Electron and positron scattering from atomic potassium

The present coupled-channels-optical method (CCO) provides a comprehensive theoretical calculation of electron-potassium atom and positron-potassium atom scattering at intermediate energies. The CCO calculations for various physical observables are compared with the convergent close-coupling method (CCC) and other theoretical results, as well as experimental data where available. The CCO provides good to fair agreement with results from the convergent close coupling calculations, which are considered to be the state of the art for alkali atoms in the electron channel.     

一种考虑几何屏蔽效应的计算“电子-分子”散射总截面的可加性规则修正方法

在考虑分子内原子间的几何屏蔽效应随电子入射能量变化的基础上, 提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的可加性规则修正方法. 利用这一修正后的可加性规则并使用“电子-C, H, O, N原子”散射总截面的实验数据, 在50—5000 eV内计算了电子被NO, N₂O, NO₂和C₂H₆分子散射的总截面, 且将计算结果与实验结果及其他理论结果进行了比较. 结果表明, 利用这一方法修正过的可加性规则进行计 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Differential cross sections for muonic hydrogen scattering on hydrogen molecules

The results of first calculations of the differential cross sections for muonic hydrogen scattering on hydrogen molecules are presented. They are functions of the initial and final kinetic energy of the system and the scattering angle. These calculations are based on the respective set of cross sections for muonic hydrogen scattering on hydrogen nuclei, obtained within the framework of the adiabatic method. The Fermi pseudopotential method is used to estimate the molecular binding effects. The influence of electrons on the cross sections under consideration is described in terms of the effective screening potential. Rotational and vibrational transitions are taken into account. The calculated molecular differential cross sections show a strong angular dependence. This effect is very significant for the electronic contributions to the cross sections, e.g. for collision energies above approximately 0.1 eV only the cross sections of small scattering angles are influenced considerably by the screening. Since these differential cross sections give detailed information about the final energies and complicated angular distributions of the scattered muonic atoms they are the proper basis for calculations concerning the deceleration of muonic hydrogen atoms in molecular hydrogen targets and for Monte Carlo simulations of different experiments in muonic physics.     

Diffraction and selective adsorption in the corrugated hard wall model

Numerical calculations are carried out for the elastic scattering of thermal energy atoms from a perfect crystalline surface with a square unit cell. The surface is treated as an infinitely-repulsive corrugated hard wall with an attractive square well in front of the hard wall. Closed, as well as open and bound, channels are included in the calculation, and multiple scattering effects are treated explicitly. It is concluded from these numerical studies that the single scattering approximation is inadequate, that closed channels must be included in any numerical calculations, and that selective adsorption can readily be understood within the framework of this simple model. Moreover, selective adsorption minima as well as maxima are found in this calculation without the necessity of invoking inelastic processes. The general quantitative agreement of the calculated results with the HeLiF diffraction experiments, the relative ease of handling more complicated corrugated surfaces, and the small amount of computer time required for these calculations suggest that the hard wall model is ideally suited in a parameterization scheme.     

K-Li冷碰撞散射特性的理论研究

本文应用量子方法和半经典方法计算了表征锂(6Li,7Li) 原子和钾(39K,40K,41K) 原子间超冷碰撞特性的散射参数,如s波散射长度,有效力程,p波散射长度等。超冷温度下6Li-39K单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面均存在显著的i波形状共振. 此外,本文应用简并内态近似获得了超精细态相互作用时的s波散射长度.     

Theoretical studies of fast He atom scattering from a W{100} surface

Theoretical calculations of the scattering of fast neutral He atoms from W{100} are presented which are directly compared to the results from recent experiments of Nielsen and Delchar. The experiments which were performed for He atom energies between 150 and 1000 eV, and for incident polar angles between 0° and 65° as measured from the surface normal, displayed peaks in the polar angle distributions at 72°, 56° and ±10°. The results from classical dynamics calculations are employed here to explain the scattering phenomena that give rise to these peaks in the polar distributions. The calculations indicate that the peak at 72° is primarily due to scattering from the first layer W atoms. The peaks at ± 10° and 56° are unusual in that there are a multitude of different collision paths that result in the He atom being scattered into the same final angle. The peaks at ± 10° result from He atoms scattering mainly from the second, third and fourth layers of W atoms. The He atoms are focused on the outward path into the near normal direction by two first and two second layer W atoms. Subsurface scattering is also responsible for the peak at 56°. In this case the channel of first and second layer W atoms that focuses the outgoing He atoms is oriented at 54.7° with respect to the surface normal. It is proposed that slight variations of the experimental data from the calculated values are due to surface reconstruction of W{100} and that a more thorough analysis could reveal the microscopic nature of this structure.     

