Front of an Electron Emitting Electrode Immersed in a Plasma with Two Electron Temperatures

The formation of a sheath in front of a negatively biased electrode (collector) that emits electrons is studied by a one‐dimensional fluid model. Electron and ion emission coefficients are introduced in the model. It is assumed that the electrode is immersed in a plasma that contains energetic electrons. The electron velocity distribution function is assumed to be a sum of two Maxwellian distributions with two different temperatures, while the ions and the emitted electrons are assumed to be monoenergetic. The condition for zero electric field at the collector is derived. Using this equation the dependence of electron and ion critical emission coefficients on various parameters ‐ like the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons ‐ is calculated for a floating collector. A modification of the Bohm criterion due to the presence of hot and emitted electrons is also given. The transition between space charge limited and temperature limited electron emission for a current‐carrying collector is also analyzed. The critical potential, where this transition occurs, is calculated as a function of several parameters like the Richardson emission current, the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

Scattering of electrons by charged dislocations in InSb-type semiconductors

Electron scattering by parallel arrays of charged dislocations in InSb-type semiconductors is considered. In the theoretical approach the nonparabolic structure of the conduction band of the semiconductors considered is taken into account in the approximation of a simplified Kane's band model. The effect of screening by free electrons of the dislocation charge is also included in the theory. The calculated relaxation time of the electrons is used to derive a formula for the dislocation-limited electron mobility in the semiconductors. Some examples of calculations of the charged dislocation-limited mobility as a function of the electron concentration with the dislocation density as a parameter are given for n-InSb at 77 K and 300 K. The ratio of the magnitudes of the charged dislocation-limited mobility to the ionized impurity-limited mobility in n-type material at low temperatures is also discussed.     

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The electron densities in the atmospheric pressure helium plasma were calculated by means of electron drift velocity and the jet velocity respectively. The electron velocity and jet velocity can be calculated by means of helium plasma jet current measured by a dielectric probe and plasma discharge current signal measured by voltage probes. The results show that the estimated electron densities of the helium plasma jet calculated from electron drift velocity and the jet velocity are in the order of 10 ¹¹ cm ^-3 and they increase with applied voltage. There is a little fluctuation in the value of the electron density along the jet axis of the plasma. This result is the same as the measured electron density in atmospheric pressure helium non-thermal plasma jet by using a Rogowski coil and a Langmuir probe. This is in one order lower than the electron density measured by microwave antenna.     

利用介质阻挡探针法测量大气压氦等离子体射流电子密度

分别利用电子的漂移速度和等离子体的传播速度计算了大气压下氦等离子体射流的电子密度。     

Monte Carlo method for finding the ionization and secondary emission coefficients and <Emphasis Type="Italic">I–V</Emphasis> characteristic of a Townsend discharge in hydrogen

Secondary cathode emission coefficients for hydrogen are determined from experimental data on breakdown by calculating the number of ionization events in gaps using the Monte Carlo method. Other parameters being calculated are the ionization rate, Townsend ionization coefficients (with demonstration of their nonlocality), velocity of electron drift toward the anode, and probabilities of electron return to the cathode. The calculated and measured values are in reasonable agreement. Gaps with pd = 0.37–17.0 Torr cm are considered. It is shown that the observed negative differential resistance of the Townsend discharge can be related to a decrease in the probability of electron return to the cathode with increasing E/N only if the value of pd is near the minimum in the Paschen curve.     

Electron density and electron redistribution in alloys—II: Electron density in alloys and intermetallic phases

Electron density for alloys which have close-packed metallic structures is calculated by assigning valence electrons to octahedral and tetrahedral interstices, a method which has been previously used for elemental metals. Some localization of electron density is proposed for β -phases when there is considerable difference in ion core sizes. This method of characterizing electron density in alloys can be used to derive structures with the amount of electron transfer if an assumption is made for the volume fraction occupied by each component of the alloy. In general, the electronic structure of intermetallic phases appears to be dominated by the correspondence of a definite number of valence electrons with the number of interstices in the metallic structure (the Hume-Rothery $\text{e}\text{a}$ ratios). The model used can also accommodate electron distributions which include both ionic and covalent components of electron density. This is the case for Laves phases and the metallic A-15 compounds. There is a preponderance of intermetallic phases where one component is a d-shell metal. Evidence is presented that in several such alloys there is a change in d-shell configuration of the elemental metal which serves to minimize size differences of the ion cores of the alloy.     

Radiative transfer in the lyman α spectral line with self-consistent electron velocity distribution

Radiative transfer in the Ly α spectral line in a stationary, plane-parallel plasma of constant temperature and electron density is studied using model H-atoms with only two bound levels and a continuum. For this purpose, the equation of radiative transfer is solved simultaneously with the steady-state equations of the atomic levels and the kinetic equation of the electrons. The numerical results indicate that, in hydrogen plasmas with temperatures T ? 12,000°K and electron densities n_e ? 10¹⁶cm^?3, the high-energy tail of the electron velocity distribution deviates from a Maxwell distribution, even in cases of rather large optical thicknesses and that therefore the deviations from local thermodynamic equilibrium are increased compared with estimates based on the assumption of a Maxwellian electron velocity distribution. This qualitative conclusion should hold in spite of some deficiencies of the model which are discussed.     

