Maarten Vos

High-energy (e, 2e) spectrometer for the study of the spectral momentum density of materials

A new spectrometer for the study of energy-resolved momentum densities is described. The (e, 2e) spectrometer uses a symmetric configuration and uses incoming energies up to 50 keV. Energy resolution and momentum resolution are 1.8 eV and 0.1 a.u., respectively. Compared to previous spectrometers this spectrometer has rather low levels of multiple scattering, and thus allows for more quantitative analysis of the data and/or the measurement of thicker samples.

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.

The use of electron scattering for studying atomic momentum distributions: The case of graphite and diamond

The momentum distributions of C atoms in polycrystalline diamond (produced by chemical vapor deposition) and in highly oriented pyrolitic graphite (HOPG) are studied by scattering of 40 keV electrons at 135°. By measuring the Doppler broadening of the energy of the elastically scattered electrons, we resolve a Compton profile of the motion of the C atoms. The aim of the present work is to resolve long-standing disagreements between the calculated kinetic energies of carbon atoms in HOPG and in diamond films and the measured ones, obtained both by neutron Compton scattering (NCS) and by nuclear resonance photon scattering (NRPS). The anisotropy of the momentum distribution in HOPG was measured by rotating the HOPG sample relative to the electron beam. The obtained kinetic energies for the motion component along, and perpendicular to, the graphite planes were somewhat higher than those obtained from the most recent NCS data of HOPG. Monte Carlo simulations indicate that multiple scattering adds about 2% to the obtained kinetic energies. The presence of different isotopes in carbon affects the measurement at a 1% level. After correcting for these contributions, the kinetic energies are 3%-6% larger than the most recent NCS results for HOPG, but 15%-25% smaller than the NRPS results. For diamond, the corrected direction-averaged kinetic energy is ≈ 6% larger than the calculated value. This compares favorably to the ≈25% discrepancy between theory and both the NCS and NRPS results for diamond.

Comparative Study of the Valence Electronic Excitations of N 2 by Inelastic X-Ray and Electron Scattering

Bound-state, valence electronic excitation spectra of N2 are probed by nonresonant inelastic x-ray and electron scattering. Within usual theoretical treatments, dynamical structure factors derived from the two probes should be identical. However, we find strong disagreements outside the dipole scattering limit, even at high probe energies. This suggests an unexpectedly important contribution from intramolecular multiple scattering of the probe electron from core electrons or the nucleus. These effects should grow progressively stronger as the atomic number of the target species increases.

Detection of hydrogen by electron Rutherford backscattering

A novel method for detection of hydrogen by an electron beam in extremely thin samples is described. Elastically scattered electrons impinging with 20-30 keV on a thin formvar film were detected at a scattering angle near 45 degrees. In these large momentum transfer elastic collisions a clear separation of the signal of hydrogen and heavier elements was found. By changing the momentum transfer we can verify that the hydrogen signal is not due to inelastic energy loss contributions. The width of the hydrogen elastic peak is much larger than the elastic peaks due to heavy elements (carbon and oxygen). The ratio of the hydrogen elastic peak and the main elastic peak is smaller than expected by 30-50% depending on the energy of the impinging electron. This could be due to electronic excitations directly coupled to the elastic collision. The stability of the formvar film under electron radiation was studied. A reduction in thickness of the film with increasing fluence, as well as the preferential depletion of hydrogen, was found. Possible improvements of the experimental configuration for this type of experiments are discussed.

Determination of the energy-momentum densities of aluminium by electron momentum spectroscopy

The energy-resolved momentum densities of thin polycrystalline aluminium films have been measured using electron momentum spectroscopy (EMS), for both the valence band and the outer core levels. The spectrometer used for these measurements has energy and momentum resolutions of around 1.0 eV and 0.15 atomic units, respectively. These measurements should, in principle, describe the electronic structure of the film very quantitatively, i.e. the dispersion and the intensity can be compared directly with theoretical spectral momentum densities for both the valence band and the outer core levels. Multiple scattering is found to hamper the interpretation somewhat. The core-level intensity distribution was studied with the main purpose of setting upper bounds on these multiple-scattering effects. Using this information we wish to obtain a full understanding of the valence band spectra using different theoretical models of the spectral function. These theoretical models differ significantly and only the cumulant expansion calculation that takes the crystal lattice into account seems to describe the data reasonably well.

Elastic electron scattering from water vapor and ice at high momentum transfer

We compare the area, peak separation, and width of the H and O elastic peak for light and heavy water, as observed in spectra of keV electrons scattered over large angles. Peak separation is well reproduced by the theory, but the O:H area ratio is somewhat larger than expected and is equal to the O:D area ratio. Thus no anomalous scattering from H was observed. Only minor differences are observed for scattering from a gaseous or a solid target. The extracted mean kinetic energy of H and D agreed within 5% with the calculated ones for ice. For the more difficult vapor measurements agreement was on a 12% level. A preliminary attempt to extract the O kinetic energy in ice agreed within 10% with the calculated values.

Electron energy loss and diffraction of backscattered electrons from silicon

Electrons backscattered from crystals can show Kikuchi patterns: variations in intensity for different outgoing directions due to diffraction by the lattice. Here, we measure these effects as a function of their energy loss for 30keV electrons backscattered from silicon. The change in diffraction contrast with energy loss depends strongly on the scattering geometry. At steep incidence on the sample, diffraction contrast in the observed Kikuchi bands decreases rapidly with energy loss. For an energy loss larger than about 150eV the contrast is more than 5 times less than the contrast due to electrons near zero energy loss. However, for grazing incidence angles, maximum Kikuchi band contrast is observed for electrons with an energy loss near 60eV, where the contrast is more than 2.5◊ larger than near zero energy loss. In addition, in this grazing incidence geometry, the Kikuchi diffraction effects stay significant even for electrons that have lost hundreds of electron volts. For the maximum measured energy loss of 440eV, the electrons still show a contrast that is 1.5◊larger than that of the electrons near zero energy loss. These geometry-dependent observations of Kikuchi band diffraction contrast are interpreted based on the elastic and inelastic scattering properties of electrons and dynamical diffraction simulations.

Electron scattering at high momentum transfer from methane: analysis of line shapes.

The measurement of the energy distribution of keV electrons backscattered elastically from molecules reveals one or more peaks. These peaks are at nonzero energy loss and have an intrinsic width. The usual interpretation of these measurements is attractively simple and assumes billiard-ball-type collisions between the electron and a specific atom in the molecule, and the scattering atom is assumed to behave as a free particle. The peak position is then related to the mass of the scattering atom, and its width is a Compton profile of the momentum distribution of this atom in the molecule. Here we explore the limits of the validity of this picture for the case of electrons scattering from methane. The biggest discrepancy is found for electrons scattering from carbon. For electrons scattering from hydrogen the effects are substantial at relatively low incoming energies and appear to decrease with increasing momentum transfer. The discrepancy is analyzed in terms of the force the atom experiences near the equilibrium position.

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.