Directional elastic peak and directional Auger electron spectroscopies — New tools for investigating surface-layer atomic structure

Intensities of elastically scattered electrons and Auger electrons emitted from a crystalline sample depend strongly on the direction of the primary electron beam with respect to the sample. Namely, distinct maxima of those intensities appear when the primary beam is parallel to one of the close-packed rows of atoms in the sample,

In an RFA analyzer, the angular distribution of the elastically scattered electrons or Auger electrons is integrated over a large scattering angle and does not influence the dependence of intensities mentioned above on the primary beam incidence angle (DEPES or DAES polar profiles). Those profiles can be obtained simply by rotation the sample positioned in front of the RFA analyzer around the axis parallel to the sample surface,

Maxima in DEPES and DAES polar profiles connected with the close-packed atom rows can be easily recognized because their position does not depend on the primary electron energy. Those maxima are the evidence for presence of the crystalline structure in the surface layer. They also enable characterization of this structure even for a crystalline layer composed of very small islands or maintained at temperatures too high to observe LEED patterns. This was shown for the Ag/Cu(111), Ag/Cu(011), and Ag/Ni(111) adsorption systems.

DEPES and DAES profiles are sensitive to the disorder in the surface layer. For example, the surface roughening transitions on the Ni(O11) and Cu(O11) faces was clearly demonstrated and the temperatures of these transitions in both cases were determined with the use of those methods.

A background on which maxima of the DAES and DEPES profiles appear is calculated using a simplified model. It is indicated that the shape of this background should contain information about the sample composition profile.     

New developments in theory of fast electron scattering in solids: Applications to microbeam analysis

The study of fast electron interaction with solids in the energy range from 100 eV to several tens of keV is prompted by quickly developing microbeam analysis techniques such as electron probe microanalysis, scanning electron microscopy, electron energy loss spectroscopy and so on. It turned out that for random solids the electron transport problem might be solved on the basis of the generalized radiative field similarity principle. The latter states that the exact differential elastic cross section in the kinetic equation may be replaced by an approximate one provided the conditions of radiative field similarity are fulfilled. Application of the generalized similarity principle to electron scattering in solids has revealed many interesting features of electron transport. Easy to use and effective formulae have been obtained for the angular and energy distribution of electrons leaving a target, total yields of characteristic photons and slow electrons escaping from a sample bombarded by fast primaries, escape probability of Auger electrons as a function of depth etc. The analytical results have been compared with Monte Carlo calculations and experiments in a broad range of electron energies and scattering properties of solids and good agreement has been observed.     

Experimental study of electron impact excitation of multiply charged ions

We report on measurements of angular differential cross sections for the excitation of multiply charged ions by electron impact. An ion beam is crossed by an electron beam; electrons which are inelastically scattered at different angles are identified by their energy loss due to the excitation process. Absolute excitation cross sections are obtained by comparing the signals of the elastic and the inelastic electron-ion scattering. Results obtained for the 3s→3p excitation of Ar⁷⁺ are discussed.     

Monte Carlo transport of electrons and positrons through thin foils

In measurements on electrons traversing matter it is important to know the transmission through that medium, their path-lengths and their angular distribution through matter. This allows one to seek improvement in techniques which employ electrons, including medical applications and materials irradiation. This work presents a simulation of the transport of beams of electrons and positrons through thin foils using an analog Monte Carlo code that simulates in a detailed way every electron movement or interaction in matter. As those particles penetrate thin absorbers, it has been assumed that they interact with matter only through elastic scattering, with negligible energy loss. This type of interaction has been described quite precisely because its angular form influences very much the angular distribution of electrons and positrons in matter. With this code it has been calculated that the number of particles, with energies between 100 and 3000 keV, which are transmitted through different media of various thicknesses as well as their angular distributions, show good agreement with the experimental data. The discrepancies are less than 5% for thicknesses lower than about 30% of the corresponding range in the tested material. As elastic scattering is very anisotropic, its angular distribution resembles a collimated incident beam for very thin foils, becoming slowly more isotropic when absorber thickness is increased.     

Electron spectroscopic imaging studies on semicrystalline and block-copolymer systems

Discrimination of elastically and inelastically scattered electrons in transmission electron microscopy (TEM) yields electron micrographs with enhanced and specific contrast. Highly resolved micrographs can be obtained in which the contrast reflects the local concentrations of specific elements. In the present work, Electron Spectroscopic Imaging (ESI) by means of a Zeiss EM 902 TEM is reported for polystyrene-block-poly(2-vinylpyridine) diblock copolymers and for a series of differently crystallized polyethylene samples. Excellent contrast can be achieved without staining. The different techniques allow easy verification of the observed morphology.     