Semi-empirical elemental sensitivities for quantitative auger electron spectroscopic analysis

A semi-empirical equation is presented which may be used to calculate the relative sensitivities of the elements for Auger electron spectroscopic analysis. The equation involves the product of a large number of theoretically or empirically derived terms, including ionization cross section. Coster—Kronig transitions, backscattered electron contribution, individual Auger transition probabilities, atomic density, electron mean free path, analyser transmission, modulation effects, detector efficiency, and peak asymmetry. The calculated sensitivities are compared with experimental relative elemental sensitivities from the Handbook of Auger Electron Spectroscopy. The close agreement obtained with experiment suggests that the semi-empirical theory is now capable of providing elemental sensitivities for analysis, in a predictive way, where reliable empirical sensitivities are missing.     

Reconstruction of bands in metallic hydrogen

The generalized theory of normal properties of a metal for the case of the properties of the electronic band of electron–phonon systems with a variable electron density of states is used to study the normal phase of metallic hydrogen at a pressure of 500 GPa and a temperature of 200 K. We calculated the frequency dependence of the real ReΣ(ω) and imaginary ImΣ(ω) parts of the self-energy part of the electron Green’s function Σ(ω), as well as the electron density of states N(ε) of the stable phase of metallic hydrogen with the I41/amd symmetry at a pressure of 500 GPa, renormalized by the strong electron–phonon coupling. It is found that the electron conduction band of the I41/amd phase of metallic hydrogen undergoes insignificant reconstruction near the Fermi level because of the renormalization by the electron–phonon coupling.     

Electron impact ionization and excitation rate coefficients in the negative glow region of a glow discharge

Some easy to use reasonable approximations for electron impact rate coefficients have been considered. The most important rate coefficients for electron collisions in noble gases are electron-neutral ionization and electron impact excitation. Electron-neutral ionization besides electron impact excitation of some states of the argon and helium atom in direct current (dc) glow discharge plasma has been calculated. The plasma parameters of electron are significant factors for computing the rate coefficients. We present first results of probe diagnostic that includes the double probe measurements of the plasma parameters, namely, electron temperature (T_e) and electron density (n_e). Electron properties obtained from the double probe characteristic curves including T_e and n_e as well as the calculated rate coefficients (ionization and excitation) were studied as a function of the axial distance from the cathode while the discharge operating parameters of voltage and pressure were varied. Two regions of the glow discharge were investigated: cathode fall region and negative glow. Particular emphasis was placed on the negative glow region.     

Line broadening studies in low energy plasma focus

Optical emission spectroscopic studies were carried out to characterise the plasma leading to the estimation of two plasma parameters, electron density and temperature. These experiments were conducted on a 2 kJ plasma device which is equipped with squirrel cage electrode configuration enclosed in a glass vacuum chamber filled with hydrogen at a pressure of 5 mbar. Spectral emissions obtained from each flash were photographed in the region of 4000–6000 Å using one metre Czerny-Turner spectrograph cum monochromator. Detailed examination of the observed features showed that theH _β andH _λ lines of hydrogen showed significant broadening of the order of 35 Å FWHM which is due to Stark effect expected in high density plasmas. Further several atomic lines of Cu and Zn from the electrode material (brass) showed broadening which was due to quadratic Stark effect. A comparative study of the broadening of lines obtained in DC arc, hollow cathode and plasma focus was made. Electron density from Stark broadened hydrogen lines and quadratic Stark Coefficient C₄ for the CuI and ZnI lines were evaluated. The excitation temperature was determined from the line intensity ratio method using CuI lines.     

Effect of small admixtures of N<Subscript>2</Subscript>, H<Subscript>2</Subscript> or O<Subscript>2</Subscript> on the electron drift velocity in argon: experimental measurements and calculations

The electron drift velocity in argon with admixtures of up to 2% of nitrogen, hydrogen or oxygen is measured in a pulsed Townsend system for reduced electric fields ranging from 0.1 Td to 2.5 Td. The results are compared with those obtained by Monte Carlo simulations and from the solution of the electron Boltzmann equation using two different solution techniques: a multiterm method based on Legendre polynomial expansion of the angular dependence of the velocity distribution function and the S _n method applied to a density gradient expansion representation of the distribution function. An almost perfect agreement between the results of the three numerical methods and, in general, very good agreement between the experimental and the calculated results is obtained. Measurements in Ar-O₂ mixtures were limited by electron attachment to oxygen molecules, which contributes to the measured drift velocity. As a result of this attachment contribution, the bulk drift velocity becomes larger than the flux drift velocity if attachment is more probable for electrons with energy below the mean value and smaller in the opposite case. Attachment also contributes to the negative differential conductivity observed in Ar-O₂ mixtures.     

Reconstruction of the conduction band in metallic hydrogen sulfide

The theory of the normal properties of a metal generalized to the case of particular properties of an electron band with a finite width for electron–phonon systems with a varying electron density of states has been used to study the normal state of the SH₃ phase of hydrogen sulfide at a pressure of 225 GPa and a temperature of 200 K. The frequency dependences of the real, ReΣ(ω), and imaginary, ImΣ(ω), parts of the selfenergy part of the Green’s function of the electron Σ(ω), as well as the electron density of states N(ε) of the Im–3m stable orthorhombic structure of SH₃ hydrogen sulfide at a pressure of P = 225 GPa, which is renormalized by the strong electron–phonon coupling, have been calculated. It has been established that a part of the electron conduction band of the SH₃ phase of hydrogen sulfide adjacent to the Fermi level undergoes renormalization-induced reconstruction in the form of a number of energy pockets with the widths equal to fractions of the characteristic phonon energies of the system.     

Das langsam beschleunigte Elektron und die Lenzsche Regel

The Slowly Accelerated Electron and Lenz's Rule The selfinduced field of a slowly accelerated electron which always tends to prevent this acceleration is calculated (without using the electrodynamical potentials) directly from Maxwells equations for the field strengths in the most simple case in which both the velocity and acceleration are small and collinear.