Measurement of surface defects by low-energy electron diffraction

Low-energy electron diffraction is capable of detection of non-periodic arrangements by spot profile analysis (SPA-LEED). All kinds of defects may be identified by a qualitative and nevertheless systematic evaluation of the diffraction pattern with respect to spot shape, background and energy dependance. It is shown that the kinematical approximation provides far reaching results. The importance and achievements of the newly developed high-resolution instrumentation is demonstrated. The quantitative evaluation provides data on defect density and distribution, which are not available otherwise, as demonstrated with examples. A comparison of imaging and diffraction techniques shows the special advandages of diffraction with respect to quantitative analysis.     

Phenomenology of low-energy ππ scattering

Exact representations of the low-energy ππ amplitudes by (for the S-waves) generalized effective range formulas and by (for the P-wave) a resonance plus background expression are derived from inverse-amplitude partial-wave dispersion relations, and applied to the phenomenological analysis of a typical set of phase shifts deduced from peripheral pion production. With the aid of rigorous crossing conditions it is found that the simplest satisfactory account of the S-waves is provided by a current algebra type solution with (Adler) zeros and small scattering lengths. Solutions without zeros (S-wave dominant) are completely ruled out. Inelasticity is important in describing the rise of the isoscalar phase shift above the σ resonance, but can be neglected below 1 GeV in other channels. The P-wave is ?-dominated, but its width is sensitive to background terms. The phenomenological importance of rigorous results is stressed.     

Theory of low-energy ππ scattering

A four-parameter family of solutions to the ππ partial-wave dispersion relations is obtained using the inverse amplitude method. Choosing the parameters consistent with phenomenological values and so as to maximize agreement with rigorous sum rules and inequalities leads to a firm prediction of the phase shifts for S and P wave scattering and for the amplitudes below threshold. Neglecting inelasticity, the I = 0S-wave resonates near $\text{s}\text{= 540}\text{MeV}$, the I = 2 S-wave falls to about ?12° near 1 GeV, and the P-wave is ?-dominated. The S-waves have subthreshold zeroes consistent with the Adler condition. Inelastic effects are estimated and found to be small below the rho meson mass.     

Analysis of the outermost atomic layer of a surface by low-energy ion scattering

Noble gas ion scattering is used to study the surface of solid targets. It is shown that this technique can be used to obtain a mass spectrum of the first atomic layer of the surface. Since the outermost atoms will largely determine the chemical reactivity and physical properties of the surface, this is an important property for an analytic tool to have. In addition to reflected ions, high-energy recoil ions are sometimes observed.Both the reflected and the recoil ions provide information about the atomic structure of the surface. The various possibilities of ion scattering are demonstrated for bromine, oxygen, sodium and potassium adsorbed on a Si(111) surface, for halogens adsorbed on Ni(100) and for GaP (110), (111) ($\text{111}$) surfaces. The influence of thermal vibrations of surface atoms and of electronic excitation on the spectra is discussed.     

Numerical experiments on low-energy ion scattering at a surface containing adsorbed impurity atoms

The energy distributions of Cs⁺ and W⁺ ions scattered, respectively, at Mo and W single crystals, the surfaces of which contain adsorbed oxygen, nitrogen, and cesium atoms, are calculated via the molecular dynamics method. It is demonstrated that the energy spectra are sensitive to the impurity atoms of a surface. When compared with the energy distributions corresponding to a pure surface, even a partial monolayer of these atoms can modify their shape. A peak in the scattered-ion intensity is commonly shifted to the lowerenergy region if the adsorbate is light and to higher energies if the adsorbate is heavy.     

Systematics of low-energy πN scattering

A simple current algebra model is shown to account for a wide range of low-energy πN experimental data including twenty-eight subthreshold expansion coefficients, six scattering lengths, and the scattering phase shifts below T_π = 300 MeV. A sigma term of σ_NN = 62 ± 10 MeV is obtained.     

The resolving power of a low-energy electron diffractometer and the analysis of surface defects

The angular distribution of intensity in a LEED diffraction spot gives information on deviations from long-range order in surfaces and overlayers. Such information is in part obscured by the finite resolution of the instrument. It is demonstrated that the instrument can detect phase correlations over distances on the surface far greater than the conventionally used “transfer width” but that this capability has little to do with the ability to resolve deviations from surface order. A quantity, “the minimum angle of resolution”, is defined to describe the resolving power of a LEED instrument. It depends, in addition to the shape and width of the instrument response function, on the accuracy of the measurement. This concept is applied to a variety of model surfaces with varying degrees of short-range order. It is demonstrated that the size of ordered domain that can be resolved depends on the type of defects present. Comparison is made with the traditional “transfer width” approach.     