Elastic scattering of electrons by benzene

Relative differential cross-sections for the elastic scattering of electrons from benzene have been measured at incident energies 300, 500, 700 and 900eV and for scattering angles between 30° and 120°. The results are discussed and compared with the independent atom model (IAM) calculations. Two different sets of scattering amplitudes for the constituent atoms of benzene were used in these calculations, one obtained from first Born approximation and the other from partial wave analysis of the Dirac equation. Only the static interaction was taken into account in the calculations. The higher the incident energy, the better is the observed agreement between experiment and theory. This indicates that at higher energies, absorption, exchange and polarization effects are not significant as compared to the static interaction and that the IAM satisfactorily predicts the interference of scattering from the individual atoms of C₆H₆.     

Bragg scattering of cooper pairs in an ultracold Fermi gas

We present a theoretical treatment of Bragg scattering of a degenerate Fermi gas in the weakly interacting BCS regime. Our numerical calculations predict correlated scattering of Cooper pairs into a spherical shell in momentum space. The scattered shell of correlated atoms is centered at half the usual Bragg momentum transfer, and can be clearly distinguished from atoms scattered by the usual single-particle Bragg mechanism. We develop an analytic model that explains key features of the correlated-pair Bragg scattering, and determine the dependence of that scattering on the initial pair correlations in the gas.     

Elastic scattering of slow positrons on atoms

The results of calculations of the elastic scattering cross section of positrons on noble gas and alkali atoms are presented. The calculations are performed within the one-electron Hartree-Fock approximation with multielectron correlations in the so-called random phase approximation with exchange taken into account. Virtual positronium formation is taken into account and proved to be very important. Arguments are presented that the positron polarization potential is repulsive for alkali atoms. The results obtained are in a reasonable agreement with experiment and with some previously reported calculations.     

Mg-CO(X1Σ+)体系的冷碰撞动力学

应用量子散射动力学方法, 研究了电场条件下Mg-CO体系的冷碰撞动力学性质, 探索了外电场对碰撞体系低场追索态的弹性和非弹性截面的影响, 为碱土金属Mg原子感应冷却CO分子提供理论预测.     

Surface structure from angular dependence of auger electron emission

Auger electrons emitted from atoms in the vicinity of a solid surface can show strong angular dependence in their emission due to elastic scattering from the surrounding ion cores of the solid. Model calculations for the emission of Auger electrons from sulphur adsorbed on a Ni (100) surface show that these angular features are very sensitive to the adsorption site of the emitting atoms, and that a study of these effects should produce a method of surface structure analysis for the location of preferred surface adsorption sites. This approach would appear to be an improvement over LEED because of its high sensitivity to structural changes and because it does not require long range order in the adsorbate layer.     

Linear photoelectron diffraction: application of a rapid approximation for surface structural studies

The linear superposition approximation proposed by Wander, Pendry and Van Hove for efficient low-energy electron diffraction calculations (“linear LEED”) is applied to photoelectron diffraction (“linear PD”). As with linear LEED, linear PD works very well for calculating the effect of displaced atoms. However, due to strong forward scattering at higher energies, linear PD requires that atoms do not move into or out of alignment. This limitation can be removed by suitable simple adjustments to the basic approximation, promising to make the method effective for structural searches of complex surfaces.     

Study of various few-body systems using Gaussian expansion method (GEM)

We review our calculation method, Gaussian expansion method (GEM), to solve accurately the Schrödinger equations for bound, resonant and scattering states of few-body systems. Use is made of the Rayleigh-Ritz variational method for bound states, the complex-scaling method for resonant states and the Kohn-type variational principle to S-matrix for scattering states. GEM was proposed 30 years ago and has been applied to a variety of subjects in few-body (3- to 5-body) systems, such as 1) few-nucleon systems, 2) few-body structure of hypernuclei, 3) clustering structure of light nuclei and unstable nuclei, 4) exotic atoms/molecules, 5) cold atoms, 6) nuclear astrophysics and 7) structure of exotic hadrons. Showing examples in our published papers, we explain i) high accuracy of GEM calculations and its reason, ii) wide applicability of GEM to various few-body systems, iii) successful predictions by GEM calculations before measurements. The total bound-state wave function is expanded in terms of few-body Gaussian basis functions spanned over all the sets of rearrangement Jacobi coordinates. Gaussians with ranges in geometric progression work very well both for shortrange and long-range behavior of the few-body wave functions. Use of Gaussians with complex ranges gives much more accurate solution than in the case of real-range Gaussians, especially, when the wave function has many nodes (oscillations). These basis functions can well be applied to calculations using the complex-scaling method for resonances. For the few-body scattering states, the amplitude of the interaction region is expanded in terms of those few-body Gaussian basis functions.     

