Kapitza-Dirac效应的最新进展

自由电子束经光的驻波散射服从布拉格(Bragg)定律的设想由Kapitza和Dirac(KD)于1933年提出．这一效应在近70年以后的2001年经Batelaan等人用实验证实．近日，电子束的KD衍射的理论以中国物理学家为主用非微扰量子电动力学做出．新理论不但解释了Batelaan的实验，还预言了若干新现象．由于玻色一爱因斯坦凝聚的发展及高科技领域的需要，原子和电子形成的物质波须由KD效应进行控制．巨大的应用前景正在使KD效应的研究由冷门变为热门．     

Hosing instability in the blow-out regime for plasma-wakefield acceleration

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)10.1103/PhysRevLett.96.165002]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)10.1103/PhysRevLett.67.991] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.     

On the semi-empirical elemental sensitivity factors for AES: ionization cross-section and electron mean free path

Two important parameters in semi-empirical theories of elemental sensitivity in Auger electron spectroscopy are the electron-impact ionization cross-section and the electron mean free path. This paper compares the Gryziński, Lotz, and Casnati et al. theories of ionization cross-section, as well as the Seah and Bench empirical theory and the Szajman et al. dielectric theory of electron mean free path, by matching the elemental sensitivity factors calculated with these parameters to experimental AES elemental sensitivities. Electron mean free paths, calculated with the dielectric theory, are presented, for the Auger electron energies of interest, for most elements in the periodic table. The best match between the semi-empirical and experimental sensitivities (with a standard deviation of 56%) was for the combination of the Casnati et al. ionization cross-section and the dielectric theory electron mean free path.     

Ultrasonics and X-ray diffraction under pressure in the Paris-Edinburgh cell

Our objective consists in validating a new set-up which will permit us to carry out simultaneously ultrasonic and X-ray diffraction measurements under pressure. To validate the results obtained by this new set-up, the elastic properties of a single crystal of germanium were studied. Our results are in good agreement with those of Goncharova et al. and McSkimin and Andreatch. The results of the present study are compared with those of Menoni et al. and obtained by X-ray diffraction in a diamond anvil cell.     

高能带电粒子直接产生电子对

本文叙述利用场强为6200高斯的磁云室,对级联簇射中的能量大于30兆电子伏的电子在铝中直接产生电子对的平均总截面进行了测量。实验上总共找到真正由电子产生的电子三岔8.1个,相应于三岔产生的平均自由路程为672±585厘米或平均总截面为(0.25±0.22)×10^-25厘米²。在数据处理中,对假三岔数目的除去作了应有的适当考虑。此外对Bhabha及Murota等的总截面公式也作了适合于于我们实验具体情况的修正,并算出总截面值,最后将它同实验值作了比较。结果我们认为实验上测得的值和Murota等的总截面值较为接近,但也不与Bhabha的截面值矛盾。     

Numerical simulation of atomic diffraction in the Raman-Nath regime

The theory of atomic diffraction from a classical standing wave light field in the presence of spontaneous emission in the Raman-Nath regime was developed by Tanguy et al. [6]. We describe the basis of computationally efficient methods for performing calculations in this regime and show their agreement with recent experimental results of Gould et al. [4].     

Site-specific recoil diffraction of backscattered electrons in crystals

A novel diffraction effect in high-energy electron backscattering is demonstrated: the formation of element-specific diffraction patterns via nuclear recoil. For sapphire (Al(2)O(3)), the difference in recoil energy allows us to determine if an electron scattered from aluminum or from oxygen. The angular electron distribution obtained in such measurements is a strong function of the recoiling lattice site. These element-specific recoil diffraction features are explained using the dynamical theory of electron diffraction. Our observations open up new possibilities for local, element-resolved crystallographic analysis using quasielastically backscattered electrons in scanning electron microscopy.     