Disordering and annealing of a Si surface under low-energy Si bombardment

Abstract

We simulated the bombardment of Si(100)(2 × 1) by Si atoms using molecular dynamics. The kinetic energies of the projectiles were 100 and 50 eV. To model the Si–Si-interactions the empirical potential of Stillinger and Weber with the two body part of the potential splined to the universal potential was used. A geometric criterion based on the Lindemann radius was defined to study damage in the Si target. We observed clusters of disordered Si atoms in the target induced by the bombardment. Large clusters of about 50 atoms are formed in the beginning of the bombardment; they shrink and decay into smaller clusters until and equilibrium cluster size of about 10 atoms is reached. Upon annealing at elevated temperature the disordered zones dissolve into point defects.     

Enhancement of scattering of surface plasmon polaritons by surface defects: Technique and results

The intensities of scattering of surface plasmon polaritons by regular surface structures significantly exceed (by several orders of magnitude) the intensities of scattering of evanescent surface waves from these structures. When plasmon polaritons are scattered by diffraction grating grooves a few micrometers wide, the scattering from the trailing edge of a grating groove greatly exceeds the scattering level from the groove leading edge. Some of our original developments are presented that make it possible to put into effect an inexpensive software-hardware optical complex with a wide dynamic range, high sensitivity, and digital lock-in detection based on the use of a computer sound card.     

Resonances in low-energy rare-gas atom scattering

Precise values of energies and widths of low-lying resonances in some rare-gas atom systems (Ar-Ar, Ne-Kr, Ne-Xe) are reported.     

Nucleon recoil for low-energy antikaon-deuteron scattering

The effect of the nucleon recoil for antikaon-deuteron scattering is studied in the framework of effective field theory. In particular, we performed a calculation of the nucleon recoil effect for the double scattering process. It is shown that the leading correction to the static term that emerges at order ξ ^1/2, where ξ?=?m _K /M _N , vanishes due to a complete cancellation of individually large terms. The resulting recoil effect for the double scattering process is found to be of order of 10–15% compared to the static term. We also developed a method to include the nucleon recoil effect in the calculation of the multiple scattering diagrams.     

Secondary defects in low-energy As-implanted Si

14 /cm² dose of As ions followed by both isochronal and isothermal annealing. The elementary defects generated first during solid-phase epitaxial recovery of implantation-induced amorphous layers at temperatures of 550 °C and/or 600 °C are {311} defects 2–3 nm long. They are considered to be transformed into {111} and {100} defects after annealing at temperatures higher than 750 °C. These secondary defects show the opposite annealing behavior to the dissolution and growth by the difference of their depth positions at 800 °C. This phenomenon is explained by the diffusion of self-interstitials contained in defects. With regard to the formation and dissolution of defects, there is no significant difference between the effects of rapid thermal annealing (950 °C for 10 s) and furnace annealing (800 °C for 10 min). Received: 14 November 1997/Accepted: 16 November 1997     

Quantum interference effect on low-energy He atomic beam scattering at the surface of He II

The specular reflection coefficient for low-energy ⁴He atoms incident on the free surface of superfluid ⁴He is calculated as a single particle motion coupled to the ripplon field in an effective surface potential. We find a characteristic dip in the reflection coefficient as an interference effect for the truncated surface potential.     

Correction to the low-energy scattering of monopoles

Following Manton, and Atiyah and Hitchin we consider approximating solutions to the dynamic Yang-Mills-Higgs equations by motions on the finite-dimensional space M_k of stable k-monopoles. For initial data transverse to M_k the approximate motion will not be geodesic motion but instead will be motion in an effective potential on M_k.     

Elastic scattering of low-energy electrons by NO

We report elastic integral, momentum transfer and differential cross sections for electron scattering by N₂O for energies up to 50 eV. These results were obtained at the static-exchange approximation with the Schwinger Multichannel Method with Pseudopotentials [M.H.F. Bettega, L.G. Ferreira and M.A.P. Lima, Phys. Rev. A 47, 1111 (1993)]. In general our results show good agreement with experimental data and with other theoretical results but some discrepancies are found. We have also found a shape resonance around 4 eV in agreement with previous calculations using the R-matrix Method of Sarpal et al. [J. Phys. B 29, 857 (1996)]. On the other hand, the existence of a resonance at about 13 eV, clearly seen by the Schwinger Variational Iterative Method [Michelin et al., J. Phys. B 29, 2115 (1996)], can not be confirmed by our calculations. At this energy, our cross sections show a broad bump with no clear resonant behavior given by the eigenphase sum. Received: 13 November 1997 / Revised: 13 March 1998 / Accepted: 9 April 1998