Low-energy Electron Energy Loss Research Articles

Monte Carlo simulation of electron dynamics in liquid water

We present a stochastic model for the energy loss of low-energy electrons (<100 eV) in water in the liquid phase. More precisely, we treat the electrons as independent particles and are thus able to model the time evolution of the kinetic energy of a single electron as a so-called pure jump process. Free electrons are created due to irradiation of an extreme ultraviolet femtosecond laser pulse. In our model, free electrons may interact with water molecules via elastic scattering and impact ionization. Moreover, we present numerical results for the kinetic energy of electrons during and after laser irradiation. Furthermore, we distinguish between primary and secondary electrons, where the latter are created by impact ionization. The numerical results show that creation of secondary electrons due to impact ionization occurs almost entirely during laser irradiation. After irradiation, only a small amount of the laser pulse energy remains in the electron system, while the majority is stored in holes of water molecules.

Processes and time-scales of energy loss of low-energy electrons (

The various processes responsible for the energy degradation and eventual thermalization of low-energy subexcitation electrons (<5eV) in liquid water are completely delineated for the first time, while previous work assumed the formation of thermalized electrons in the 10−11–10−12s for liquid water at room temperature. Chief among these are intramolecular vibrational and rotational excitation, dielectric interaction and excitation of intermolecular vibration through H-bond stretching and bending. Cross-sections for these processes are taken from experiments or derived theoretically. Individual time scales are obtained for subexcitational and subvibrational regimes in water at room and higher temperature.

Monte Carlo simulation of the energy loss of low-energy electrons in liquid water

A Monte Carlo code that performs detailed (i.e. event-by-event) simulation of the transport and energy loss of low-energy electrons (∼50–10000 eV) in water in the liquid phase is presented. The inelastic model for energy loss is based on a semi-empirical dielectric-response function for the valence-shells of the liquid whereas an exchange corrected semi-classical formula was used for K-shell ionization. Following a methodology widely used for the vapour phase, we succeeded in parametrizing the dielectric cross-sections of the liquid in accordance with the Bethe asymptote, thus providing a unified approach for both phases of water and greatly facilitating the computations. Born-corrections at lower energies have been implemented in terms of a second-order perturbation term with a simple Coulomb-field correction and the use of a Mott-type exchange modification. Angular deflections were determined by empirical schemes established from vapour data. Electron tracks generated by the code were used to calculate energy- and interaction-point-kernel distributions at low electron energies in liquid water. The effect of various model assumptions (e.g., dispersion, Born-corrections, phase) on both the single-collision and slowing-down distributions is examined.

Variation of the ELNES Under Channeling Conditions

The orientation dependence of the fine structure of the Al L1 and L2,3 electron energy loss (EELS) edges in (α-Al2O3 has been investigated by measurements with a dedicated scanning transmission electron microscope (VG HB501 STEM, 100 keV acceleration voltage). α-Al2O3 is an anisotropic solid with a complicated alternating stacking sequence of fee Al and hcp O planes along the [0001] direction [1]. This distingiushes the [0001] direction crystallographically, as the highest-order three-fold rotation axes (C3) of the trigonal crystal structure are parallel to [0001], whereas all other symmetry elements are of lower order. Group theory predicts, that more stringent symmetry selection rules apply when electronic transitions are excited by irradiation parallel to the low-index [0001] zone axis than by irradiation along any other arbitrary direction.Yet, even for a low-energy EELS edge (θE = 0.4 mrad) both scattering parallel and perpendicular to the incident beam direction are likely.

Atomic Theory of Electron Energy Loss from Transition Metal Oxides

Low-energy electron energy loss (LE-EELS) spectra have been calculated by a generalization of $R$-matrix methods to solid state systems. The scattering processes involved in LE-EELS can be explained by an atomic model. The contribution of exchange scattering in the calculated loss spectra is important over a large energy range of incident electron energies and is comparable in strength with direct scattering. The ratio of spin-flip to nonflip intensity in LE-EELS depends on the magnetic structure of the target, the type of transition involved (triplet-triplet or triplet-singlet), and the scattering geometry.

A study of Cu growth on an yttria-stabilized ZrO2(100) surface

An yttria-stabilized cubic crystal ZrO2(100) surface was studied by low-energy diffraction and electron energy loss spectroscopy (EELS). The deposition of Cu onto this surface was followed by Auger electron spectroscopy and EELS. The results show that Cu on the surface exhibits Volmer-Weber mode growth at room temperature. The EEL spectra from clean and Cu-covered surfaces are discussed on the basis of the electronic band structure.

