Relativistic effects in low energy electron scattering from atoms

Recent relativistic calculations on low energy elastic scattering of electrons from atoms are reviewed. Two aspects are emphasized : mathematical transformations and numerical methods used in the solution of the Dirac equation and the summation of the partial wave series ; and the results of relativistic calculations, in particular the behaviour of the cross sections and spin polarizations for different atoms, different energies of the incident electron, and in different approximations. The effects of the screening of the nucleus by the atomic electrons, electron exchange (including the use of approximate exchange potentials) and the distortion of the atom by the incident electron are discussed. Detailed results for cross sections and spin polarizations are given for mercury and the inert gases. The differences arising from use of the Dirac equation instead of the Schrödinger equation are considered.     

Calculated beam intensities for low energy positron diffraction

The intensity of the specular beam for low energy positron diffraction (LEPD) from the Cu(111) surface is calculated in the energy range 20–150 eV. Close agreement is found with the first experimental data for LEPD published recently. This result indicates that LEPD may be exploited as a technique for determining surface structures in the near future. It is suggested that LEPD may have advantages in surface structure analysis in comparison with low energy electron diffraction (LEED).     

Low energy electron diffraction with position-sensitive detection: Intensity measurements and comparison with positron diffraction for NaF(001)

A novel low energy electron diffraction (LEED) system incorporating a position-sensitive detector has been used to measure beam intensities as a function of incident electron energy. The detector characteristics, and the control of the incident beam focus and current are described. The (0,0)-beam intensity for NaF(001) surface was measured over the incident energy range of 2–150 eV, and the results were compared with the corresponding results for low energy positron diffraction (LEPD) [A.P. Mills and W.S. Crane, Phys. Rev. B31 (1985) 3988] as well as earlier LEED results. The LEED plots were found to contain two types of peak not present in corresponding LEPD ones: (a) narrow peaks attributable to beam-threshold interferences, and (b) odd-order Bragg peaks. The absences of these types of peak in LEPD are attributed tentatively to (a) the large incident angular divergence in the LEPD measurement cited, and (b) the relatively weak scattering of low energy positrons by Na⁺ ions, respectively.     

Intermediate changes in high energy elastic scattering from atoms

Correction of the Glaubner T-matrix to account for target particle motion and energy change in elastic scattering of charged particles from atomic systems is found to vanish to leading order.     

Polarization and exchange in electron scattering by atoms in solids

Two corrections are employed to the theoretical calculations of the cross-sections of elastic scattering of electrons by atoms in solids. The corrections are due to target polarization and electron exchange, of which the exchange is found to be more significant in the range of incident energies 100 to 1000 eV. Both the corrections are introduced through model potentials, for Li atoms as well as Al atoms in the solid state.     

Differential cross sections for elastic and inelastic positron scattering from alkali atoms

We have investigated positron scattering from the alkali atoms lithium, sodium, potassium and rubidium, focusing on the calculation of differential cross sections for elastic scattering and impact excitation. The results obtained with the two-state and five-state close-coupling method as well as the first-order distorted wave Born approximation are compared with each other and with some selected results for electron scattering. Numerical problems due to lack of, or numerically inaccurate, calculation of partial waves with large angular momenta are solved through semi-empirical fitting/extrapolation procedures.     

Differential cross section surfaces for low energy scattering of electrons and positrons from rare gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (1/r ⁴) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.     

Dynamics of positron scattering from lithium,sodium and potassium atoms in hot and dense plasmas

The dynamics of positron scattering from the ground state of lithium, sodium and potassium atoms in hot and dense plasmas has been investigated by applying a two-state approximation that includes elastic scattering and rearrangement scattering. The wave functions and energies of the target alkali atoms have been determined quite accurately within the framework of the ’method of model potential’. An inclusive study was made on the effects of plasma screening on the differential and total cross sections for elastic scattering and positronium formation in the ground state for the incident positron energy lying within 10 eV to 500 eV. For the unscreened case, our present results agree reasonably with the results of other calculations. It was found that the cross sections suffer considerable change due to the effects of the background plasma.     

Low energy muonic hydrogen reactions with heavy atoms

Average s-wave cross-sections appropriate for liquid hydrogen temperatures are calculated with the aid of an in-coming wave boundary condition for the muon transfer process μ^-p + Z → (μ^-Z)¹ + p on heavy atoms. The surface-correlated nature of the heavy atom X-rays is emphasized.     