Spin-flip cross sections in muonic hydrogen scattering on hydrogen molecules

The results of calculations of the total cross sections of spin-flip processes in low energy muonic hydrogen scattering on hydrogen molecules are presented. These calculations are based on the respective set of cross sections for muonic hydrogen scattering on hydrogen nuclei, obtained within the framework of the multichannel adiabatic method. All combinations of the three hydrogen isotopes are considered. Molecular binding effects are described in terms of the Fermi pseudopotential method. Electron screening effects are calculated in the distorted wave Born approximation. Rotational and vibrational transitions of the molecules, due to collisions with muonic hydrogen atoms, are taken into account. The molecular and electron screening corrections do not exceed a few tens per cent for lowest collision energies.     

Two-time Green's function method in quantum electrodynamics of high-Z few-electron atoms

The two-time Green's function method in quantum electrodynamics of high-Z few-electron atoms is described in detail. This method provides a simple procedure for deriving formulas for the energy shift of a single level and for the energies and wave functions of degenerate and quasi-degenerate states. It also allows one to derive formulas for the transition and scattering amplitudes. Application of the method to resonance scattering processes yields a systematic theory for the spectral line shape. The practical ability of the method is demonstrated by deriving formulas for the QED and interelectronic-interaction corrections to energy levels and transition and scattering amplitudes in one-, two-, and three-electron atoms. Numerical calculations of the Lamb shift, the hyperfine splitting, the bound-electron g factor, and the radiative recombination cross section in heavy ions are also reviewed.     

Differentielle Streuquerschnitte bei der Wechselwirkung neutraler Moleküle

An apparatus to measure differential scattering cross sections using crossed molecular beams is described and results for the scattering of potassium atoms by different heavy atoms and molecules are presented. The experimental data are compared with theoretical calculations for a Buckingham-(exp-six)-potential and the parameters of this potential are determined. To carry out this comparison, it was necessary to average the existing calculations of E. A.Mason over the Maxwellian velocity distribution in the potassium beam. These averaged differential cross sections are tabulated for various angles and temperatures in the appendix of the present paper. Using crossed beams of molecules containing halogens, we can observe chemical reactions. Methods to determine total and differential reaction cross sections are discussed.     

Simulation of the lattice dynamics of an Al-Cu-Fe icosahedral quasicrystal

The total and partial vibrational spectra of aluminum, copper, and iron atoms in an Al-Cu-Fe icosahedral quasicrystal are calculated by the recursive method. The calculations are based on the 1/1 crystal approximant. The interaction of atoms in the Al-Cu-Fe quasicrystal is described within the EAM model. The calculated spectra are in satisfactory agreement with the experimental data on neutron inelastic scattering.     

Theoretical studies of antihydrogen formation in collisions between positronium atoms and antiprotons

The current state of the theoretical methods that are used in calculations of cross sections for the production of antihydrogen in collisions between antiprotons and positronium atoms is reviewed. A broad outline of available methods together with the results of recent computations are presented. The main emphasis is made on the general close-coupling approach that allows any reaction channel to be taken as the initial state of the collision system. In this way, and on account of charge-conjugation invariance, the formation of antihydrogen in collisions between antiprotons and positronium atoms becomes linked to positron-hydrogen scattering and the same computational methods can be applied to either reaction. The review gives references to recent papers on the subject.     

一种计算中、高能电子被分子散射总截面的修正势方法

在考虑分子内成键原子间的电子云重叠效应的基础上，提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的修正势方法.利用可加性规则及Hartree-Fock波函数，使用这一修正过的复光学势，在30—5000eV内对电子被4个等电子(Z=18)分子(HCl，H₂S，PH₃和SiH₄)散射的总截面进行了计算，并将理论计算值与实验结果及其他理论值进行了比较.结果表明，利用这一修正过的复光学势及可加性规则进行计算，所得理论值与实验结果更为接近. 关键词： 电子散射 总截面 可加性规则 束缚原子     

Delbrück scattering

The various calculations of the Delbrück scattering amplitude are presented. This is followed by some generalities on the elastic scattering of photons by heavy atoms and finally by a review of the experiments together with a comparison with the calculations.