电子衍射条件对Si(111)外延时反射式高能电子衍射强度振荡的影响

本文研究了不同电子衍射条件对Si(111)外延时的反射式高能电子衍射(RHEED)强度振荡的影响,在保持生长条件不变的情况下,沿[112]方位观测时,不同入射角下其强度振荡的相位和初始瞬态响应变化很大,甚至会发生180°相位变化,而在[011]方位观测时,其相位的变化不明显,结合Si(111)面的RHEED强度摇摆曲线测量结果,表明这种与电子衍射条件有关的振荡特性变化,实际上反映了由电子多重散射机理引起的RHEED强度振荡两种情形,对RHEED强度的初始瞬态响应机理也作了探讨。 关键词：     

Thickness difference: a new filtering tool for quantitative electron diffraction

A new way of filtering electron diffraction patterns has been discovered. Patterns from slightly different specimen thicknesses beyond the mean free path for inelastic scattering are subtracted. Only thickness sensitive information (dominantly elastic) remains. Thermal diffuse scattering and Borrmann effects are removed in addition to the inelastic signal eliminated by conventional energy filtering. One application is quantitative convergent beam electron diffraction without an energy filter. Structure factors for alpha - Al(2)O(3) have been measured with an average uncertainty of 0.25%.     

Inelastic scattering of slow electrons in solids

A theory of the inelastic scattering of slow electrons in solids due to excitation of interband transitions is developed. It is shown that both nondirect and direct transitions occur which can be described by a generalization of the formalism used in solid state optics. Experiments with 30–200 eV electrons scattered from Si (111) surfaces with well defined surface structures as determined by low energy electron diffraction confirm the theoretical predictions. They indicate that the inelastic scattering of slow electrons can be understood in terms of the three-dimensional band structure of solids and suggest the use of inelastic low energy electron scattering as a tool for band structure analysis.     

Angular pattern of electron diffraction by reflection in the plasmon channel of energy losses

We have constructed a theory for the excitation of plasmons by a fast charged particle that undergoes diffraction in a single crystal and then is scattered elastically and incoherently through a large angle. The theory allows the 30-year-old experimental results that have seemed strange to be explained. An increase in the diffraction contrast in the channel of inelastic electron scattering related to the excitation of a bulk plasmon compared to the diffraction contrast of elastically and incoherently reflected electrons was observed in these experiments. Based on this theory, we show that the excitation of a surface plasmon affects only slightly the angular diffraction pattern, leaving it almost the same as that for elastically reflected electrons. These peculiarities of elastic and inelastic diffraction can be used to identify the type of energy plasma loss.     

Resonance scattering of electrons from orientated O2 physisorbed on graphite

We have studied inelastic electron scattering from a saturated monolayer of O₂ physisorbed on graphite at 25K. The cross-section for excitation of the v = 0–1 vibrational transition in O₂ peaks at 8.5–9eV, and is attributed to the ⁴Σ_u negative ion resonance. The angular distribution of electrons inelastically ejected out of the resonance orbital has been measured, and is found to peak at 15° from the normal to the crystal for several different incident electron beam angles, in accord with the theory of resonant electron scattering by an orientated molecule. We discuss the implications of this measurement for a determination of the orientation of the O₂ molecule on the surface, taking account of possible multiple scattering effects.     

Dynamische Theorie der elastischen Elektronenbeugung unter Verwendung komplexer Atomformfaktoren

The two-beam approximation of dynamic electron diffraction in crystals is deduced from successive scattering by two-dimensional gratings. The scattering-amplitude caused by the single grating (phase-grating) is determined by the atomic scattering factors of the single atoms. The use of real atomic scattering factors leads here to the same solutions as the twobeam theory, using the Schroedinger-equation. Anomalous absorptions effects are described in the usual theory by introducing a complex lattice potential into the Schroedinger-equation, taking into account inelastic scattering. A more exact calculation of only elastic scattering by a single atom results in complex atomic scattering factors. Using the complex atomic scattering factors one can describe the anomalous absorption, if recursion-formulae for successive scattering are applied. On the other hand, the transformation to differential equations leads to results, which are not in agreement with experiments. The influence of inelastic scattering for the anomalous absorption can be neglected compared with the complex atomic scattering factors.     

