The electron ionization rate in multilayered semiconductor structures

We present theoretical results of the electron impact ionization rate in GaAs/AlGaAs multiquantum well structures as a function of applied electric field for various geometries, i.e., well and barrier widths. In addition, we present preliminary measurements of the current-voltage characteristics of MBE grown devices which demonstrate very low leakage current as well as sharp breakdwon behavior. It is found that the net ionization rate, determined by averaging over the constitutent GaAs and AlGaAs layers, approaches the weighted average of the constituent bulk rates at high electric field strengths; the potential discontinuity is relatively unimportant. The electron ionization rate within the well regions alone is still higher than that in bulk GaAs, but is insufficiently enhanced to compensate for the much lower rate in the AlGaAs layers. As the field is lowered to 250.0 kV/cm, the average ionization rate in the multiquantum well structure becomes larger than in the bulk.     

Measurements of electric-field strengths in ionization fronts during breakdown

Using laser-induced fluorescence-dip Stark spectroscopy, we performed time-resolved, direct measurements of electric-field strengths during the breakdown phase of a low-pressure, pulsed discharge in xenon. With this experimental technique we could for the first time quantitatively measure the time evolution of the driving force of the plasma breakdown process: the electric field. Moving ionization fronts were measured with submicrosecond resolution. These ionization fronts were sustained by a spatially narrow, rapidly moving region of strong electric field.     

磁性多层膜磁特性的表面效应

利用Monte-Carlo方法和转移矩阵法研究了具有不同表面交换耦合J_s和薄膜厚度 磁性多层 膜的表面和尺寸对磁相变的影响.模拟结果表明，系统的相变温度随薄膜层数的变化取决于J_s/J(J为体内交换耦合)，当J_s/J大于某一临界值时，由于表面磁 有序先于体内磁有序 ，系统的相变温度随薄膜层数的增多而降低，反之，表面磁无序可与体内磁有序共存，系统 的相变温度随薄膜层数的增多而升高；当J_s/J较小时，随J_s增大 ，系统的居里温度缓慢 升高，趋近于体内相变温度，而当J_s/J较大时，随J_s增大，系统的 居里温度 呈线性升高.模拟结果与用转移矩阵法推导出的结果相当符合，且很好地解释了实验事实. 关键词： 磁星多层膜 交换耦合 Monte-Carlo模拟 转移矩阵法     

Measurements of frequency of ionization waves in axial magnetic field

The observations of the frequency change of selfexcited ionization waves in an axially electric field are present. The typical results obtained in Ne discharge are discussed.On leave fromCharles University, Dept. of Electronics and Vacuum Physics, Prague, Ke Karlovu 5, 121 16 Praha, Czechoslovakia.     

New surface ionization processes

The results of analysis of processes including surface ionization are presented. It is found that these processes do not fit into the classical theory of surface ionization. This concerns the processes with diffusion exchange of particles between the adsorbed layer and the bulk of the emitter, catalytic processes on the emitter surface involving individual centers, as well as processes occurring in the adsorbed layers under illumination, electron bombardment, and in electric fields. We consider the results of analysis of surface ionization of alkali metal atoms and organic molecules on the surface of gold intermetallide (Na _x Au _y ), considerably extending information about this phenomenon.     

Measurements on the fermi surface of PdSb

Measurements of the de Haas-van Alphen effect in the metallic compound PdSb (NiAs structure) have been performed. By means of a Fourier expansion analysis we have been able to ascribe most of the frequency branches to three sheets of the Fermi surface. These show a partial resemblance to results on the isomorphic and isoelectronic compound AuSn of Edwardset al.     

Measurements of multiphoton ionization coefficients with ultrashort ultraviolet laser pulses

Multiphoton ionization coefficients of atmospheric gases were measured with ultrashort ultraviolet laser pulses. The values were obtained using two different experimental setups and the pressure range covered four orders of magnitude. The coefficients were pressure-independent and consistent with numbers predicted by the nonresonant Keldysh theory. Received: 13 June 2000 / Revised version: 5 October 2000 / Published online: 7 February 2001     

用共振多光子电离技术测量分子动力学参数

理论上推导了央三光子共振四光子电离过程中，电离流与多光子动力学参数的关系。首次提出用共振多光子离技术获得三光子吸收截面，四光子电离截面以及无辐射弛豫速率参数法。实验测量了气相甲苯分子里德堡态的上述动力学参数。结果在理论预期的数量级范围内。     

Broadband light absorption using a multilayered gap surface plasmon resonator

A broadband visible light absorber composed of multiple metal-dielectric-metal (MMDM) layers is proposed and numerically investigated. Dielectric layers of different thicknesses in the MMDM structure lead to multiple plasmon resonances at different wavelengths; as a result, efficient broadband absorption can be achieved under optimized conditions. We found that an average simulated absorption of 93 % was obtained over the entire visible spectrum of 400–700 nm by controlling the geometric parameters. Furthermore, the origin of the broadband absorption was studied, and the effects of the diameter and pitch of the pattern on the absorption were investigated. Our proposed structure with a periodic array of circular patterns represents a novel candidate for future applications in photovoltaic cells and thermal emitters.     

