电子入射角度对聚酰亚胺二次电子发射系数的影响

采用具有负偏压收集极的二次电子发射系数测试系统, 对聚酰亚胺样品的二次电子发射系数与入射电子角度和入射电子能量的关系进行了测量. 测量结果表明, 在电子小角度入射样品的情况下, 随着入射角度的增加, 二次电子发射系数单调增加, 并符合传统的规律, 但是在电子大角度入射时, 却与此不符合. 测量显示, 出现偏差时对应的临界电子入射角度随着入射电子能量的降低而减小. 采用简化的电子弹性散射过程和卢瑟福弹性散射截面公式对这种偏差的出现进行了分析, 并推导出修正后的二次电子发射系数的计算公式. 修正后的二次电子发射系数的计算结果更加符合实验结果.     

Triple differential cross section for autoionisation of helium by electron impact at 100 eV

Cross sections for autoionisation of helium by electron impact have been measured by an electron-electron coincidence technique at an incident electron energy of 100 eV and a scattering angle of 15°. The variation of the direct ionisation cross section and the resonance parameters for the (2s2p)¹P autoionising level, as functions of ejected electron angle, are compared with the results of recent theoretical calculations.     

A 90° laser scattering experiment for measuring temperature and density of ions and electrons in a cold dense theta pinch plasma

In a 90° scattering experiment performed on a plasma with a density of some 10¹⁷cm^?3 and a temperature of some eV a scattering spectrum is expected which consists essentially of an intense central part and two very weak satellite peaks. In this investigation the spectrum of the central part (ion line) was resolved. From the ion line alone which was in good agreement with calculated spectra, the ion temperature, electron temperature and electron density were determined. At some plasma parameters only a few particles were included in the Debye volume. Also in these cases no deviations from calculated spectra have been observed. In addition the position and half-width of the satellites were measured. The position of the satellites yields an independent value of the electron density which agrees very well with that determined from the ion line. The half-width of the satellites was much broader than expected theoretically due to fluctuations of the electron density within the scattering volume. The measurement was performed on a theta pinch plasma. The energy of the condensor bank was 7.5 kJ, the voltage 18 kV. As a light source aQ-switch laser was used with a power of about 100 MW. Because of beam stops to prevent stray light only a fourth of the whole power reached the plasma.     

Coupling to the elastic channel in electron impact spectroscopy: A simple second born model and its application to excitation of the 61P1 state of mercury and to the excitation of molecules

The matrix element in the infinite channel close coupling approximation responsible for coupling to the elastic channel in electron impact inelastic encounters is investigated. The contribution from the imaginary part of the energy denominator in the elastic coupling matrix element for dipole allowed transitions is shown to yield large angle differential cross sections in good agreement with experiment. This coupling mechanism predicts that the shape of the inelastic differential cross section will be dominated by the shape of the elastic cross section in the large angle high energy limit. In fact the coupling matrix element exhibits a dependence on incident energy, k², and momentum transfer, K, of the form 1/kK² which is in agreement with the theoretical predictions of Huo and means that in the limit of high incident energy the non-first-Born elastic coupling will dominate the angular dependence of the inelastic differential cross section at large scattering angles. In the case of molecular electron impact spectra it is shown that the analog of the Massey—Moore coherence features depending on the symmetry of the states involved in the excitation process will also occur in the coupling contribution. It is suggested that this new mechanism for producing coherent features in inelastic differential cross sections may be the explanation of the coherent features observed experimentally by Karle and Swick.It can be concluded on the basis of the results obtained here that the coupling to the elastic cross section provided by the imaginary contribution from the second Born energy denominator is sufficient to explain presently available experimental data on the large angle differential cross section and spin polarization. The simple coupling model was found to be inadequate to explain the small angle differential cross section in the range 10° < θ < 30° even at incident energies as high as 400 eV. The calculations also showed significant differences between first and second Born calculations at zero scattering angle. No conclusion can be drawn about this observation as all the omitted terms should make significant contributions in the small angle region. It is important to again emphasize that the large angle scattering even in the limit of high incident electron energy will be completely dominated by the coupling to the elastic channel^{7, 11}. On the basis of this work it appears that the coherent structure in the large angle inelastic differential cross section observed by Swick and Karle^{12, 13} at incident electron energies in the keV region may well be due to coupling to the elastic channel.     