Self-oscillations and bunching of striations in ionospheric modifications

Based on the theory of stationary striations created in ionospheric modifications by powerful radio waves, as developed by Gurevich et al. [Phys. Lett. A 206 (1995) 247], the existence of self-oscillations of striations and of a large scale nonlinear structuring in a modified region is predicted. The theory is shown to be in agreement with existing observational data. Direct in situ rocket measurements of the electron temperature inside the striations, analogous to the recent plasma density measurements [M.C. Kelley et al., J. Geophys. Res. 100 (1995) 17 367], are desirable.     

大角度会聚束电子衍射的新方法

会聚束电子衍射是测定晶体点群、空间群的有效方法。要清楚地判断会聚束电子衍射图样的对称性,电子束的会聚角要大,但衍射盘不能重叠。本文提出了一种新方法来获得不重叠的大角度会聚束电子衍射。新方法是使电子束离焦,固定样品,从而保留了样品的同心倾动位置,作大角度倾动时不会失去原来感兴趣的区域。新方法在原理上和田中等人提出的方法相同,但优越性更大。同时也研究了大束斑(≥2μm)大角度会聚束电子衍射。 关键词：     

Experimental determinations of beta attenuation in planar dose geometry and application to ESR dating of tooth enamel

Two experiments were performed to determine the distribution of beta dose in a target medium as a function of distance from a planar source (2π geometry). In Experiment 1 planar absorbers were used to simulate depth in the target medium, and the attenuated dose that penetrated the intervening absorbers was detected using thermoluminescence from CaSO₄:Tm. In experiment 2 pellets of powdered tooth enamel were situated in a cavity in solid tooth enamel. Dose response of the electron spin resonance (ESR) signals in the pellets allowed direct detection of dose distribution within the target medium. The dose distributions were found to agree more closely with Monte Carlo calculations and with one-group transport theory (Prestwich et al., 1997 and Brennan et al., 1997) than with approximations used widely in ESR dating applications (Grün, 1986). Although the latter were supported by experiments by Aitken et al. (1985), the results of experiment 1 provided data to suggest that departure from 2π geometry was unacceptably large in the Aitken et al. (1985) configuration. The beta doses obtained by one-group theory in typical target volumes of enamel were 17 to 40% lower than those obtained using the Grün approximation. It was concluded that one-group transport theory provides the best available routine tool for calculations of beta dose attenuation in near-planar target materials (e.g. tooth enamel and shell) irradiated by natural sources, although Monte Carlo methods are better and could eventually be introduced into the ROSY ESR dating program (Brennan et al., 1997). It was also concluded that a revision of previously published ESR ages using one-group transport for beta dosimetry will be needed. For cases where the U concentrations in enamel are generally low (less than a few ppm), previously published ESR ages on tooth enamel are systematically too young by 5–30%.     

A method for thin foil thickness determination by transmission electron microscopy

With the intention of determining the local thickness within a crystalline thin foil specimen, by means of transmission electron microscopy (TEM), a method previously proposed by Zuo and Shi [J.M. Zuo, Y.F. Shi, Microsc. Microanal. 7 (Suppl. 2) (2001) 224-225] was applied. Using the convergent beam technique, with the incident beam parallel to a zone axis with low indices, diffraction patterns were obtained for some aluminum alloys with low solute content. These patterns were contrasted with those obtained from simulations based on the dynamic theory with Bloch's waves formalism. The local thickness of the thin foil was then obtained by visually comparing the simulated patterns with the experimental one.Comparison of the proposed method with that based on the analysis of two-beam convergent beam patterns [P.M. Kelly, A. Jostsons, R.G. Blake, J.G. Napier, Phys. Stat. Solidi (a) 31 (1975) 771-780] and with that based on the ratio of intensity of the zero loss peak to the total intensity in an electron energy loss spectrum [R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, second ed., Plenum Press, New York, 1996] was carried out. A very good agreement between thicknesses determined using the different methods was found. The sensitivity of the method of Zuo et al. was found to be about 1 or 2 nm. The advantages and limitations of the different methods are discussed. The method of Zuo et al. can provide fast and reliable results and can be applied in all modern instruments.     