Field surface ionization of nanosized molecular complexes

The changes in the parameters of the temperature dependences of field surface ionization ion currents upon an increase in the electric field strength at the emitter surface is estimated for systems in which the ions under investigation are the end product of dissociation of multimolecular complexes with a large number of degrees of freedom. Expressions for the maxima on the temperature dependence of ion currents of field surface ionization are derived. The sizes of molecular nanocomplexes formed on the emitter surface are estimated and their thermodynamic stability is analyzed.     

Measurement of the surface density of states by field ionization

It is shown that recent measurements of field ion energy distributions from clean tungsten surfaces probe the density of metal states in the vicinity of the surface. We find j(ω) = (2π/kh)Σ_m| ∫ d³rψ_m(r)γ_z|²δ(ω??_m), where j(ω) is the ion current a ω, ψ_mand ?_m are electronic metal eigenfunctions and eigenvalues in the presence of the external electric field used in field ionization and γ(z) is a function which is large near the noble gas atom. An explicit expression for γ(z) is given in the text. It is estimated that tungsten metal states with values of k₆ at least as large as 0.5 Å^?1 make an appreciable contribution to j(ω) where k₆ is the electron momentum parallel to the surface.     

Effects of surface and interface energies on the bending behavior of nanoscale multilayered beams

A modified continuum model of the nanoscale multilayered beams is established by incorporating surface and interface energies. Through the principle of minimum potential energy, the governing equations and boundary conditions are obtained. The closed-form solutions are presented and the overall Young's modulus of the beam is studied. The surface and interface energies are found to have a major influence on the bending behavior and the overall Young's modulus of the beam. The effect of surface and interface energies on the overall Young's modulus depends on the boundary condition of the beam, the values of the surface/interface elasticity constants and the initial surface/interface energy of the system. The results can be used to guide the determinations of the surface/interface elasticity properties and the initial surface/interface energies of the nanoscale multilayered materials through nanoscale beam bending experiments.     

半无界分层介质表面任意面源激发的弹性波场

研究了半无界层状介质自由表面任意形状的面源产生的弹性波场。首先,我们将层状介质中的传递矩阵理论从二维推向三维空间,在频率域研究了任意面源激发的三维弹性波理论问题。然后,深入研究了Rayleigh波和Love波的激发与传播特性,发现Rayleigh波和Love波的传播速度在与自由表面平行的平面内与传播方位角θ无关,但其激发强度却强烈地依赖于传播方位角θ。最后,我们具体研究了矩形源、无穷长条形源和圆盘激励出的弹性波场,并通过数值计算给出了Rayleigh波和Love波的位移指向性分布图。     

Elastic waves excited by a plane source on the surface of a multilayered medium

Elastic waves excited by a plane piezoelectric source with an arbitrary shape on the surface of a multilayered medium have been studied for the first time in this paper. On the basis of Abzo-zena [Geophys. J. R. Astron. Soc. 58, 91-105 (1979)] and Menke [Geophys. J. R. Astron. Soc. 59, 315-323 (1979)], the propagator matrix for the elastic wave field in multilayered medium is extended from two- to three-dimensional (3D) space. 3D elastic wave field is investigated and the displacement-stress response for the boundary conditions is obtained. The propagation of elastic wave in multilayered media is analyzed in 3D space in the frequency domain. The P-SV and SH modes corresponding to the poles are studied. The excitation and propagation of the modes are analyzed further. It is found that the propagation velocities of the P-SV and SH modes do not depend on the propagation azimuth theta in the plane parallel to the free surface of the multilayered medium while the displacement amplitudes are strongly dependent on the azimuth theta. The directional distribution functions of the modes are independent of the medium parameters and the modes and dependent on the shape and excitation fashion of the source. Finally, as an example, the displacement fields of the P-SV and SH modes excited by a rectangle source are analyzed. The displacement representation and numerical results of the directivity distribution functions for the P-SV and SH modes are obtained.     

Measurements of double differential cross sections in electron impact ionization of helium and argon

Double differential cross sections (angular distributions and energy loss spectra) have been measured of electrons after ionizing electron collisions with helium at primary energiesE ₀ between 25 eV and about 260 eV and with argon atE ₀=75, 150 and 200 eV. The spectra have been measured with an energy analyzing collector system of constant transmission. It was found that for high collision energies (E ₀≧ 80 eV) the outgoing electrons belong to one of the two energetically well separated groups, either thefast electrons, which are scattered mainly in forward direction or theslow electrons which are distributed isotropically into all angles. At low primary energiesE ₀ no separation into groups is possible. Several findings indicate the qualitative applicability of the binary collision model.     

Special features of strain-induced surface roughness formed in specimens with curvilinear geometry of the hardened surface layer-substrate interface

The paper studies the formation of strain-induced roughness on an initially flat surface of materials with a modified surface layer in tension. The dynamic boundary value problem of mechanics is solved numerically by the finite difference method for plane strain. The curvilinear surface layer-substrate interface is specified explicitly in the calculations. The mechanical response of the steel substrate and surface layer is described by elastoplastic models with isotropic hardening and elastic-brittle fracture models, respectively. The surface roughness shape and amplitude are found to depend on the thickness of both elastic and high-strength plastic layers with sinusoidal geometry of the hardened layer-substrate interface.     

Measurements of surface coordinates and slopes by shearography

This paper represents an optical method which is based on speckle-shearing interferometry for the measurement of surface coordinates and slopes. The technique enables generation of carrier fringes which measure surface coordinates, or displacement derivative fringes which measure surface slope. The theory of the method as well as some experimental results are presented.