Dynamics of hyperthermal collisions of O(3P) with CO

The dynamics of O(3P) + CO collisions at a hyperthermal collision energy near 80 kcal mol-1 have been studied with a crossed molecular beams experiment and with quasi-classical trajectory calculations on computed potential energy surfaces. In the experiment, a rotatable mass spectrometer detector was used to monitor inelastically and reactively scattered products as a function of velocity and scattering angle. From these data, center-of-mass (c.m.) translational energy and angular distributions were derived for the inelastic and reactive channels. Isotopically labeled C18O was used to distinguish the reactive channel (16O + C18O 16OC + 18O) from the inelastic channel (16O + C18O 16O + C18O). The reactive 16OC molecules scattered predominantly in the forward direction, i.e., in the same direction as the velocity vector of the reagent O atoms in the c.m. frame. The c.m. translational energy distribution of the reactively scattered 16OC and 18O was very broad, indicating that 16OC is formed with a wide range of internal energies, with an average internal excitation of approximately 40% of the available energy. The c.m. translational energy distribution of the inelastically scattered C18O and 16O products indicated that an average of 15% of the collision energy went into internal excitation of C18O, although a small fraction of the collisions transferred nearly all the collision energy into internal excitation of C18O. The theoretical calculations, which extend previously published results on this system, predict c.m. translational energy and angular distributions that are in near quantitative agreement with the experimentally derived distributions. The theoretical calculations, thus validated by the experimental results, have been used to derive internal state distributions of scattered CO products and to probe in detail the interactions that lead to the observed dynamical behavior.     

Structure and dynamics of CO overlayers on a hydroxylated metal oxide: the polar ZnO(0001) surface

The adsorption and desorption of CO on the hydroxylated, O-terminated polar ZnO(0001) surface has been studied using He-atom scattering. The experimental results reveal the formation of a physisorbed ordered CO overlayer. In addition to recording angular distributions of elastically scattered He atoms, also the dynamical properties of the CO overlayer have been investigated using inelastic He-atom scattering. With the aid of electronic structure calculations a loss peak with an energy transfer of 7.2 meV is assigned to the frustrated translation of the CO molecule normal to the surface.     

Electron correlation and charge density study of N2 and O2 by high energy electron scattering

The differential elastic scattering cross sections of N₂ and O₂ for 29 keV electrons have been measured. The experiment was performed using a Möllenstedt type energy analyzer to isolate the elastically scattered electrons. The difference between the measured results and calculations from molecular Hartree-Fock wave functions reveals the electron correlation in the molecules. Using the previously measured total scattering data, the inelastic scattering cross sections are derived. Several potential energies of the target are evaluated from the cross sections. Results at small angles are analyzed in terms of molecular moments and diamagnetic susceptibilities. The scattering behavior at small angles of the N₂ measurement agrees well with several ab initio calculations.     

Electron energy loss spectroscopy of liquid glycerol

The inelastic scattering of low energy electrons from liquid glycerol has been studied. For the first time, electron energy loss spectra of liquids are sufficiently well resolved to permit the identification of vibrations corresponding to individual bonds, namely the C–H and O–H stretching vibrations in glycerol. The angular distribution of the specular peak is very broad, indicating the absence of long-range order at the surface of the liquid. The measurement of the loss signals as a function of the primary electron energy suggests a hybrid mechanism of excitation. The excitation mechanism for the O–H vibration has a stronger impact character as compared to the C–H vibration. A negative ion resonance of glycerol is found at a primary energy of 8 eV. The signal intensities measured as a function of the specular angle of the electron beam appear to be influenced by the angular dependence of the dipole and impact scattering cross-section and a possible preferred orientation of the C–H and O–H groups at the surface of the liquid.     

Determination of ash content in coals by gamma reflection method

The effective atomic number Z dependence of the ratio R of the intensity of elastically and inelastically scattered X-rays has been employed in the determination of the ash contents of some coal samples from Hungary, Poland and Russia. The results obtained by this method compared favorably with those obtained by the combustion method. This fast, simple and nondestructive method can therefore be used for on-line quality control of coal produced in a mine.This work was supported by the Hungarian Research Foundation (Contract No. 016713).     

Angular distribution of photoelectrons with regard to non-dipole effects in photoionization and elastic electron scattering in solids

The angular distribution of photoelectrons from a semi-infinite solid sample is considered with regard to non-dipole transitions in photoionization with polarized and unpolarized radiation and elastic scattering. Non-dipole transitions are taken into account up to the octupole approximation. Calculations were carried out based on transport theory and Monte Carlo methods. Good agreement between two independent approaches is obtained. All angular parameters, necessary to calculate the angular dependence of intensities of photoelectrons for all elements Z < 100 in a wide range of ionizing radiation energies, are calculated in the relativistic approximation.     

“Photofluorescence” by electron impact: determination of the relative oscillator strength for formation of the B2σ State of CO+

An experiment is reported in which inelastically scattered electrons, ions and photons emitted by excited ions have been detected in triple coincidence. We studied the formation of the B²σ state of CO⁺. The coincidence intensity has been converted into a relative oscillator strength for formation of the B²σ state from threshold up to 60 eV.     

