Entstehung von Alkalimetallen bei der Elektronenbestrahlung von Alkalihalogeniden

We have observed additional peaks in the energy loss spectra of NaBr, NaI, KF, KI, and RbF foils, which are caused by the electron impact. They are identified as volume and surface plasma losses of alkali metal particles. The energy loss values are in accord with energy loss data of alkali metal foils. Electron diffraction patterns of the irradiated alkali halide foils showed additional rings of alkali metal agglomerates, the size of which are estimated to be in the range of 100Å.     

The diamond surface: II. Secondary electron emission

The total secondary electron emission spectrum from diamond has been examined, and details of the Auger spectra, characteristic loss spectra and K-level ionisation loss spectra have been presented. These features from the clean diamond surface were contrasted to those from graphite and amorphous-carbon. The fine structure in the carbon Auger spectra were compared with the line shape calculated using the band structure model, and the chemical sensitivity of the Auger spectra was demonstrated. A high energy Auger satellite peak in the diamond spectrum was ascribed to the Auger transition occurring from a doubly ionised carbon atom. This result was substantiated by the observation of loss peaks associated with the singly ionised level. The characteristic energy losses were assigned to interband transitions and plasma losses, and have been compared to optical data where possible.     

Surface and bulk plasmon coupling observed in reflection electron energy loss spectra

Reflection electron energy loss spectra have been measured for a semiconductor and some metals (Si, Cu, Ag and Au). A novel procedure is presented to rigorously decompose the spectra into contributions corresponding to surface and volume excitations. The resulting distributions of energy losses in an individual surface loss are in good agreement with theory. In particular, the begrenzungs effect occurring at the boundary of a solid state plasma, i.e. the reduction of the intensity of bulk modes due to the coupling with surface modes, can be clearly observed in the retrieved energy loss distribution.     

Charakteristische Energieverluste bei der Streuung mittelschneller Elektronen an Aluminium-Oberflächen

The energy distributions of electrons scattered by variously treated (111) surfaces of aluminium crystals and their dependence on scattering angle has been measured by an electrostatic analyser with a spherical condenser. Etched surfaces show in the diffraction maxima discrete energy losses with intensities similar to those of thin aluminium foils. Electrolytically polished surfaces (H₃PO₄+CrO₃ bath) show no diffraction patterns of Al, and the energy distribution is that of aluminium oxide. After removal of the oxide film, the diffraction pattern of even surfaces appears. In the diffraction maxima the intensity of the energy loss of 6·7 eV is high; the intensity of the energy loss of 14·9 eV increases with increasing order of the diffraction maxima. The angular distributions of the intensities of the discrete energy losses near the diffraction spots are similar to those observed at thin foils and agree with theory.     

Electron energy loss spectroscopy of TiC,ZrC and HfC

The dielectric properties of commercial TiC, ZrC and HfC powders were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) were obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions across the energy gap, these spectra can be interpreted on the basis of existing energy-band calculations. Comparison between experimental results and theory shows good agreement.     

Joint experimental and computational study of aluminum electron energy loss spectra

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.     

Study of the order–disorder transition and martensitic transformation in a Cu–Al–Be alloy by EELS

Changes in 3d states occupancy associated with order–disorder transition and martensitic transformation in a Cu–Al–Be alloy was investigated by electron energy loss spectroscopy (EELS) in both high energy and low energy loss regions. From the high energy loss region, the Cu L_2,3 white-line intensities, which reflect the unoccupied density of states in 3d bands, was measured for three states of the alloy: disordered austenite, ordered austenite and martensite. It was found that the white-line intensity remains the same during order–disorder transition but appears slightly smaller in martensite, indicating that some electrons left Cu 3d bands or some hybridization took place during phase transformation. From the low energy loss region, the optical joint density of states (OJDS) was obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions, these spectra can be used to probe changes in the electronic band structure. The analysis shows that during the martensitic transformation, the peaks positions and relative intensities in the OJDS spectra undergoes noticeable changes, which are associated with interband transitions.     

