High-resolution Electron Energy Loss Spectrometer Research Articles

Absolute vibrational cross sections for 1-19 eV electron scattering from condensed tetrahydrofuran (THF).

Absolute cross sections (CSs) for vibrational excitation by 1-19 eV electrons impacting on condensed tetrahydrofuran (THF) were measured with a high-resolution electron energy loss spectrometer. Experiments were performed under ultra-high vacuum (3 × 10(-11) Torr) at a temperature of about 20 K. The magnitudes of the vibrational CSs lie within the 10(-17) cm(2) range. Features observed near 4.5, 9.5, and 12.5 eV in the incident energy dependence of the CSs were compared to the results of theoretical calculations and other experiments on gas and solid-phase THF. These three resonances are attributed to the formation of shape or core-excited shape resonances. Another maximum observed around 2.5 eV is not found in the calculations but has been observed in gas-phase studies; it is attributed to the formation of a shape resonance.

Focus on Laboratory Equipment

Focus on Laboratory Equipment

Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy

New experiments made possible with a commercial transmission electron microscope (TEM) equipped with a high-resolution electron energy loss spectrometer (EELS) are presented. With this commercial system, a 100 meV energy resolution using a sub 2 nm probe or 500 meV at a 0.20 nm probe are possible, in combination with other modern techniques available for TEMs. In this paper a number of explorative examples of the first results are shown. The benefit of the increased resolution for detecting more details in near edge structures are shown for the Ti K edge in TiO 2 (brookite) and for the N K edge in cubic and hexagonal GaN. The bandgap of GaN is studied in both crystal structures, as well as the dependency of the low-loss spectrum on the momentum transfer direction in diffraction mode.

EELS performance measurements on a new high energy resolution imaging filter

In this paper we characterize the spectroscopy performance of a new high resolution electron energy-loss spectrometer for transmission electron microscopy, recently installed in our lab. With the event of commercially available monochromated transmission electron microscopes, equipped with improved spectrometers, experiments became feasible that allow the study of chemical bonding and electronic structures of atoms and solids at the nanometer level with an energy resolution in the range of 0.2–0.3eV. This significant enhancement in resolution, however, can only be fully harnessed with an optimized microscope and spectrometer setup and in a suitable local environment. We attempt to quantify some of the limiting factors on our system with emphasis on spectrum mixing, spectrometer aberrations and transmissivity and we sketch proper working conditions in order to achieve the desired performance.

Bond Formation in Reactions of Solid Cyclopropane Induced by Low-Energy Electrons

Reactions of cyclopropane and propene in the condensed phase have been induced and monitored simultaneously using a conventional high-resolution electron energy loss spectrometer. Reaction products are identified from both the vibrational and the electronic spectra of the irradiated solid samples. Propene is identified as the major initial reaction product from its characteristic vibrational bands. Subsequent reactions are shown to lead to the formation of longer chains. Some of these chains contain conjugated diene units. This latter product cannot be the result of standard oligomerization reaction steps following the formation of primary radicals by dissociative processes alone. Subsequent collision processes leading to additional H ion must be invoked to explain the formation of the diene units.

High-Resolution Electron Energy Loss Spectroscopy of Cs Vibration on the Ru (0001) Surface

By using a newly implemented high-resolution electron energy loss spectrometer with better resolution, a new loss at a energy near to double energy of the Cs-Ru stretching vibration, and its dependence on Cs coverage for Cs overlayer on Ru (0001) were observed. Since the energy is slightly smaller than the double energy of the Cs-Ru vibration (intrinsic loss), overtone excitation must be responsible for this new loss observed, but two-phonon process is not excluded.

Surface state transitions on the reconstructed diamond C(100) surface

The diamond C(100) surface was investigated with a high-resolution electron energy loss spectrometer (HREELS) in both the electronic excitation and surface vibrational excitation regimes. The 0–15 eV electronic excitation spectra on the bare C(100) surface revealed three dominant loss features at 3.5, 4.7 and 5.9 eV. These peaks have been attributed to two mid-gap surface states. These states were absent on the H-terminated surface, indicating that they were localized at the surface and were due either to π-bonded dimers or dangling bonds on the surface. The separation between the states, and their possible positions relative to the valence band maximum, were compared to recent surface band structure calculations, and provide evidence for symmetric dimers on the C(100) surface.

