Helium scattering from metal surfaces: potential determination and crystallography

We describe a method for determining the interaction potential between a helium atom and a metal surface from diffraction data. Results are presented for He/Ni(110) and He/Cu(110). For He/Cu(110) the potential 3s in good agreement with a first principles calculation but this is not true for He/Ni(110). We also show that a surface relaxation of 10% produces measurable changes in intensities.     

Enhanced Raman scattering on metal surfaces

We present a theory which describes enhanced Raman scattering from molecules adsorbed on metals. The enhancement is due to the screening fields induced by the optically polarized adsorbate in the presence of the substrate. These fields interact coherently with the applied optical field and the vibrational motion of the adsorbed molecule. The enhancement is shown to be coverage dependent and also rather sensitive to the optical dielectric function of the substrate. Predicted enhancements compare well with experiment.     

The thermal attenuation of elastic scattering of helium from copper single crystal surfaces

The dependence of diffraction peak intensities upon temperature for the scattering of helium from various copper surfaces: Cu (111), (100), (110), (113), (115) and (117), has been experimentally determined for two incident energies (21–63 meV) and a large range of incidence angles. For the close-packed faces (111) and (100), the data are consistent with a Debye-Waller formalism involving an effective surface mean square displacement <u²_eff〉. The quantitative fit with the data is good if anharmonic effects are properly taken into account. This result establishes that a generalized Debye-Waller formalism is, at least as a first approximation, relevant to the helium-surface diffraction. For the rougher surfaces the agreement remains good up to a threshold temperature above which the intensities are always lower than predicted by the model. It is proposed that this may be due to some kind of thermal roughening of the surface.     

Multiple light scattering from metal and dielectric rough surfaces

A numerical study is done on light scattering from one-dimensional random rough surfaces supporting both dielectrics and metals. The influence of the corrugation on the Brewster angle as well as on the angular distribution of transmitted intensity into the dielectric is investigated. The authors also obtain enhanced backscattering in the light reflected from metallic surfaces. Finally they discuss the influence of roughness on the drop of reflectance due to polariton absorption.     

Nonadiabatic electronic response in atom scattering from metal surfaces

The response of metal electrons to a localized perturbation varying in time is singular; this dynamic property may influence the observability of diffraction peaks in atom scattering from metal surfaces.     

Multiple light scattering from metal and dielectric rough surfaces

Abstract

A numerical study is done on light scattering from one-dimensional random rough surfaces supporting both dielectrics and metals. The influence of the corrugation on the Brewster angle as well as on the angular distribution of transmitted intensity into the dielectric is investigated. The authors also obtain enhanced backscattering in the light reflected from metallic surfaces. Finally they discuss the influence of roughness on the drop of reflectance due to polariton absorption.     

Unified treatment of fluorescence and raman scattering processes near metal surfaces

We present a general model study of surface-enhanced resonant Raman scattering and fluorescence focusing on the interplay between electromagnetic effects and the molecular dynamics. Our model molecule is placed close to two Ag nanoparticles and has two electronic levels. A Franck-Condon mechanism provides electron-vibration coupling. Using realistic parameter values for the molecule we find that an electromagnetic enhancement by 10 orders of magnitude can yield Raman cross sections sigma(R) of the order 10(-14) cm(2). We also discuss the dependence of sigma(R) on incident laser intensity.     

Brewster's scattering angle in scattered waves from slightly rough metal surfaces

Brewster's scattering angle in electromagnetic wave scattering from slightly random metal surfaces is investigated by means of the stochastic functional approach. While there are dips due to Brewster's scattering angle in scattering profiles from dielectric surfaces, Brewster's scattering angle does not exist in scattering from metal surfaces. However, the dips can exist in scattering from rough metal surfaces with the optically denser medium to convert evanescent wave into radiative wave.     

Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar⁺ ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10¹⁶–10¹⁷ ions/cm² ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.     