Model for the capture of secondary electrons by a fast ion

A capture process is discussed which is strongly assisted by the relaxation of energetic secondary electrons inside a solid target. The following processes are incorporated into the model (in a simplified form): — the production of secondary electrons by a fast projectile inside the target (in binary encounter approximation of the projectile with free target electrons), — the dynamics of the secondary electrons (in continuously slowing down approximation without elastic scattering), — the reduction of the screening of the projectile charge at the exit surface (in sudden approximation and neglecting the image charge). The capture efficiently populates Rydberg states with high angular momenta and gives high coherences between Rydberg states. The application to convoy electrons gives contributions only to the innermost part of the cusp shaped velocity spectra which is possibly caused by the neglect of elastic scattering of the secondary electrons.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Energy loss of scattered ions at glancing-angle incidence on the crystal surface

Experimental findings of glancing-angle scattering of energetic ions are reviewed with particular emphasis on the energy loss of ion during scattering. The position-dependent stopping power of the surface which is derived from the angular dependence of observed energy loss is explained in terms of single and distance electron excitations of surface electrons. For the ions with velocities less than the Fermi velocity of target valence electrons, it is shown that the stopping power of a surface is determined only by the elastic collisions of the ion with valence electrons. From the analysis of the energy losses of 12–30 keV He ions reflected from the (001) surface of SnTe, a method is proposed to derive the electron density distribution averaged over the plane parallel to the surface from this position-dependent stopping power.     

Multiple scattering of light from polymer dispersed liquid crystal material

Light scattering from polymer dispersed liquid crystal (PDLC) material has been studied experimentally and by Monte Carlo simulation. Light scattering was measured as a function of both scattering angle and cell thickness. The cell thicknesses of practical interest are in an intermediate regime where neither single scattering nor light diffusion applies. Both the angular and the thickness dependence of the scattering intensity can be described accurately by a Monte Carlo simulation of multiple scattering from a homogeneous distribution of independent scatterers. The model smoothly interpolates between the single scattering limit for thin cells and the diffusion limit for thick cells. It can easily be extended to include any specific feature of a scattering display system.     

Size distribution analysis of polymer latex systems by use of the extrema in the angular light scattering pattern. III. Unpolarized and horizontally polarized scattered light

The extrema in the unpolarized and the horizontally polarized angular scattering patterns are used as a basis for size-distribution analysis of polymer latex systems composed of single, spherical, optically isotropic particles. The method of analysis is similar to that recently proposed for vertically polarized scattered light and the polarization ratio, where the modal diameter and a distribution width parameter are determined from the experimental angular scattering pattern and prepared theoretical diagrams obtained by Mie theory calculations. The method of analysis, based on all four scattering functions, is illustrated by use of a Dow polystyrene latex sample.     

FORWARD SCATTERING PROPERTIES OF HUMAN EPIDERMAL LAYERS

Abstract From an optical point of view the outermost skin layers contain numerous structures by which penetrating radiation may be scattered as well as absorbed. The nature and strength of this scattering may strongly influence the extent of penetration. We illuminated samples of stratum corneum and full-thickness epidermis with collimated radiation and measured the angular intensity distribution of the transmitted radiation; we did this in the ultraviolet for several angles of incidence, and in the visible for perpendicular incidence only. Skin samples were obtained from the skin of the lower back and upper leg of Caucasian volunteers. Epidermis and subsequently stratum corneum were separated by chemical methods. In the case of stratum corneum, the angular intensity distribution of the transmitted radiation peaks strongly at all wavelengths, in approximately the direction of the incident radiation, that has been refracted at the surface of the sample. With full-thickness epidermis, the distribution of the transmitted radiation also peaks, though less strongly than with stratum corneum. These features suggest a forward oriented scattering mechanism. Both in the case of stratum corneum and full-thickness epidermis, the angular distribution flattens towards the shorter wavelengths and with increasing thickness. The wavelength dependence suggests that both scattering and absorption increase towards the shorter wavelengths. The existence of a thickness dependence indicates that volume scattering occurs. Hydration of stratum corneum is found to influence its scattering properties. Dry samples scatter less than hydrated samples. The consequences of our findings for modelling skin optics are briefly discussed.     

Direct observation of the angular distribution of the chemiluminescence from Ba + N2O → BaO + N2

The angular distribution of the chemiluminescent reaction Ba + N₂O → BaO + N₂ has been investigated by photographing directly the chemiluminescence from this reaction in a crossed beam experiment. It was found that the lifetime of the reactively scattered chemiluminescent BaO molecules is sufficiently long (≈ 10^?s δ) to allow the observation of the angular distribution. From the dependence of this distribution on R and ? where R is the distance from the scattering center and ? the laboratory scattering angle, we conclude that under single collision conditions the chemiluminescence arises preferentially from highly excited vibrational-rotational levels of the A′¹ Π state of BaO.