Dissociative resonance activation of cyclopropane monolayers on copper: Evidence for CH and CC bond scission

Energy selective electron beam modification of cyclopropane on Cu(110) and Cu(111) single crystal surfaces was performed using the electron beam intrinsic to a high resolution electron energy loss (HREELS) spectrometer. Vibrational spectra of the resulting adsorbed dissociation fragments were recorded using the same HREELS spectrometer. Electron irradiation in the range 10–18 eV of weakly adsorbed cyclopropane was found to lead to the formation of chemisorbed dissociation fragments. HREELS and complementary thermal desorption data were used to identify the chemisorbed products as metallacyclobutane and cyclopropyl groups. The formation of these two surface species implies the existence of two dissociation channels involving CC and CH bond scission, respectively. Variations in the cross section for electron induced modification indicates the presence of a dissociative attachment resonance centered at approximately 10 eV. Cyclopropyl formation and ring opening of cyclopropane at 10 eV are attributed to competition between two decay channels of a core-excited resonance. The cross section for metallacyclobutane formation, at 10 eV, was evaluated to be roughly 0.8×10−17 cm2. The ratio of cyclopropyl to metallacyclobutane surface species, formed at 10 eV, was found to be 4:1.

Construction of a high-resolution electron energy loss spectrometer

A new version of a high-resolution electron energy loss spectrometer has been constructed. Major modifications are the enlargement of the analyzer, gold coating of all the electrode surfaces facing the electron beam, and the reduction of magnetic fields. A high detector current of 4 pA has been obtained together with the narrow energy half width of 1.2 meV (FWHM) in the straight-through mode.

Electron optical analysis of a high-resolution electron energy loss spectrometer with a retarding Wien filter

The transfer matrix up to the second-order aberrations has been formulated for an electron optical systems of a monochromator and an energy analyzer. Both of these instruments have two magnetic round lenses and a retarding Wien filter. The filter consists of a deceleration lens, a Wien filter, and an acceleration lens. The optimum excitation of the round lens is found to be that which provides parallel exit of electrons from the acceleration lens. The excitation of the Wien filter must be adjusted to focus the beam on the slit. The computed results are useful for finding the optimum operating conditions and for explaining experimental results of the high-resolution electron energy loss spectrometer.

Low-energy electron diffraction with energy resolution

Low-energy electron diffraction (LEED) is a well established technique for evaluation of the surface atom arrangement. The information is derived from the elastically scattered electrons. Usually the thermal diffuse background between diffraction spots is assumed to be constant and subtracted as a constant. For spot profile analysis, which provides the arrangement of units like islands or domains, the elastic intensity has to be measured throughout the Brillouin zone. The usual LEED systems (with energy resolution of some eV) cannot distinguish between elastic and thermal diffuse scattering. For that purpose a new instrument has been developed. We combined the deflection unit from a high-resolution LEED system with 127° analyzer from a high-resolution electron energy loss spectrometer (EELS) in a suitable manner. The new instrument was checked with the Si(111), Al(111), and Al(111)+O2 surface. The energy resolution (ΔE=6.7 meV) allows separation of a large fraction of phonon losses. The momentum resolution (transfer width 150 nm) is the same as other high-resolution LEED systems. As a result the first LEED system with both high momentum and high energy resolution is presented (ELS-LEED).

A multi-channel accumulator

A multi-channel accumulator is described, which provides reduced data acquisition times compared with existing multi-channel scalers and multi-detector systems. The instrument has been implemented using a single application-specific integrated circuit and a memory array, so that it may be mounted as a single-card plug-in for a system controller. Tests indicate that the instrument will accumulate data from the detector at rates corresponding to up to 25.6 million pulses per second. This represents a fivefold improvement over the previous best reported count rate. The application of the accumulator in a high resolution electron energy-loss spectrometer is considered although the specification is sufficiently general to allow its use in other experimental apparatus.