Inclusive production of neutral strange particles in\bar pp interactions at 32 GeV/c

Multiplicities of charged particles produced in the forward and backward hemispheres are investigated for proton-nucleus and proton-nucleon collisions in the range from 20 to 400 GeV/c. Weak, but growing with incident beam momentum, forward-backward multiplicity correlations are observed which cannot be reduced to the inelastic diffraction. These correlations are more pronounced in proton-nucleus than proton-nucleon interactions. The experimental data are discussed in the frameworks of current cluster and multiple scattering models. It is shown that multiple scattering models describe well the forward-backward multiplicity correlations in nuclear production.     

Electron scattering cross section measurements in a variable pressure scanning electron microscope

Scattering of the incident electron beam in the variable pressure scanning electron microscope (VPSEM) affects the ability to perform quantitative chemical measurements. However, the manner in which the sum of the elastic and inelastic scattering cross sections varies as a function of gas type and accelerating voltage in the VPSEM is not well understood. A dual Faraday cup was constructed to measure the scattered fraction of the primary beam as a function of gas pressure, working distance, and accelerating voltage in air, water vapor, and argon environments. Experimentally measured values of the scattering cross section agree with previous experimental work, and agree within a factor of two with those values calculated carefully from theory.     

Temperaturabhängigkeit des Streuuntergrundes im Elektroneninterferenzdiagramm polykristalliner Silberschichten

The filtered and unfiltered intensities of electrons (30 to 50 kev) scattered in thin polycrystalline silver foils (300 to 1400 Å) were measured by means of a retarding field apparatus in the temperature range from 170 to 380 °K. Further the absorption coefficients due to the inelastic and the different elastic scattering processes were determined. It is shown, that the diffuse elastic scattering intensity increases with temperature for all values of thickness and electron energy. From the measured absorption coefficients the contributions of the different elastic scattering processes to the total background intensity are calculated. It turns out, that the main contribution is due to multiple scattering processes including both Bragg reflections and thermal diffuse scattering. Furthermore it is shown, that in contrary to the filtered diagram the background intensity of the unfiltered diagram increases with temperature only for larger scattering angles. In the small angle region a reversed temperature behaviour is observed. This effect is caused by that part of inelastically scattered electrons at low angles coming directly from the primary beam and from the low order diffraction rings.     

An alternative scheme of angular‐dispersion analyzers for high‐resolution medium‐energy inelastic X‐ray scattering

The development of medium‐energy inelastic X‐ray scattering optics with meV and sub‐meV resolution has attracted considerable efforts in recent years. Meanwhile, there are also concerns or debates about the fundamental and feasibility of the involved schemes. Here the central optical component, the back‐reflection angular‐dispersion monochromator or analyzer, is analyzed. The results show that the multiple‐beam diffraction effect together with transmission‐induced absorption can noticeably reduce the diffraction efficiency, although it may not be a fatal threat. In order to improve the efficiency, a simple four‐bounce analyzer is proposed that completely avoids these two adverse effects. The new scheme is illustrated to be a feasible alternative approach for developing meV‐ to sub‐meV‐resolution inelastic X‐ray scattering spectroscopy.     

New opportunities for quantitative analysis as applied to reflected electron energy loss spectroscopy of Fe/Si structures

The feasibility of determining the elemental composition, chemical state, and element distribution across the depth in a subsurface region using the computer simulation of the electron inelastic scattering cross section is demonstrated with iron layers on silicon substrates. Analysis is carried out based on the dielectric theory and on the experimental determination of the product of the electron inelastic mean free path by the inelastic scattering cross section from reflected electron energy loss spectra.     