Surface and interface properties of cleaned and Ag-covered YBa 2Cu 3O 7−x films

As-received YBa 2Cu 3O 7−x (YBCO) films are generally covered by approximately one monolayer of chlorine. Such YBCO films, which were first cleaned by annealing in oxygen or additionally by electron-stimulated desorption and subsequently covered with Ag at room temperature, were studied by using Auger electron (AES), low-energy electron energy-loss (LEELS), and photoemission spectroscopy (UPS). AES and LEELS signals of YBCO were detected even for nominal silver coverages Θ = 20 nm. This indicates that Ag overlayers grow extremely nonuniform. The formation of Ag islands is concluded from the coverage dependence of the AES lines of the substrate as well as of silver. For small Ag coverages, the Ag(MVV) Auger lines were found to be shifted by approximately −1.4 eV with respect to the Ag(350/355 eV) signals recorded with thick overlayers. On clean samples, we conclude the initial formation of Ag 2O up to gQ ≈ 0.2 nm and a growth of metallic Ag islands for Θ > 1 nm only. On Cl-contaminated YBCO surfaces, a distinctly different behavior is observed. We infer an initial formation of AgCl which prevents an oxidation. Accordingly, energy losses caused by bulk-plasmon excitations in silver metal appear on YBCO:Cl samples already for Θ ≈ 0.2 nm. Our UPS data indicate that the Ag clusters show a polycrystalline morphology.

Studies on as-grown, Cl-covered surfaces of YBa 2Cu 3O 7-δ films

Surfaces of YBa 2Cu 3O 7-δ (YBCO) films were studied at room temperature by using Auger electron (AES), low-energy electron energy-loss (LEELS), and ultraviolet photoemission spectroscopy (UPS). The films were obtained from five different sources and were prepared by laser ablation, sputtering, or metal-organic chemical vapor deposition on ZrO 2, MgO, and LaAlO 3 substrates. All films were found to be covered by approximately one monolayer of chlorine. After annealing in pure oxygen, no carbon contaminations were detected any longer while chlorine was found to persists. The chlorine coverage could be almost completely removed by electron-stimulated desorption (ESD). Subsequent annealing of such ESD-cleaned YBCO surfaces in ultrahigh vacuum gradually restored the Cl coverage. This surface segregation clearly indicates the presence of Cl in the bulk of the YBCO films investigated. By using UPS with 21.2 eV photons, an intense valence-band peak with a binding energy of 4.4 eV was observed with Cl-contaminated but not with ESD-cleaned YBCO surfaces. The strongly reduced photoionization cross section of this peak for 40.8 eV photons demonstrates its Cl(3p) character.

Energy loss of low-energy electrons to nonabrupt metal surfaces.

The interaction of a charged particle with a semi-infinite medium bounded by a flat surface is analyzed as a problem of metal optics. The reflection at the surface of the electromagnetic fields excited by the passing beam is investigated including retardation and spatial dispersion in the hydrodynamic approximation. The soft decay of the electron density at the metal-vacuum interface is modeled. Using measured values of the dielectric function, the calculations are found to agree with experimental results of electron-energy-loss distributions for Ag targets in interaction at a distance. Further predictions of the theoretical model are discussed.

Nitride layers on GaAs(110) surfaces

Nitride layers were prepared on cleaved GaAs(110) surfaces at room temperature by using two different experimental approaches. GaN surface films were obtained by adsorption of nitrogen atoms via an anion exchange, while AlN films were deposited by simultaneous evaporation of aluminum and exposure to a beam of ammonia molecules (reactive molecular beam epitaxy). The layers grown were investigated by using Auger electron, low-energy electron energy-loss, ultraviolet photoemission. soft X-ray and X-ray photoemission spectroscopy as well as a Kelvin probe. To identify the formation of the nitride layers, some of the experimental tools were also applied to GaN(0001) and AlN standards.

Adsorption of atomic nitrogen at GaAs(110) surfaces

The interaction of nitrogen atoms with cleaved GaAs(110) surfaces was studied at room temperature by using Auger electron, low-energy electron energy-loss, and uv photoemission spectroscopy (AES, LEELS, UPS) as well as a Kelvin probe. For reasons of comparison AES and EELS were also applied to a GaN(0001) sample. At low exposures a chemisorption of the nitrogen atoms at the GaAs(110) surface was observed. For larger exposures, the experimental results obtained are indicating the formation of a GaN surface layer via an anion exchange.

A simple cooled Si(Li) electron spectrometer

The construction of an electron spectrometer using a 80 mm 2 × × 3 mm cooled Si(Li) detector has been described. The system resolutions (fwhm) for 115, 194 and 624 keV electrons were found to be 0.88, 1.06 and 1.50 keV, respectively. The resolution (fwhm) for 5.8 MeV α-particles was found to be 12.0 keV. The Si(Li) detector did not show any deterioration in performance after several warming and cooling cycles over a period of two years. The thickness of the Si dead layer on the detector face was estimated from the energy loss of low-energy electrons and was found to be 0.6 μm. The spectrometer described here can be used for the measurement of electron lines above 20 keV energy. Also because of its simple design the spectrometer can be used for α−e − and γ−e − coincidence experiments.