Streuung von 25 keV-Elektronen an Gasen

The angular distribution of 25 kev electrons scattered elastically and inelastically by molecular hydrogen has been measured in the angular range 7·10^?4≦?≦4,3·10^?2. By the separation of the inelastically scattered electrons observation of deviations from the Debye Ehrenfest theory of the electron diffraction by molecules at small angles is possible. These deviations are due to the alteration of the electron density distribution of the hydrogen atoms induced by the bonding. The energy loss spectra at different scattering angles (energy resolution ≈1 ev) shows a strong peak atΔE=12,6 ev. At larger angles forΔE>15 ev a continuum appears. That part of the inelastic processes which leads to ionization of the molecule is raising with increasing scattering angle. Normalization of the experimental values to the theoretical elastic differential cross section enables comparison of the angular distribution of the 12,6 ev energy loss with the distribution calculated byRoscoe. The shape of the experimental curve is in fairly good agreement with the calculated one but the experimental values at small angles are 20–30% higher. For zero angle the energy loss spectrum is taken with better resolution (≈0,04 ev). It shows vibrational states of the Lyman and Werner band and higher terms. The probability of the excitation of some vibrational states of the Werner band (square of the overlapp integral) calculated here is inspite of the required approximations in excellent agreement with the measurement, while Hutchisson's result fails for the Lyman band.     

纳秒激光脉冲在空气中聚焦的临界自由电子密度问题

实验研究了高功率纳秒量级激光脉冲在空气中聚焦时的能量透过率随输入激光脉冲能量变化的规律,发现在纳秒激光脉冲聚焦半径相同的情况下,激光脉冲的能量透过率随入射激光脉冲能量的变化可分为三种情况：当入射激光脉冲能量较低时,激光脉冲能量全部通过;当入射激光脉冲能量增大后,激光脉冲的能量透过率由近100%迅速减小;当入射激光脉冲的能量进一步增加时,激光脉冲的能量透过率继续缓慢变小.用临界自由电子密度以及所对应的临界时间点对上述实验现象进行了理论分析得到了如下结论：当自由电子密度未达到临界自由电子密度时,多光子电离过程起主要作用,而当自由电子密度超过临界自由电子密度后,逆韧致吸收过程起主要作用,临界时间点是入射激光脉冲与空气作用过程中自由电子密度达到临界自由电子密度的时刻.入射激光脉冲能量决定了临界时间点在脉冲作用时间上的位置,临界时间点的位置决定了激光脉冲的能量透过率.可以通过测量激光脉冲的能量透过率来计算出临界自由电子密度,从而确定出激光脉冲在空气中聚焦时的能量透过特性. 关键词： 临界自由电子密度 临界时间点 多光子电离 逆韧致吸收     

Fine-structure in secondary electron emission from Cu(0 0 1)c(2 × 2)-Cl: Experiment and theory

Normal emission secondary electron spectra excited by photons of energy ?ω = 21.2 eV and by electrons of 100 eV kinetic energy have been measured with high resolution from Cu(0 0 1) and Cu(0 0 1)c(2 × 2)-Cl. Calculations were performed using a “one-step” multiple scattering formalism for the semi-infinite system. The close agreement between experimental and theoretical fine structure reveals the nature of individual features. Our results demonstrate that in general bulk-derived features in the secondary electron fine structure cannot be directly associated with critical points in the band structure.     

Electron mean free path and conduction-band density of states in solid argon

In this paper, we apply a theoretical model developed by Plenkiewicz et al. for analyzing the current I_t transmitted by an ultrathin dielectric film as a function of incident electron energy E for solid argon. The analysis of I_t(E) in the elastic scattering region (0–12 eV) allows one to determine the electronic conduction-band density of states and to calculate the electron scattering mean free path as a function of energy. Comparison of our results with various solid argon band structure calculations is also presented.     