Low energy adsorbate vibrational modes observed with inelastic helium atom scattering: CO on Pt(111)

We report on new low energy loss and gain features which appear in the time-of-flight distributions of He atoms scattered from CO adsorbed on Pt(111). On the basis of calculated normal mode energies we assign the 6 meV features to the excitation of the CO vibrational mode corresponding to a hindered translation of the upright molecule parallel to the surface. The energy and intensity of the 6 meV mode as a function of coverage, sample temperature and scattering conditions are investigated. At high coverages, concurrent with the appearance of a well ordered (4 × 2) overlayer structure, the 6 meV mode is observed to broaden and shift in energy to about 7.4 meV. The dispersion curve measured at high coverages is flat suggesting that there is no direct coupling between hindered translations of adjacent CO molecules.     

Resonant scattering of ultracold atoms in low dimensions

Low energy scattering amplitudes for two atoms in one- and two-dimensional atomic waveguides are derived for short range isotropic and resonant interactions in high partial wave channels. Taking into account the finite width of the resonance which was neglected in previous works is shown to have important implications in the properties of the confinement induced resonances. For spin polarized fermions in quasi-1D waveguides, it imposes a strong constraint on the atomic density for achieving the Fermi Tonks Girardeau gas. For a planar waveguide, the characteristics of the 2D induced scattering resonances in p- and d-waves are determined as a function of the 3D scattering parameters and of the waveguide frequency.     

乙炔分子的低能电子弹性散射研究

本文采用R矩阵方法研究低能电子与乙炔分子的弹性散射截面及其动力学过程.研究采用静态交换势模型(SE)和静态交换加极化势模型(SEP)两种方法.结果显示当前SEP模型预测的弹性散射截面与已有的实验值和理论结果符合的很好.SE模型预测了一个位于4.75 eV,宽度为3.0 eV的π形状共振态.该共振态在SEP模型中的位置降低为2.65 eV,宽度为1.5 eV.它来自B_(2g)和B_(3g)对称性贡献,理论预测共振态位置与实验值符合的很好.我们讨论了考虑不同的虚轨道对共振态的影响,说明预测精确的共振态的位置需要准确的处理极化效应.基于SEP模型得到的K矩阵元,我们使用POLYDCS代码计算的散射微分截面(DCS),和实验值符合的较好.除此之外,我们还报道了2 eV时转动分辨的态-态微分截面.     

The scattering of He atoms from an ordered layer of Xe atoms on Cu(001) with energy analysis

The scattering of He atoms from an ordered layer of Xe atoms deposited on Cu(001) is described. Energy analysis was carried out using a LiF crystal analyser. Energy gain and loss events were observed, the values of which were independent of the parallel momentum transfer. Results are shown for varying degrees of coverage where at large coverages a change in the energy spectrum of the scattered atoms was observed. Adsorption of Xe on a Cu surface damaged by ion bombardment yielded no inelastic information — only an elastic component was detected.     

Influence of surface conditions on thermal positron reemission spectra from W(100)

Our measurements trace the temporal dependence of positron reemission spectral features, namely, the reemission intensity, energy and angular distributions with post-cleaning time and oxygen exposure. The unwanted inelastic component in the reemission spectra can be kept at less than 6 % of the total reemitted positrons during long and continuous operation by simply ensuring the cleanliness of the sample with better vacuum level. Simultaneously the other optimized spectral features of the beam can be maintained except for the reemission spectral intensity which decreased with increasing time after cleaning. Even though oxygen at high temperature is used normally in the cleaning process of the tungsten moderators, exposure of the sample to oxygen at room temperature caused an exponential reduction in the reemission yield with exposure time.     

Virtual Compton scattering at low energies with a positron beam

The European Physical Journal A - Using a transport model that includes a first-order chiral phase transition between the partonic and the hadronic matter, we study the development of density...     

Surface analysis with low energy ion scattering

Principles and applications of low energy ion scattering for surface analysis are presented. Basic features are the binary collision concept, the scattering cross-sections and the ion neutralization process. The potential and the limitations of the method are outlined. Some pertinent experimental aspects are considered. In a number of examples the performance of the technique is demonstrated for qualitative and quantitative composition analysis and for studies of surface structures. Finally a few comparisons are made with other techniques, such as AES, LEED, or SIMS.     

Intensity correlation of fluorescence radiation from impurity atoms in solids in the low temperature limit

The influence of the linear electron-photon coupling on the intensity correlation function of the resonance fluorescence light from an impurity atom in a solid is studied. It is found that this coupling can lead to a stronger anticorrelation of the emitted photons in comparison with the case of a free atom.