Michael Went

Investigation of binary compounds using electron Rutherford backscattering

High-energy (40keV) electrons, scattering over large angles, transfer a small fraction of their kinetic energy to the target atoms, in the same way as ions do in Rutherford backscattering experiments. The authors show here that this energy transfer can be resolved and used to determine the mass of the scattering atom. In this way information on the surface composition for thicknesses of the order of 10nm can be obtained. The authors refer to this technique as “electron Rutherford backscattering.” In addition the peak width reveals unique information about the vibrational properties (mean kinetic energy) of the scattering atoms. Here the authors demonstrate that the method can be used to identify a number of technologically important compounds.

A trap-based positron beamline for the study of materials

We present the design of a new positron beamline for lifetime and Doppler studies of materials. The design is based on the Surko buffer gas trap system and is capable of producing sub-nanosecond pulses with a spot size at the sample of 10 mm. The present timing resolution of ≈800 ps allows lifetimes as small as 500 ps to be resolved. Typically count rates are between 100 and 150 counts per second.

Extraction of cross-sections of inelastic scattering from energy spectra of reflected atomic particles

REELS spectra of the electrons reflected off niobium are measured with energy resolution <0.5 eV within the 5–40 eV energy range of the probing beam. The measurements were performed for the scattering angles θ = 45° and θ = 120° by means of two electron guns. The process of energy losses is described within the framework of a model with three different energy loss laws: surface, intermediate, and bulk layers are considered. Differential cross-sections of inelastic scattering are represented in the form of simple equations.

Quantitative measurements of Kikuchi bands in diffraction patterns of backscattered electrons using an electrostatic analyzer

Diffraction patterns of backscattered electrons can provide important crystallographic information with high spatial resolution. Recently, the dynamical theory of electron diffraction was applied to reproduce in great detail backscattering patterns observed in the scanning electron microscope (SEM). However, a fully quantitative comparison of theory and experiment requires angle-resolved measurements of the intensity and the energy of the backscattered electrons, which is difficult to realize in an SEM. This paper determines diffraction patterns of backscattered electrons using an electrostatic analyzer, operating at energies up to 40 keV with sub-eV energy resolution. Measurements are done for different measurement geometries and incoming energies. Generally a good agreement is found between theory and experiment. This spectrometer also allows us to test the influence of the energy loss of the detected electron on the backscattered electron diffraction pattern. It is found that the amplitude of the intensity variation decreases only slowly with increasing energy loss from 0 to 60 eV.

Elastic electron scattering from methane at high momentum transfer

We describe elastic electron scattering data at high momentum transfer (between ≈20 and ≈40 au) from methane and Xe. Under these conditions there is a significant recoil energy transferred to the target and electrons scattered elastically from methane are separated into two peaks: one due to electrons scattered from carbon, and one due to electrons scattered from hydrogen. The separation of these peaks is within a few per cent identical to what is expected for scattering from isolated C and H atoms. The peak due to electrons scattered from C, is again shifted compared to the peak of electrons scattered from Xe. The Xe, C and H peaks all have clearly different widths. The C and H peak areas are compared. Their relative intensity shows no substantial deviation (<10%) from what is expected based on either simple Rutherford cross sections, or state-of-the-art elastic scattering calculations. The latter observation is in strong contrast to electron scattering results from a gaseous equimolar H2–D2 mixture and from electron and neutron scattering results from polymers at similar momentum transfer.

Electron spectroscopy using two-dimensional electron detection and a camera in a single electron counting mode

A brief description is given of an economical implementation of the read out of a two-dimensional detector in an electron spectrometer by a charge coupled device camera, using a pulse counting mode. Count rates up to 10 kHz can be handled in this way. A comparison with results obtained using a resistive anode detector is given for the case of electron scattering from Xe atoms. Good agreement was obtained between both detection techniques, establishing the validity of the method described here.

Direct measurement of spectral momentum densities of ordered and disordered semiconductors by high energy EMS

High Energy solid state electron momentum spectroscopy (EMS) is capable of directly measuring spectral functions of ordered and disordered solid matter. In this paper we investigate the spectral functions for the group IV semiconductors Ge and Si. We attempt to resolve the electronic structure differences in amorphous, polycrystalline and crystalline atomic arrangements of the semiconductors. We examine the experimental differences in polycrystalline and amorphous Ge, and draw conclusions as to the similarities/differences between the two states of matter.

Spin Asymmetries for Elastic Scattering in Krypton at Intermediate Energies

Measurements of the spin asymmetry for elastic scattering of spin-polarized electrons from krypton are presented, for incident energies in the range 20–200 eV. The measured spin asymmetries are generally small, and do not exceed 0.25 in magnitude at any energy or angle. The experimental results are compared with calculated values of the Sherman function, obtained by solution of the Dirac–Fock equations. The calculations have been performed with the inclusion of polarization and dynamic distortion potentials, and with the addition of an absorption potential to model inelastic processes. For incident energies of 50, 60 and 65 eV, the calculated values of the Sherman function are shown to be extremely sensitive to the details of the model, with the addition of the absorption potential producing dramatically different results.

Characterisation of metakaolin-based geopolymers using beam-based and conventional PALS

The nano-porosity of metakaolin-based geopolymers and the effect of heat-treatment on porosity have been studied with conventional and beam-based positron annihilation lifetime spectroscopy (PALS). Conventional PALS found significant nano-porosity in the geopolymers, as indicated by the presence in the PALS spectrum of two long lifetime components, τ3 = 1.58 ns and τ4 = 47 ns, associated with pore diameters of approximately 0.5 and 3 nm respectively. The lifetime of the shorter component was found to decrease monotonically with successive heat treatments of 300°C and 600°C. Beam-based PALS, conducted at 5 keV, also indicated two long lifetime components, τ3 = 4.84 ns and τ4 = 54.6 ns. These are significantly longer than those observed by conventional PALS and the monotonic decrease of τ3 with successive heat treatments was not observed. As the beam-based PALS probed only the near-surface region, with an average implantation depth of about 350 nm, these results suggest that the near-surface structure may vary significantly from that of the bulk. This could be an inherent property of the samples or an artefact caused by surface effects or sample outgassing.

Spin‐Resolved Collisions of Electrons with Rubidium Atoms: A Search for Relativistic Effects

The search for relativistic effects in electron‐alkali scattering is currently a topic of considerable interest. The A2 spin asymmetry parameter is a direct measure of relativistic effects in the electron‐atom collision process, as it is entirely dependent on the spin‐orbit effect. We present measurements of the A2 spin asymmetry for the 5S → 5P transition in rubidium at incident energies of 15, 20, 30 and 50 eV and for elastic scattering at 15, 20, 30, 50 and 80eV. Our results indicate that under these collision conditions, relativistic effects are measurable, in qualitative agreement with the available theory.