The high-energy critical minimum in elastic electron scattering by argon

The position of high-energy critical minimum in elastic electron-argon scattering was investigated both experimentally and theoretically. Differential cross-sections (DCSs) were measured as a function of both incident electron energy (40-150 eV) and scattering angle ( ), in small steps around the critical minimum. The position of the high-energy critical minimum in elastic electron-argon scattering was experimentally found to be at eV and . To cover the energy and angular ranges of the present experiment, relevant relativistic ab initio calculations were carried out, based on the Dirac-Hartree-Fock method with the exchange calculated exactly. Target polarization is described by an ab initio potential taken from relativistic polarized orbital calculations. The calculated position of the high-energy critical minimum is eV, . It was shown that even slight difference of fixed scattering angle close to the critical point could affect significantly the energy dependent DCS. Discussion of behavior of DCS in the vicinity of the critical minimum was performed including convolution analysis in both energy and angle.Received: 31 October 2003, Published online: 20 April 2004PACS: 31.15.Ar Ab initio calculations - 34.80.Bm Elastic scattering of electrons by atoms and molecules     

合肥光源逐圈束流位置监测系统中的定时系统

定时系统是合肥光源逐圈束流位置监测和相空间测量系统中的重要组成部分，其主要目的是为数据采集卡提供与环内电子束团同步的时钟信号。该系统包含高速ECL时钟成形、分频电路、程控延时电路、控制模块等部分。可实现延时范围0～220ns，最小延时步距0.5ns，调整精度0.1ns，时钟抖动小于200ps。系统控制软件基于客户/服务器结构，操作灵活、方便。     

Characteristics of potential-resonance elastic scattering of slow electrons by calcium atoms

The characteristics of elastic scattering of electrons by atomic systems are described by the potential resonance method in terms of the energy dependence of integral-type cross section S taken using a hypocycloidal electron spectrometer. The effect of ² D shape resonance on the run of the curve S(E) is demonstrated with scattering of slow (of energy no higher than 2 eV) electrons by calcium atoms. The energy and width of the resonance are derived from the extrema (minimum and maximum) of the experimental curve S(E). This dependence also serves to find the electron scattering differential cross sections. Two slightly differing scattering angles are taken into account. The differential cross sections derived from the experimental data qualitatively agree with theoretical results, albeit being somewhat lower.     

On the influence of elastic scattering on asymmetric xp-signal distribution

In the standard equation for the strength of photoelectron signals the angular dependence of emitted photoelectrons is represented by the asymmetry parameter. As the geometry has a number of characteristic angles, γ, for example due to a finite solid angle of electron detection, one expects an effective asymmetry parameter β* instead of the theoretical β. But elastic electron scattering causes much more of a spread from the characteristic angle γ to a spectrum of possible γ-values. This paper aims to obtain quantitative results allowing a quantification of the influence of elastic scattering on the theoretical asymmetry parameter β. For photoelectrons with kinetic energies within 1–1.5 keV and elements up to Z = 46 the result is β* = (1.0688 ?0.0235Z+0.000188Z²)β     

Polarization of slow electrons (100–2000 ev) elastically scattered from heavy atoms

The angular and energy dependence of the Mott polarization for slow electrons is explained in terms of the complex zeros of the scattering amplitudef(z), wherez is the cosine of the scattering angle. The trajectories in the complexz-plane, on which the zeros move if the energy is varied, are shown in the case of scattering from mercury atoms. The properties of these trajectories imply that there are, in fact, certain combinations of energy and angle, for which total polarization occurs. Calculations for target atoms lighter than mercury show, that the number of such combinations becomes smaller and the corresponding energy values are shifted to lower energies. The possibility is also discussed to extract the complex zeros and the other relevant parameters from the experimental data rather than from theoretical calculations, which to some extent are sensitive to the model of the atomic potential.     

高能电子辐照绝缘厚样品的表面电位动态特性

采用数值计算和实验测量相结合的方法, 阐明了高能电子束照射下绝缘厚样品的表面电位和电子产额动态特性. 结果表明: 由于电子在样品内部的散射和输运, 沿着深度方向, 空间电位先缓慢下降到最小值, 然后逐渐升高并趋近于零; 随着电子束照射, 样品的表面电位逐渐下降, 可至负千伏量级, 电子总产额逐渐增大至一个接近于1的稳定值; 电子束停止照射后, 长时间放置下, 表面电位将逐渐升高, 但带电并不会消除; 表面电位随电子束能量的升高近似线性下降, 随入射角的增大而升高, 而随样品厚度的增大仅略有下降.     

Phase of reflection of very low energy electrons at crystal surfaces

The amplitude coefficients of electron reflection at crystal surfaces are complex numbers, each of which may be characterized by a reflection intensity (the squared modulus of the coefficient) and a phase. The phase of reflection of very low energy (? 10 eV) electron reflection is described on the basis of existing theory, and experimental approaches to phase determination are reviewed. Theoretical properties of the phase are described on the basis of the two-beam dynamical theory of diffraction. The model considered is an idealized substrate crystal with an attached selvedge (surface region). The indirect effect of inelastic scattering (absorption) is included by going to complex values of the electron energy or of the surface-normal component K of the propagation vector. In the absence of a selvedge the phase is determined solely by the band structure of the substrate crystal. If a selvedge is present there are large additional effects on the phase associated with zeros of the amplitude coefficient of reflection on the complex K plane. The experimental approaches considered are: (1) measurement of the kinetic energy distributions of ions produced in the field ion microscope, and (2) measurement of the periodic deviations from the Schottky line in field-assisted thermionic emission and photo-emission. Recent results of phase determination for W (011) surface by method (1) are summarized and compared with theoretical expectations.     

The Qweak: Precision Measurement of the Proton’s Weak Charge by Parity Violating Experiment

The Qweak experiment at the Thomas Jefferson National Accelerator Facility measures the parity violating asymmetry in longitudinally polarized electron scattering from the proton at very low momentum transfer, Q ² = 0.026 (GeV/c)², at an incident electron beam energy of 1.16 GeV. With this measurement and the earlier results of the parity violating elastic scattering experiments, the Qweak experiment determines the weak charge of the proton, ${Q^p_{\rm W}}$ , with a 4% combined statistical and systematic error. This measurement will be used to determine the weak mixing angle, ${\sin^2\theta_{\rm W}}$ , that is predicted by the Standard Model from the Z ⁰ pole down to lower energies. Qweak will determine ${\sin^2\theta_{\rm W}}$ to a 0.3% relative precision, providing a competitive measurement of the running of this quantity. Moreover, if there is a significant deviation of the weak mixing angle from the Standard Model prediction, then the Qweak experiment will give a glimpse of possible extensions of the Standard Model.     

A WIDE-RANGE MAGNETIC SPECTROMETER FOR ELECTRON SCATTERING IN MEDIUM ENERGY RANGE

A non-focusing magnetic spectrometer design for electron scattering in medium energy region is proposed.The positions read out from the position-sensitive detectors in the spectrometer are used for track reconstruction and momentum measurement by means of a computer program.The construction of this spectromenter is rather simple and there is no special technique and element for correction to aberration.It is suitable for usage of spectrometer with large solid angle and wide momentum range.The momentum resolution,momentum range and acceptance in our case are calculated by Monte Carlo simulation.     

Spin dependent 2D electron scattering by nanomagnets

The 2D scattering problem of an electron by a magnetized nanoparticle is solved in the Born approximation with account of the dipole-dipole interaction of the magnetic moments of electron and nanomagnet. The scattering amplitudes in this problem are the two-component spinors. They are obtained as functions of the electron spin orientation, the electron energy and show anisotropy in scattering angle. The initially polarized beam of electrons scattered by the nanomagnet consists of electrons with no spin flipped and spin flipped. The majority of electrons with no spin flipped are scattered by small angles. The majority electrons with spin flipped are scattered in the vicinity of the scattering angles π/2 and 3π/2. This can be used as one more method of controlling the spin currents.     

Probing interlayer coupling in twisted single‐crystal bilayer graphene by Raman spectroscopy

Twisted bilayer graphene, in which interlayer interaction plays a critical role in this coupled system, is characterized for its angle‐dependent electronic and optical properties. Here, we present a systematic Raman study of single‐crystal twisted bilayer graphene grains, with the spectra of each bilayer graphene precisely correlated to its twist angle using combined transmission electron microscopic technique. Van Hove singularities develop as a result of band rehybridization at the crossing Dirac cones of the two layers, giving rise to a critical twist angle that determines the energy separation between the saddle points in the band structure and the resonance Raman spectra accordingly. The 2D mode becomes sensitive to the twist angle, showing the angle‐dependent position, peak width, and intensity. Our results interpreted in the framework of angle‐dependent double resonance scattering provide an important experimental perspective in understanding the coupled bilayer graphene system. Copyright © 2014 John Wiley & Sons, Ltd.