22MeV质子在28Si上弹性及非弹性散射微观分析

通过22MeV质子与²⁸Si的散射实验,测量了质子弹性散射和在21+(1.78MeV)及41+(4.62MeV)激发态上的非弹性散射微分截面.采用动量空间DWBA理论,利用由电子散射实验得到的核结构信息,和低能区的密度依赖、能量依赖的复数有效相互作用对数据进行了分析.在没有任何可调参数和归一因子的情况下,理论计算与实验数据的符合情况是令人满意的.     

Fresnel diffraction in deuterium

In Glauber's theory of high energy scattering the wavefunction inside the nucleus is given by geometrical optics. The theory is extended to lower energies by incorporating the effects due to Fresnel diffraction, and applied to elastic and inelastic scattering off deuterium.     

Weak localization of inelastically reflected electrons under auger emission conditions

A new type of weak localization of electrons emerging during electron emission is considered. It is manifested in singularities of the angular spectra of particles reflected inelastically from a solid and causing Auger ionization of the atoms. The orientational dependences in this case appear as a result of interference of two types of processes. In one case, an electron from the primary beam penetrates the solid, undergoes inelastic scattering, ionizes an atom, and is then scattered elastically through a large angle, after which it leaves the solid. In the other case, elastic scattering of an electron precedes its inelastic scattering due to the Auger ionization of an atom. The azimuthal angular dependences of currents created by inelastically reflected electrons contain information on new processes of weak localization of particles.     

Radiation corrections to the electron inelastic scattering by an intense stationary laser beam

By solving the Dirac-Lorentz equation numerically with a perturbative predictor-corrector method, we find that the electron inelastic scattering by an intense stationary laser beam is heavily modified by the radiation reaction when the electron is injected upon the laser beam with a large crossing angle (θ>10°). This is the first time the Dirac-Lorentz equation is applied to study the interaction of an electron with a real laser beam in the Hermite-Gaussian mode.     

Multiple scattering theory for slow neutrons (from thermal to ultracold)

The general theory of neutron scattering is presented, valid for the whole domain of slow neutrons from thermal to ultracold. Particular attention is given to multiple scattering which is the dominant process for ultracold neutrons (UCN). For thermal and cold neutrons, when the multiple scattering in the target can be neglected, the cross-section is reduced to the known value. A new expression for inelastic scattering cross-section for UCN is proposed. Dynamical processes in the target are taken into account and their influence on inelastic scattering of UCN is analyzed. Received 21 July 1999     

Emissionsmikroskopische Messung der Ortsverteilung von 20 keV-Elektronen nach Streuung in dünnen Al- und Ge-Aufdampfschichten

Electrons incident on a thin film are displaced laterally when leaving the target by elastic and inelastic scattering. The widening of the current density distribution of an electron beam with an initial energy of 19.5 keV and an initial Gaussian distribution with a half width of about 0.6 microns by plural and multiple scattering as well as in the region of complete diffusion (corresponding to mass thicknesses ranging from 40–500 µg/cm²) is measured for evaporated films of Al and Ge. From these results the increase in half width of an infinitely fine primary electron beam (δ-function response) is computed and plotted. In order to measure the current density distribution of the scattered electrons it is converted to an equivalent distribution of secondaries using a cathode lens image converter. These distributions are magnified and scanned electronically or recorded photographically.     

Elastic and inelastic scattering components in the angular intensity distribution of He scattered from graphite

The angular intensity distribution of thermal energy He beam scattered from highly oriented pyrolytic graphite (HOPG) surface has been measured by means of supersonic molecular beam scattering technique in the wide surface temperature range. To separate the elastic and inelastic scattering components, simple analysis method has been developed by applying the classical binary collision theory of the hard cube model (HCM). From the extracted elastic scattering component in the scattering distribution, the Debye temperature of the HOPG surface has been derived as 590 ± 30 K. On the basis of the HCM analysis for the extracted inelastic scattering components of He beam, the effective mass for the HOPG surface has been found to be 72 u (six carbon atoms).     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.