Characteristic energy loss and Auger electron spectra of GaP (110)

The characteristic energy loss and Auger electron spectra of clean GaP (110) have been measured with a four grid retarding field analyser. A peak in the loss spectrum has been found at 11.2 eV which is probably due to a surface plasma loss. The remaining structure has been assigned to direct interband transitions, to single and double bulk plasma losses and to d-band transitions by analogy with previous optical and electron transmission studies. Suggestions are made as to the origin of the peaks in the Auger spectrum and changes in the spectrum in the presence of oxygen are discussed.     

Die optischen Eigenschaften von Gold,Silber und Gold-Silber-Legierungen zwischen 2 und 40eV aus Energieverlustmessungen

The energy loss functions Im \(\left( { - \frac{1}{\varepsilon }} \right)\) of Au, Ag and Ag-Au alloys are determined from energy loss spectra of fast electrons, using different methods in the energy regions 2 to 4 eV and 4 to 40 eV. The optical constants ?₁ und ?₂ are calculated from the energy loss functions by Kramers-Kronig analysis. The experimental results show a different dependence in the optical behaviour of the alloy on the alloying component: if Au is added to Ag, the effect on the optical properties is stronger than in the reverse case. Between 50 and 70 at.% Au in the alloy the specific properties of Ag and Au vanish completely. An interpretation of the energy loss maxima as surface plasma oscillation, volume plasma oscillation and interband transition is attempted.     

Cerenkov losses: a limit for bandgap determination and Kramers-Kronig analysis

Measuring low energy losses in semiconductors and insulators with high spatial resolution becomes attractive with the increasing availability of modern transmission electron microscopes (TEMs) equipped with monochromators, C(s) correctors and energy filters. In this paper, we demonstrate that Cerenkov losses pose a limit for the interpretation of low energy loss spectra (EELS) in terms of interband transistions and bandgap determination for many materials. If the velocity of a charged particle in a medium exceeds the velocity of light, photons are emitted leading to a corresponding energy loss of a few electronvolt. Since these losses are strong for energies below the onset of interband transitions, they change the apparent loss function of semiconductors and insulators, with the risk of erroneous interpretation of spectra. We measured low energy losses of Si and GaAs with a monochromated TEM demonstrating the effect of sample thickness on Cerenkov losses. Angle resolved EELS and energy filtered diffraction patterns (taken without a monochromator) show the extremely narrow angular distribution of Cerenkov losses. The latter experiment provides a method that allows to decide whether Cerenkov radiation masks the very low loss signal in EELS.     

Phononenspektroskopie von Ammoniumchlorid und Ammoniumbromid mit keV-Elektronen

The high resolution energy loss spectrum of NH₄Cl in the spectral region of internal lattice vibration has been reinvestigated. In order to assure the assignement made, the spectra of the deuterated ammonium halides ND₄Cl and ND₄Br has been studied too. The structure of the films, which were investigated, was checked by electron-diffraction and by electron microscopy. The deuterated films show the shift of energy losses towards lower energy losses as expected. Unshifted losses are assigned to partially deuterated material. Two types of spectra are observed for NH₄Cl depending on the size of the crystals and presumably on the perfectnes of the lattice. Unlike the external vibrations, which are strongly influenced by surface effects, at least for the strongest bands good agreement is found with the absorption measurement concerning the positions of the peaks.     

Energieverluste monochromatisierter 30 keV-Lithiumionen an Argon und Äthylen

The energy loss spectra of 30-keV Li₇ ⁺ ions after interaction with argon and ethylene have been studied. Wien filters were used as monochromator and energy analyzer, an energy resolution of 0.2 eV was achieved. The ion energy loss spectra obtained differ from those taken with electrons at the same primary energy mainly in two points: In the ion spectra energy losses are found with strong intensities corresponding to optically forbidden transitions which are not excited by fast electrons. Furthermore in the energy range beyond the first ionization limit energy losses appear which are due to charge transfer into excited states of the lithium atom and re-ionization.     

Electron loss spectra from a W(001)surface

A high resolution energy dispersion analysis of the electron loss spectra (ELS) from a W(001) surface has been done with a specular reflexion geometry. The non-dispersive ELS correlates well with a combination of the bulk and surface loss functions determined from recent optical data whereas the dispersive ELS showed significant differences. Multiple plasmon losses were demonstrated while for the interband losses a momentum selection method has been demonstrated to separate scattering in the surface region from that in the bulk.     

