Electron Energy-loss Technique Research Articles

Investigation of radiation-induced transformations in thin NbN films by analytical electron microscopy

This work demonstrates implementation of low energy electron energy loss technique (EELS) in scanning transmission electron microscopy (STEM) to investigate the changes of free electron density at room temperature in ultra-thin NbN films under composite ion beam irradiation up to the deses of ∼3 d.p.a. for nitrogen atoms. It was found the constant value of the free electron density ∼1.6 ·1029 m-3 in this dose range while the irradiated material was characterized by metal type of electrical conductivity.

Photoabsorption of hydrocarbons in Titan's atmosphere

AbstractTitan, the largest satellite of the planet Saturn, has a thick atmosphere which consists of nitrogen (N2) and methane (CH4). In 2004, the Cassini-Huygens mission observed the occultation of two stars through the atmosphere of Titan and measured ultraviolet (UV) absorption spectra. Through these spectra it was possible to identify the molecular species contained in this environment. In the present work, we have simulated a spectrum of this atmosphere using some molecules such as CH4, C2H2, C2H4, C2H6, C4H2, and C6H6. Our cross sections data were experimentally obtained using the electron energy-loss technique, where the electron energy-loss spectra, measured high incident energies and in small scattering angles, are similar to photoabsorption spectra. The comparison of our synthetic spectrum with that measured by Cassini shows that this method is very efficient for identifying molecules as well as estimating abundances.

Plasma bands in graphite

The π + σ plasmons in a strongly oriented pyrolitic graphite were studied experimentally using the characteristic electron energy loss technique with an angular resolution in the reflection mode. It was established that the bulk intralayer π + σ plasma bands have two branches with a positive and a negative dispersion, respectively. Each dispersion branch of the π + σ plasmons is a doublet. The interlayer plasmons are characterized by a positive dispersion coefficient only. The results are interpreted qualitatively based on the special features of the electronic band spectrum of graphite. The doublet arises because both dipole and multipole interband transitions similar in intensity are excited in graphite exposed to electron bombardment.

Generalized oscillator strength for the 1B 2u← 1A g transition and the observation of forbidden processes at the C 1s spectrum of the naphthalene molecule

The angular dependence of the valence and carbon 1s excitation in the naphthalene molecule has been studied using the electron energy-loss technique. The valence shell spectra were measured at 1.0 keV incident energy, 0–50 eV excitation energy range and in the 1.25–12.0 degrees angular range. The energy resolution was 1.0 eV (full-width at half-maximum (FWHM)). Absolute values were determined, for the first time, for the elastic differential cross section and for the generalized oscillator strength (GOS) and inelastic differential cross section for the 1B 2u← 1A g transition (5.9 eV). The carbon 1s spectrum (270–330 eV) was also measured, at 1.3 keV and at an energy resolution of approximately 1.0 eV (FWHM). Dipole-forbidden processes have been observed, both at the valence and C 1s excitation spectra. In the latter case, a previously unreported, dipole forbidden transition, has been observed at 290.3 eV.

Total and elasticelectron scattering cross sections from ozone at intermediateand high energies

Total cross sections for electron scattering from O3molecules in the energy range 350-5000 eV have been measuredfor the first time. The experimental method used was based onthe measurement of the attenuation of a collimated electronbeam through an O3-O2 mixture in combination with useof an electron energy loss technique to determine the purity ofthe ozone sample. Differential and integral elastic crosssections have also been calculated using a scattering potentialin the framework of the independent-atom model. The presenttheoretical and experimental results are compared with earliercalculations available in the literature.

Valenceelectronic excitation of the SiF4 molecule: generalized oscillatorstrength for the 5t2 → 6a1 transition and ab initiocalculation

The electronic excitation of the silicon tetrafluoride (SiF4) moleculehas been studied using the angle-resolved electron energy-loss technique,at 1.0 keV incident electron energy, in the 0-50 eV energy range with anangular range of 1.5°-20.0°. The absolute generalizedoscillator strength (GOS) for the 5t2→6a1 electronictransition, located at 13.0 eV, has been determined. A minimum has beenobserved in the GOS for this transition at K2 = 1.4 au. We have alsodetermined the absolute elastic and inelastic differential cross sectionsat 1 keV. In order to help in the interpretation of the experimentalresults, ab initio calculations have been performed for the verticalvalence transitions and ionization energies for the SiF4 molecule.Configuration-interaction calculations, including single and doubleexcitations (CI-SD) and the symmetry-adapted-cluster expansion (SAC)were used. The CI-SD approach was also employed to obtain the opticaloscillator strength for the 5t2→6a1 transition.

Absolute Measurements of Electron Excitation Cross Sections in C3+

Spectral emissions from C3+ are of importance in solar, planetary and astrophysical plasmas. In these plasmas the population of excited states is often dominated by electron impact excitation. Accurate cross sections for this process are required to determine parameters such as electron and ion densities. The 2s 2S–2p 2P transition in C3+ is of particular interest as it allows direct comparisons among the results of several different experimental and theoretical groups. Employing the electron energy-loss technique we have measured cross sections to several eV above threshold, a region where there is still significant disagreement among previous results.

Coadsorption of K and O on Ni(100) by HREEL investigation

The coadsorption of oxygen on a potassium precovered Ni(100) surface was investigated by the high resolution electron energy loss technique. The interaction between the coadsorbate atoms takes place only for a K coverage of 0.38 ML corresponding to the completion of the first K layer. At this K coverage, an intense vibrational feature, at 180 meV, is a consequence of the oscillating motion of the atoms of the Ni–K–O clusters, particularly, of the component vibrating along the surface plane.

Observation of inner-shell triplet states of CO 2 and N 2O and of inner-shell fine structure in Kr and Xe

Inner-shell singlet-triplet transitions in CO 2 and N 2O have been observed by the electron energy-loss technique at low values of the incident electron energy, typically 1.4 times the excitation energies of the states. The same technique has been used to observe fine structure in transitions near the M 4.5 edges in Kr and the N 4.5 edges in Xe.

Differential electron scattering cross sections for the 3 (2)S to 3 (2)P0 h, K transitions in MG II - Comparison of experiment and theory

Angular differential electron scattering cross sections are reported for the unresolved inelastic 3s (2)S to 3p (2)P0 h, k transitions in Mg II for the first time. Relative differential cross sections have been measured at 35 eV and 50 eV in the angular range of Theta between 6 and 17 deg using the newly developed electron energy loss technique in a crossed electron-ion beam geometry. Theoretical values have been calculated in a five-state close-coupling approximation in which 3s, 3p, 3d, 4s, and 4p states were included, and to which measurements were normalized at Theta = 12 deg.

Surface extended energy loss fine structures above the K-edge of oxygen on Al

Surface Extended Energy Loss Fine Structures (SEELFS) have been detected above the K-edge of oxygen adsorbed on an Al sample. This is the first evidence that the local geometry of adsorbates on metal surfaces can be investigated with the electron energy loss technique in the reflection mode. This experimental technique is highly surface sensitive and accurate bond lengths can be obtained. The oxidation model implied by our results for the OAl system is in agreement with data in the literature.

The observation of electric-dipole-forbidden inner-shell transitions in N2and Ar by the electron energy-loss technique

By using low-energy electron impact excitation the authors have observed the first example of an inner-shell single-to-triplet transition, namely the transition from the ground state to the (1s)-1( pi 2p) 3 Pi state of N2. The parameters omega e and Re of the triplet state have been deduced from the observed vibrational structure of the transition. The parity-forbidden transition to the (2p32/)-14p state of Ar has also been studied, and the intensity of this transition relative to that of the electric-dipole-allowed transition to the (2p32/)-14s state has been measured as a function of the incident electron energy.

Electron energy band shapes from angular dependent electron energy loss spectroscopy

Because the electron energy loss technique can be used to excite interband transitions with a finite momentum transfer, the possibility exists of using this experiment to determine the shapes as well as separations of energy bands. Angular dependent electron energy loss measurements allow the frequency- and wavevector-dependent dielectric function epsilon (q, omega ) to be determined and the relationship of this function to the joint density of valence and conduction states J(q, omega ) is discussed. To examine the feasibility of determining energy band shapes, J(q, omega ) is calculated for typical energy bands in one, two and three dimensions. Rules relating the q dependence of J(q, omega ) to the band shapes are obtained and the sensitivity of the dependence is found to decrease markedly on going from two to three dimensions.

An investigation of inner-shell excited states near the L2,3edges of Cl2using electron impact excitation with high resolution

Inner-shell excitation of molecular chlorine has been studied by the electron energy-loss technique, at an incident electron energy of 1.5 keV and with an energy resolution of 65 meV. Excitation energies of states in the vicinity of the L2,3 ionisation edges have been accurately determined. Two series of states have been observed, and their relative energy separation and transition strength show that the inner-shell vacancy is localised at one of the chlorine atoms. Information about the 'equivalent-core' molecule ArCl has been deduced from these results.

Nitrogen K-shell excitation in N2, NO and N2O by high-resolution electron energy-loss spectroscopy

The excitation of nitrogen 1s electrons has been observed in N2, NO, and N2O by using the electron energy-loss technique at an incident energy of 1.5 keV, and with an energy resolution of 0.075 eV. The energies, widths and vibrational structures of the inner-shell excited states have been obtained. The spectroscopic constants (internuclear separation Re, vibrational constants omega and omega x and quantum defect delta ) of the inner-shell excited states of N2 have been derived and have been found to be similar to those of the equivalent core states. The energies and vibrational structures of the inner-shell excited states of NO and N2O have also been interpreted in terms of the equivalent core model.

Carbon K-shell excitation in small molecules by high-resolution electron impact

The excitation of 1s carbon electrons has been observed in CO, CH4, CF4, CO2, COS, C2H2and C2H4by means of the electron energy-loss technique with high resolution (70 meV in the 300 eV excitation energy range) and at an incident electron energy of 1.5 keV. The energies, widths and vibrational structures of excited states corresponding to the promotion of 1s carbon electrons to unoccupied valence and Rydberg orbitals have been obtained. The validity of the equivalent-core model, and the role of resonances caused by potential barriers, are discussed.

Plasmon excitation energies in ZnO, CdO, and MgO

A study has been made to experimentally determine the plasmon excitation energies in the oxides of Zn, Cd, and Mg using the reflection electron energy loss technique. The measurements were made on clean single-crystal surfaces using electrons of 250–1000-eV energy. Distinction was made between surface- and bulk-plasmon excitation by recording the changes which occurred in the energy loss spectrum as the surface was selectively contaminated. Surface-plasmon excitation energies of 17.0, 15.9, and 14.7 eV and bulk-plasmon excitation energies of 18.8, 22.8, and 22.0 eV were measured for ZnO, CdO, and MgO, respectively. Comparison is made with similar data obtained from optical reflectivity and transmission electron energy loss measurements.