用反应窗理论研究离子与原子碰撞的态选择微分截面

基于描述低能离子与原子碰撞的分子库仑过垒模型,详细阐述了与入射离子速度相关的反应窗理论,并对影响势垒变化的平均径向速度做了修正.根据该理论,计算了C5 -He和He2 -He碰撞体系单电子俘获过程的态选择微分截面,并分别与Kamber等人和Mergel等人的实验结果进行了比较.     

Analyse du contraste d'un sous-joint de torsion (0 0 1) dans le silicium en MET à deux ondes

A quantitative analysis of the image of a low angle (0 0 1) twist boundary in silicon is performed using the two-beam dynamical theory of electron diffraction. The contrast features are discussed as functions of the thickness of the foil and possible elastic relaxation effects of the low angle twist boundary in the thin foil. To cite this article: R. Bonnet et al., C. R. Physique 3 (2002) 657–663.     

共面不对称条件下低能电子碰撞电离Ar(3p)的三重微分截面

采用修正的扭曲波玻恩近似方法, 在共面不对称几何条件下, 计算了能量为70.8 eV电子与Ar(3p)碰撞电离的(e, 2e)三重微分截面, 并与最新实验数据进行了比较. 结果表明电子交换效应和后碰撞相互作用在低能电子碰撞的(e, 2e)过程中起着重要作用. 关键词： (e,2e)三重微分截面 扭曲波玻恩近似 后碰撞相互作用 电子交换效应     

Xe(4d~(10))(e,2e)反应三重微分截面的理论研究

采用修正后的扭曲波玻恩近似理论,计算了共面不对称几何条件下Xe(4d~(10))(e,2e)反应的三重微分截面.散射电子能量为1000 eV,敲出电子能量为20 eV,散射电子角度分别固定在2°,4°和7.5°.理论计算与Avaldi等人的实验结果和扭曲波玻恩近似理论计算进行了比较,发现出射电子之间的后碰撞相互作用较弱,极化效应在反应过程中起着重要作用.     

High-Order Above-Threshold Ionization of H2+ in Intense Laser Field

The nonperturbative quantum electrodynamies method proposed by Fu et al. [Phys. Rev. A 75 （2007） 063419] is employed to study the high-order above-threshold ionization （ATI） of a diatomic molecule. Based on this frequency-domain theory, the high-order ATI process can be regarded as ATI followed by laser-assisted collision, where the total transition amplitude is the coherent summation of the contributions from each ATI channel. The angular-resolved ATI spectrum, which agrees with the results by Becket et al. based on the time-domain method, is obtained by this frequency domain theory. Furthermore, it is demonstrated that the interference characteristics representing the molecular structure in the ATI spectrum originates from the recollision of the electron with two-centre ion in each ATI channel.     

Electron Dispersion in Liquid Alkali and Their Alloys

Ashcroft's local empty core (EMC) model pseudopotential in the second-orderperturbation theory is used to study the electron dispersion relation, theFermi energy, and deviation in the Fermi energy from free electron value forthe liquid alkali metals and their equiatomic binary alloys for the firsttime. In the present computation, the use of pseudo-alloy-atom model (PAA)is proposed and found successful. The influence of the six different formsof the local field correction functions proposed by Hartree (H), Vashishta-Singwi (VS), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F), andSarkar et al. (S) on the aforesaid electronic properties is examined explicitly,which reflects the varying effects of screening. The depth of the negativehump in the electron dispersion of liquid alkalis decreases in the orderLi →K, except for Rb and Cs, it increases. The results of alloys are in predictive nature.     

Auger kinetic energies and electronic relaxation phenomena in atoms and solids

A semi-empirical theory has been developed to calculate the kinetic energy of Auger electrons resulting from radiationless transitions in both free atoms and metals. Experimental electron binding energies and calculated two-electron interaction and relaxation energies are used. Relaxation energies are determined by means of hyper-Hartree—Fock hole-state calculations. To account for extra-atomic relaxation phenomena in metals, it is assumed that conductionband electrons occupy free-atom-like screening orbitais. The relationship of the present theory to recent work of Shirley et al., Larkins, Kim et al. and Watson et al. is discussed. The dependence of the Auger cross-relaxation energy on the ionicity of compounds is briefly discussed.