Energieverlustspektren der Alkalihalogenide und der Metalle Cu,Ag und Au und Vergleich mit optischen Messungen

Energy loss spectra of 50 keV electrons have been measured in transmission of the alkali halides and of Cu, Ag, and Au at zero scattering angle. These spectra are compared with the quantity ?Im(1/ε) determined from optical measurements. The general agreement is good, especially in the low energy loss region. In the case that the foil thicknesses could be measured, the absolute intensity of ?Im(1/ε) is found to be equal or up to three times larger than from optical data. In addition to the sharp low energy peaks (excitons), characteristic sharp peaks are found at higher energies: 60 eV for the Li-halides, 33 eV for the Na-halides, and 20 eV, 17 eV, and 13 eV for the K-, Rb-, and Cs-halides respectively. These peaks are very little shifted by the different anions in the crystal lattice and may be due to excitons caused by alkali band transitions. We have further examined several peaks in the loss spectra of LiF, KCl, KBr and the Na-halides: 1. The angular dependence of the loss intensity is in agreement with theory. 2. The dispersion of the peaks is much smaller than for plasma losses. 3. The temperature dependence of the peak energy has been found quite different for different peaks.     

Characteristic energy loss structure of solids from x-ray photoemission spectra

Energy loss peaks in x-ray photoemission spectra of nine metals are presented. No strong evidence for intrinsic plasmon structure was observed. Spectra from the free electron-like metals, Al, Li, and Na under ultra-high vacuum (10^?11 torr) conditions show intense bulk plasmons and surface plasmons. Systematic variations in the characteristic energy losses are reported for the series Ag to Te. In addition to losses that may be attributed to plasma oscillations of the 5s5p bands, with N = 1 and 2 electrons, respectively, both Ag and Cd show additional high-energy losses that may arise through 4d-shell participation, with N = 11 and 12.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Surface plasma resonances in small spherical silver and gold particles

With 51 keV electrons surface plasma losses have been investigated on small spherical Ag and Au particles embedded in a medium with dielectric constant? _a=2.37. The surface loss of particles with radii of about 50 Å is found at 2.99 ±0.03 eV for silver and 2.34±0.03 eV for gold being in good agreement with calculated values. For larger radii the loss shifts to higher energy values which agrees qualitatively with the theory of Fujimoto and Komaki for the free electron gas. The optical extinction bands of the particles have been measured, too. Their maxima are shifted to lower energies, in accordance with Mie's theory, if the particle size increases sufficiently. Comparison of the energy values of the “electric” extinction bands with those of the surface plasmons shows, that they correspond in theory and experiment, if the particles are small. This confirms, that the optical colloid extinction of Ag and Au may be interpreted as plasma resonance absorption and emission of the particles.     

Electron energy loss study of TiC [111]

The electron energy loss (EEL) spectra of TiC (111) were measured over a wide range of electron primary energies. The electron energy losses below 16 eV were analyzed using the theoretical band calculations of Price and Cooper [11]. The volume and surface plasma excitations were identified from their electron primary energy dependence. Energy losses due to core electrons autoinization effects were identified above 35 eV. We observed a difference in the electronic structure of the surface vs the bulk of TiC. The temperature dependence of the EEL spectra was studied between 300 to 1250 K. The reaction of the TiC surface with ethylene and oxygen was also investigated. The ethylene bonding to the TiC surface was found to be very weak. There is evidence of the formation of surface defects on the TiC (111) surface at high temperatures.     

Untersuchung der Anregung innerer Elektronen von Alkalihalogenidmolekülen im Energieverlustspektrum von 25 keV-Elektronen

The energy loss spectra of 25-keV electrons after interaction with alkali halide vapors were measured. For the energy losses in the energy rangeE?6eV the positions of the peaks are consistent with light absorption measurements considering the energy resolution of the loss spectra. At higher energy peaks were observed, which correspond to the excitation of inner electrons belonging to the alkali atoms. From electron diffraction diagramms it follows, that for the lithium halides the concentration of dimers is considerable.