Surface phonon spectroscopy of Ni(111) studied by inelastic electron scattering.

We report the experimental determination and theoretical analysis of the surface phonon dispersion relations for clean Ni(111) along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}M\ifmmode\bar\else\textasciimacron\fi{} symmetry direction. The surface phonon spectra were obtained with a high-resolution electron energy loss spectrometer operating in the off-specular impact scattering regime. Kinematic conditions were varied in order to selectively examine the Rayleigh mode and ``gap'' mode, as well as contributions from bulk phonons. Comparison of the experimental surface phonon dispersion relations and inelastic scattering cross sections with lattice dynamical and quantum multiple scattering calculations demonstrate that tensile surface stress is present at the level of 1.6\ifmmode\pm\else\textpm\fi{}0.2 N/m, and that the intraplanar surface force constant is (11\ifmmode\pm\else\textpm\fi{}3)% softer than in bulk nickel.

Multichannel detection high-resolution electron energy loss spectrometer

A new general-purpose high-resolution electron energy loss spectrometer has been constructed and tested. Novel features of the electron optics include multichannel energy detection and tandem four-element zoom lenses that incorporate beam defining apertures. Compact design of the lenses permit scattering measurements in which the total scattering angle is less than 40°. This feature permits probing bulk phonon modes. The lens system permits electronic adjustment of the analyzer collection angle, and yields high transmission at retardation/acceleration ratios of 400:1. Performance characteristics include simultaneous detection of 50 energy channels at 4-meV energy resolution and target currents exceeding 2×10−10 A. Separate monochromator and analyzer goniometers permit access to out-of-plane scattering configurations which enables probing odd symmetry shear surface modes.

Design and performance of a double-pass high-resolution electron energy loss spectrometer

The design, characterization, and performance of a high-resolution electron energy loss spectrometer for surface studies is described. The spectrometer is based on a double-pass 127° cylindrical deflector analyzer for both the monochromator and analyzer stages. An overall resolution of 4 meV is obtained with a current of 5×10−11 A measured at the sample. The resolving power of the double-pass deflector, in the specific geometry utilized, is significantly greater than that of either deflector. The observed energy resolution on metal and semiconductor surfaces is contrasted; it is found that electronic excitations lead to noninstrumental broadening on semiconductor surfaces.

Design and construction of a high-stability, low-noise power supply for use with high-resolution electron energy loss spectrometers

The design and construction of a high-stability, low-noise power supply which provides potentials for the lens and analyzer elements of a 127° Ehrhardt-type high-resolution electron energy loss spectrometer (HREELS) is described. The supply incorporates a filament emission-control circuit and facilities for measuring electron beam current at each spectrometer element, thus facilitating optimal tuning of the spectrometer. Spectra obtained using this supply are shown to have a four-fold improvement in signal-to-noise ratio and a higher resolution of the vibrational loss features when compared with spectra taken using a previously existing supply based on passive potential divider networks.

Generalized oscillator strengths in the electron impact spectra of rubidium and caesium

Studies of generalized oscillator strengths as a function of incident electron kinetic energy within the range 33·5-100·5 eV have been made for rubidium and caesium using a crossed-beam high-resolution electron energy-loss spectrometer. Absolute generalized oscillator strengths have been derived by normalizing the resonance transition to the optical oscillator strength. For rubidium the reduced matrix elements (5 2Sparallelr2parallel6 2S) and (5 2Sparallelr2parallel7 2S) were found to be 44 A.U. ±25% and 15 A.U. ±40%, respectively, and the transition probabilities A(4 2D -> 5 2S) and A(5 2D -> 5 2S) found to be 204 s-1 ±10% and less than 24 s-1, respectively. For caesium the reduced matrix element (6 2Sparallelr2parallel7 2S) was found to be 57 A.U. ±20%, and the transition probabilities A(5 2D -> 6 2S), A(6 2D -> 6 2S) and A(7 2D -> 6 2S) were found to be 49 s-1 ±10%, 94 s-1 ±20% and 80 s-1 ±40%, respectively.