Corrugating and anticorrugating static interactions in helium-atom scattering from metal surfaces

We perform a density-functional-theory calculation of the static repulsive potential of He scattering off a noble and a simple metal surface. The classical turning point of He on Cu(111) is found to be closer to the metal when the adatom is at top than at bridge site (anticorrugating effect). The potential of He on Al(111) is instead corrugated. By comparing the results of the two systems, we conclude that the He-metal anticorrugating effect occurs when the kinetic energy difference for He at top and bridge sites is larger than the electrostatic one, and an induced localized dipole on He is formed.     

Computer simulations and rainbow patterns of alkali ion scattering from metal surfaces

In a previous article [Surface Sci. 172 (1986) 90] the necessity of 3-dimensional computer simulations has been shown for the modelling of the scattering of 10–100 eV K⁺ from W(110). The measured triple differential cross section shows a complicated peak structure which can only be understood with the help of trajectory calculations. A simple model potential is given that can describe the scattering at 35 eV. The calculated trajectories show from which impact regions in the surface unit cell the peaks in the spectra originate. This mapping also allows a rainbow analysis for a triple differential cross section.     

Grazing incidence scattering of vibrationally excited H2 molecules from metal surfaces

We have studied the scattering of vibrationally excited H₂ molecules from metal surfaces under fast grazing incidence conditions, by means of quasi-classical calculations based on six-dimensional potential energy surfaces. We show that, in spite of the fast parallel motion, the reorientation of the molecule along the trajectory plays a fundamental role on the scattering, being responsible for the nonmonotonic behavior observed as a function of the normal incidence energy, similar to that observed under slow normal incidence conditions. The present study has allowed us to further prove that the interaction between a H₂ molecule and an ordered metal surface under fast grazing incidence conditions is, in general, governed by the normal momentum of the molecule.     

An approach to the scattering of electromagnetic waves by rough metal surfaces

Summary A method to describe the scattering of an electromagnetic wave by a rough metal surface whose imperfections are not small with respect to the wave-length is proposed. The surface is simulated as a plane uniformly covered by hemispheres of equal radii, the material is supposed to be a good conductor and the incident wave to propagate normally to the plane. The use of an addition theorem for spherical waves together with some statistical considerations allows the boundary conditions to come out in a simple form.

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Riassunto Si propone un metodo per il calcolo dello scattering della radiazione elettromagnetica da parte di una superficie metallica rugosa avente imperfezioni non piccole rispetto alla lunghezza d’onda. La superficie è schematizzata mediante un piano ricoperto uniformemente da semisfere di egual raggio. Si suppone che il materiale sia buon conduttore (abbastanza da giustificare l’approssimazione di Leontovich) e che la radiazione incida normalmente al piano. L’uso di un teorema di addizione per le onde sferiche insieme ad alcune considerazioni di carattere statistico permette di scrivere le condizioni al contorno in forma semplice.

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Резюме Предлагается метод для описания рассеяния электромагнитной волны на шероховатой металлической поверхности, неоднородности которой не являются малыми по сравнению с длиной волны. Поверхность представляется в виде плоскости, покрытой равномерно полусферами одинакового радиуса. Предполагается, что материал поверхности представляет хороший проводник, а падающая волна распространяется нормально к плоскости. Использование теоремы дополнительности для сферических волн и статистического описания позволяет записать граничные условия в простой форме.

     

Orientation dependence in the dissociative scattering of hydrogen molecules from metal surfaces

We investigate the molecular orientation dependence of the dissociation probability of neutral molecular hydrogen scattered from metal surfaces. The probability distributions are calculated in the cases when positive and negative hydrogen ions (ion pair) are formed and when two neutral hydrogen atoms are formed. On the basis of numerical results, it is found that a parallel orientation preference of probability distributions shows a tendency to be suppressed in the case when an ion pair is formed as compared with the case when two neutral hydrogen atoms are formed.     

Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.     

Inelastic scattering of electrons by helium

Measurements of the angular distribution and total cross section for all inelastic processes in the scattering of intermediate and high energy electrons by helium atoms are analyzed using the first Born approximation.