Elastic backscattering of electrons from surfaces

A Monte Carlo algorithm is proposed for calculating the elastic reflection coefficient, η_E, for elements. The algorithm accounts for the multiple elastic scattering of electrons in solids. The calculated values of η_E compare well with the literature data for elements with atomic number up to 47 and at primary energies above 2 keV. The proposed Monte Carlo method makes it possible to determine the functional relation between η_E and the inelastic mean free path, λ. This relation turned out to be non-linear, arid it deviates from a similar relation based on published earlier single elastic scattering model. The deviation is especially pronounced for elements with medium atomic numbers. The calculated function $\text{η}{}_{\mathrm{<mtext>E</mtext>}}\text{=}\text{f}\text{(λ)}$ offers a convenient method for determining the inelastic mean free path. The values of λ derived in the present work from published experimental values of η_E compare very well with the literature data.     

Spectroscopy of near-threshold electrons from surfaces

A time-of-flight technique for two-dimensional spectroscopy of near-threshold electrons from surfaces is described, and the properties of the method are illustrated using selected results from clean surfaces, and mono- and multilayer adsorbates. The analysis of two-dimensional data yields easy access to inner (bulk) and outer (surface) ionization edges and exciton series, for valence and core electron levels including satellite states, and to final state effects. Strong interaction between bulk and surface states is revealed in particular for samples with negative electron affinity. Core-level threshold electron spectra from monolayers adsorbed on metal surfaces exhibit strong substrate-mediated suppression of post-collision interaction-induced line broadening and shift, each of which can be quantitatively explained by image charge screening. Received: 27 April 1998/Accepted: 25 August 1998     

Heavy-ion-induced desorption of deuterium from titanium

A titanium-layer of approximately 90 nm thickness has been homogeneously loaded with deuterium by implantation and diffusion. Desorption was studied by use of²⁸Si ions of up to 20 MeV energy (and other ions). The spectrum of the elastic-recoil-detection method serves as indication of the released and retained amount of deuterium. The retained deuterium is recorded as a function of the employed ion dose and is converted into a desorption rate as a function of the retained deuterium concentration. — A one-dimensional theory considering transitions from vacuum to surface and surface to Ti volume (and back) leads to a system of equations, the solutions of which allow adaptation to the experiment and therefore an evaluation with respect to certain characteristic parameters. — Some of the results might be influenced by the high concentration of C and O impurities.     

The emission of correlated electrons from surfaces

This work offers a theoretical framework for the treatment of the simultaneous many-electron excitation in solids and surfaces. Starting from the transition operator, an expansion is presented that treats all involved interactions on equal footing. The method is applied to the simultaneous two-orbital excitation and the results are compared with recent experimental findings. It is concluded that such studies provide a useful tool for the investigation of inter-electronic coupling. Received: 23 March 1999 / Accepted: 25 June 1999 / Published online: 23 September 1999     

Reflection of keV-energy electrons from multilayer surfaces

The energy spectra of electrons reflected from multilayer targets are studied theoretically and experimentally. A self-consistent theory of electron reflection from multilayer surfaces is constructed. Simple analytical models of electron reflection that illustrate the feasibility of the depth profiling of multilayer targets are presented. The energy spectra of electrons normally incident on Nb/Si and Nb/Al/Nb/Si targets and reflected from them at an angle of 45° to the normal are taken. A method for the depth profiling of such structures is elaborated.     

Heavy-ion-induced electronic desorption of gas from metals

During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE_{e}/dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering.     

The emission of electrons from hot silicon surfaces

The emission of electrons from hot silicon surfaces is reviewed in an effort to present a summary of the state of research in this field. The theoretical aspect of the problem is outlined briefly, and experimental results covering the temperature and field dependence of the saturation current density, as well as the anisotropy and the energy distribution of the emission are presented.     

Energy spectra of electrons backscattered from bulk solid targets

Energy distribution spectra of backscattered electrons in the range 5–25 keV are obtained experimentally. An inverse problem of the reconstruction of the true electron spectrum is solved taking into account the instrument response function of the spectrometer; on the basis of the obtained solution, we specify functions of the real energy distribution of the electrons backscattered from homogeneous and layered samples.     

Features of bremsstrahlung from relativistic electrons in solid targets

It has been shown that the classical Bethe-Heitler result can be inapplicable for describing the bremsstrahlung spectrum of relativistic electrons in real solid targets.     

XPS data for sputter-cleaned In0.53Ga0.47As,GaAs, and InAs surfaces

Core level binding energies and Auger parameters were determined for In, Ga, and As in the three compounds In_0.53Ga_0.47As, GaAs, and InAs. The surfaces were cleaned by 1.5 keV Ar ion bombardment. Under this condition the radiation-induced defects are small. In the case of GaAs the Ga and As3d levels become comparable with available data for chemically cleaned surfaces. The high Ga deficiency of chemically cleaned In_0.53Ga_0.47As surfaces could not be observed. Sputter cleaned surfaces seem to be closer to the bulk composition.     

High-order optical harmonic generation from solid surfaces

During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Scattering of linear molecules from solid surfaces

Scattering of linear molecules, which are internally deactivated initially, from solid surfaces is investigated classical mechanically. The solid is modeled with a three-dimensional isotropic Debye solid whose surface is essentially flat but thermally roughened by lattice vibration, and the molecule is described as a rigid body, which interacts with the solid via an exponential potential wall and a stationary attractive potential well. A variation in the interaction energy due to thermal roughening of the surface and due to nonsphericity of the molecule being regarded as perturbations on the system, a second order perturbation theory is developed to construct a Gaussian velocity distribution function of molecules scattered off the exponential wall and still moving inside the potential well. Among the molecules in the distribution, those with sufficient energy to escape from the potential well are regarded as being scattered into the free space. This model shows that the major differences between the scattering of monatomic molecules and that of linear ones result from less normal momentum transfer to the linear molecules than to the monatomic ones.     

Phonons and electrons at metal surfaces

Some aspects of the coupling between phonons and electrons and the interaction between electrons at metal surfaces are reviewed. Surface science techniques as diverse as electron energy loss spectroscopy, angle-resolved photoelectron spectroscopy, and scanning tunnelling microscopy are employed to study these interactions. Electron–phonon and electron–electron coupling are discussed in terms of renormalized phonon dispersion relations, and increased decay rates of electronic excitations. PACS 63.20.Kr; 68.35.Ja; 68.37.Ef; 68.43.Pq; 72.10.Fk; 72.15.Qm; 73.20.At     

Structure dependent stimulated de-excitation of Auger electrons from clean Be surfaces

High energy satellites observed from the Be KVV Auger Spectrum are assigned to energy coupling from collective bulk plasma oscillations (hω_p?19eV) with emerging Auger electrons. These high energy satellites reflect the valence-bands structural characteristics which arise from an ordered and disordered phase because of the different surface preparation procedures employed.     

Total external reflection of X-rays from solid surfaces

Technical Physics - The reflection of X-rays from solid surfaces is comprehensively studied using the measurements of patterns of total external reflection and X-ray diffraction with the aid of a...     

Photoemission from solid surfaces in intense laser fields

We present a Floquet analysis of photoemission of electrons from a solid surface due to its interaction with a laser beam. The method we use is based on a three-dimensional quasifree electron model. The model takes account of the changes in the effective mass of the electron inside and outside the solid and in the reflected and refracted beams due to the presence of the surface. It is shown that the photoemission of electrons from the solid surface may take place not only with no absorption of photons but also along with the emission of extra photons.     

Thermal desorption of large molecules from solid surfaces

We use molecular-dynamics simulations and importance sampling to obtain transition-state-theory rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces.