Characteristic Energy Loss Research Articles

Non-Abelian feature of parton energy loss in energy dependence of jet quenching in high-energy heavy-ion collisions

One of the non-Abelian features of parton energy loss is the ratio $\Delta E_g/\Delta E_q=9/4$ between gluon and quark jets. Since jet production rate is dominated by quark jets at high $x_T=2p_T/\sqrt{s}$ and by gluon jets at low $x_T$, high $p_T$ hadron suppression in high-energy heavy-ion collisions should reflect such a non-Abelian feature. Within a leading order perturbative QCD parton model that incorporates transverse expansion and Woods-Saxon nuclear distribution, the energy dependence of large $p_T\sim 5-20$ GeV/$c$ hadron suppression is found to be sensitive to the non-Abelian feasture of parton energy loss and could be tested by data from low energy runs at RHIC or data from LHC.

Absolute Cross Sections for the Electron-Induced Formation of Unsaturated Hydrocarbons from Solid Cyclopropane

Electron-stimulated reactions in solid films of cyclopropane, condensed on Ar spacers deposited on a Pt substrate, were induced and monitored simultaneously using a high-resolution electron-energy-loss spectrometer. Absolute cross sections for the formation of propene, including similar unsaturated species, were independently determined as σ p = (7.2′ 3.9) x 10 - 1 7 cm 2 and σ p = (9.6 ′ 4.8) x 10 - 1 7 cm 2 at an incident electron energy of 15 eV using both characteristic electronic and vibrational energy losses, respectively. The cross section obtained as a function of the film thickness of cyclopropane deposited on a fixed 5-layer spacer of Ar was found to increase above 3-layer coverage. This suggests that intermolecular reactions are not only involved, but also increase the production rate of unsaturated compounds in multilayer films.

Electron energy loss spectroscopic investigation of Co metal, CoO, and Co 3O 4 before and after Ar + bombardment

X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (ELS) have been used to study a sputter-cleaned polycrystalline Co metal foil and CoO plus Co 3O 4 powders before and after ion bombardment. The ELS data have been collected using primary beam energies ranging from 100 to 1150 eV in order to distinguish between surface and bulk features and to facilitate determination of peak locations. It is shown that ELS spectra from these reference materials are considerably different and should, consequently, allow species identification in future catalyst characterization studies. The main surface features obtained from Co metal are observed at 3.7 and 7.0 eV, while the major bulk energy losses are located at 8.2, 9.8, 23.1, 25.7, and 28.1 eV. Despite the collected data on Co ELS and the electronic structure of Co metal, unambiguous peak assignments are difficult. Interband transitions in the energy loss region under investigation originate from the relatively localized Co 3d filled states, but the limited information about the empty states in the conduction band make specific assignments of the energy loss features in the Co ELS spectra challenging. XPS analysis of as-entered CoO and Co 3O 4 reveals that both samples consist mainly of Co 3O 4, in agreement with a Co 3O 4-terminated CoO surface. After prolonged Ar + sputtering both oxides are CoO-like according to high-resolution spectra of the Co 2p binding energy region. In agreement with the XPS data the ELS spectra obtained from as-entered CoO and Co 3O 4 are very similar. In contrast, ELS spectra obtained from CoO and Co 3O 4 after prolonged Ar + sputtering are considerably different. The ELS spectra obtained from CoO are not altered significantly with sputtering suggesting that the Co 3O 4 layer cannot be removed using Ar + bombardment. The sputtered Co 3O 4 instead gives rise to ELS spectra that appear CoO-like. The major ELS features in the spectra obtained from Co 3O 4 are located at 2.5, 6.7, 22.1, and 25.0 eV, while those obtained from Ar +-sputtered Co 3O 4 (CoO) are observed at 7.0, 8.6, 18.3, 21.9, and 36.1 eV. The relative intensities of the features at 6–8 eV and 22–25 eV are considerably different in the two sets of spectra. The intensity ratio of the peaks at 6.7 and 25.0 eV in Co 3O 4 is significantly higher than the 7.0 to 21.9 eV peak intensity ratio of CoO. Although several of the energy losses in the two spectra overlap and make identification difficult despite the large intensity difference, there are peaks that are specific for each compound, i.e. energy losses at 2.5 and 28.5 eV in Co 3O 4 and at 36.1 eV in CoO. Consequently, ELS can be used to differentiate the two cobalt oxides and information complementary to XPS data can be obtained from the ELS data.

Propagation of τ-neutrinos and τ-leptons through the Earth and their detection in underwater/ice neutrino telescopes

If muon neutrinos produced in cosmological sources oscillate, neutrino telescopes can have a chance to detect τ-neutrinos. In contrast to ν μs the Earth is completely transparent for ν τs thanks to the short life time of τ-leptons that are produced in ν τ N charged current interactions. τ-lepton decays in flight producing another ν τ (regeneration chain). Thus, ν τs cross the Earth without being absorbed, though loosing energy both in regeneration processes and in neutral current interactions. Neutrinos of all flavors can be detected in deep underwater/ice detectors by means of C̆erenkov light emitted by charged leptons produced in ν interactions. Muon and τ-leptons have different energy loss features, which provide opportunities to identify τ-events among the multitude of muons. Some signatures of τ-leptons that can be firmly established and background free have been proposed in literature, such as `double bang' events. In this paper we present results of Monte Carlo simulations of τ-neutrino propagation through the Earth accounting for neutrino interactions, τ energy losses and τ decays. Parameterizations for hard part and corrections to the soft part of the photonuclear cross-section (which contributes a major part to τ energy losses) are presented. Different methods of τ-lepton identification in large underwater/ice neutrino telescopes are discussed. Finally, we present a calculation of ν τ double bang event rates in km 3 scale detectors.

Adsorbate-induced surface rearrangement of the system Pd/Mo(1 1 1)

The thickness effect of Pd-adlayer on thermal stability of the Mo(1 1 1) surface has been investigated. It is found for θ>3 ML that heating to T>700 K results in faceting of the type {2 1 1}. At 920 K a reversible transition from the faceted to the planar surface occurs, which is accompanied by meaningful changes in work function opposite in sign to those measured upon the irreversible surface evolution during heating a new layer. It is also found that the faceting process is accompanied by changes in characteristic electron energy loss spectra with the loss energy of the faceted surface higher by 0.7 eV than that of the planar one. The dependence of desorption energy has been determined and four adsorption states found with their related energies of 4.82, 4.47, 4.13 and 3.92 eV/atom, respectively, in the range 0< θ<3 ML.

Quantum and classical aspects of activated surface diffusion

The specific features of classical and quantum activated diffusion of a particle over a surface, modeled by a one-dimensional periodic potential, are analyzed in the low-to-moderate friction limit, in which the kinetics of the process is determined by the energy relaxation. Different models for the energy transition probability are considered with special emphasis on the exponential model which leads to significant simplification of the problem. New expressions are presented for the escape rate, mean squared path length and diffusion coefficient of an activated particle whose energy exchange dynamics is described by an exponential kernel. A universal behavior pj∼j−3/2 exp(−Δj) (where Δ depends only on the friction strength) is found for the distribution pj of diffusive hopping lengths j. It is identical for classical and quantum activated diffusion, does not depend on the details of the model used or on the characteristic energy loss of the particle to the bath. Quantum effects (tunneling) demonstrate themselves only in the absolute values of hopping rates, which for the weak damping regime considered in this paper, lead to a decrease of rates and, thus, the diffusion coefficient. This quantum suppression of diffusion is shown to be equivalent to an effective increase in the activation barrier, caused by quantum above barrier-reflection.

Low-energy electron-energy-loss spectroscopy of electronic transitions in solid carbon dioxide

We report electron-energy-loss spectra of solid films of CO2 for electronic transitions induced by 15, 19.4, and 25 eV incident electrons. All spectra were obtained under sufficiently small electron exposures so as to avoid sample damages. The use of a low-energy electron along with the backscattering geometry give access to spin- and symmetry-forbidden transitions while the effect of the condensed phase makes it possible to modify the energy, ordering, and magnitude of most gas-phase transitions. The most noticeable observation is the disappearance of all sharp energy-loss peaks attributed to a Rydberg series of CO2 in the gas phase. In contrast, transitions to the molecular valence Δu3,1 and Σu−3,1 states are located virtually at the same energy as in the gas phase. The strong dipole-allowed valence Σu+1 transition is found shifted to lower energy by about 0.3 eV while transitions to mixed Rydberg-valence Πg3,1 and Πu1 states are both shifted to higher energy by about 0.4–0.5 eV. The lowest valence Σu+3 transition is ascribed to the lowest energy-loss feature in the solid at 7.9 eV.

Excitation of spin waves at the Fe(100) surface by spin-polarized electron scattering

Inelastic scattering of low-energy electrons off Fe(100) surfaces shows a strong spin dependent energy loss feature in the range of 100--350 meV due to the excitation of spin waves. The highly asymmetric line shape is attributed to the excitation of a continuum of bulk spin waves due to nonconservation of perpendicular momentum in the scattering process.

The electronic structure and lithiation of electrodes based on vanadium–oxide nanotubes

The effect of lithium insertion (lithiation) in electrodes of vanadium oxide nanotubes has been studied using resonant soft-x-ray emission spectroscopy (RSXES) and soft x-ray absorption spectroscopy (SXAS). Spectra at the V L edges were recorded ex-situ on a series of cycled electrodes that had been discharged to different potentials (3.0, 2.5, 2.0, and 1.8 V). Different chemical states upon discharge effectively correspond to different doping levels: the lower the cell potential, the higher the lithiation. The RSXES and SXAS spectra reflect the occupied and unoccupied electronic states, respectively. Spectral changes are observed as a function of the reduction process of vanadium by the electrochemical process. Lithiation induces a gradual enhancement of features attributed to occupied V 3d states in the RSXES spectra, indicating that the intercalated charges occupy bands of V 3d. Both SXAS and RSXES results suggest a partial reduction to V3+ at potentials below 2.0 V and the presence of a mixture of three different oxidation states. RSXES spectra also reveal the low energy-loss feature of 1 eV, suggesting strong correlation of the V 3d orbitals.

Assessment of erosion of the ITER divertor targets during type I ELMs

This paper presents the results of a preliminary assessment conducted to estimate the thermal response and erosion lifetime of the ITER divertor targets clad either with carbon-fibre composite or tungsten during type I ELMs. The one-dimensional thermal/erosion model, used for the analyses, is briefly described. It includes all the key surface heat transfer processes such as evaporation, melting, and radiation, and their interaction with the bulk thermal response, and it is based on an implicit finite-difference scheme, which allows for temperature-dependent material properties.The cases analysed clarify the influence of several ELM parameters on the heat transfer and erosion processes at the target (i.e. characteristic plasma ELM energy loss from the pedestal, fraction of the energy reaching the divertor, broadening of the strike-points during ELMs, duration and waveform of the ELM heat load) and design/material parameters (i.e. inclination of the target, type and thickness of the armour material, and for tungsten only, fraction of the melt layer loss). Comparison is made between cases where all ELMs are characterized by the same fixed averaged parameters, and cases where instead the characteristic parameters of each ELM are evaluated in a random fashion by using a standard Monte Carlo technique, based on distributions of some of the variables of interest derived from experiments in today's machines.Although uncertainties rule out providing firm quantitative predictions, the results of this study are useful to illustrate trends. Based on the results, the implications on the design and operation are discussed and priorities are determined for the R&D needed to reduce the remaining uncertainties.

Lithium ion insertion in nanoporous anatase TiO2 studied with RIXS

Lithium insertion into nanoporous anatase TiO2 electrodes has been studied using resonant inelastic soft-x-ray scattering spectroscopy (RIXS). We have measured Ti 2p absorption, L2,3-emission and resonant inelastic scattering spectra of pristine anatase TiO2 and Li inserted TiO2. Inelastic scattering features in RIXS spectra of pristine TiO2 are assigned to charge-transfer excitations. Low energy-loss features for Li inserted TiO2 electrodes, appearing within t2g bands, due to d–d excitations, indicate the presence of Ti3+ states. The experimental observations suggest strong electron correlation in anatase TiO2 introduced by the Li-insertion.

Optical properties of imperfect In2Se3

Spectra of complete sets of optical functions for α-and β-In2Se3 in the range of 0–20 eV were calculated using experimental reflection spectra and the Kramers-Kronig relation. Special features in the spectra of optical functions for both In2Se3 phases were analyzed. The spectra of both permittivity and characteristic electron energy losses were decomposed into elementary transverse and longitudinal components using the combined Argand diagrams. The main parameters of the electron transitions for these components were determined. The structure of the components was compared with the structure of the expected spectrum of interband transitions.

Spectra of fundamental optical functions of BeSe

Spectra of sets of fundamental optical functions of BeSe were derived for the range of 0–25 eV. Calculations were performed using an experimental ɛ2(E) spectrum and integral Kramers-Kronig relations. Using combined Agrand diagrams, the spectra of permittivity and characteristic energy losses of electrons were decomposed into elementary transverse and longitudinal components. The main parameters of these components were determined. The results obtained were compared with theoretical spectra of optical functions calculated using the OPW and APW methods and with the energy bands obtained in an ab initio GW approximation.

Characterization of tungsten oxide films produced by reactive pulsed laser deposition

Tungsten oxide thin films have been prepared by reactive pulsed laser deposition (PLD). Substrate heat treatment and oxygen partial pressure during growth are correlated with Auger electron (AES), X-ray photoelectron (XPS), electron energy loss (EELS) and transmittance spectroscopies. Electronic and mass densities, composition and chemical states are strongly dependent of the deposition conditions. No significant change in the oxygen content in films as a function of substrate or annealing temperature is detected. However, the colored state turns out to be associated to the degree of chemical disorder in the samples, as evidenced by the peak shape of the W 4f transition. Also, the strength of a characteristic energy loss at 6–7 eV appears to be related to the presence of the colored state.

Electron channeling, structure factor phases, polarity and atom site determination in crystals

We relate element characteristic electron energy losses and the accompanying X-ray emission under electron channeling conditions to the phases of structure factors. We discuss examples illustrating that structure factor phase information can be extracted from two-beam channeling experiments. This information has been used by several investigators to determine the absolute orientation of non-centrosymmetric crystals. We further discuss how the success of electron channeling in locating atoms in the crystal unit cell, ALCHEMI, is tied to phase information beyond the conventional one. From the perspective of diffraction-based crystallography, ALCHEMI provides information about the phase with which a particular element scatters into a reflection. Therein lies the strength of the ALCHEMI technique as compared to relying on diffraction intensities for the location of small concentrations of elements and neighbors in the periodic system.

Polytype determination at the SiC–SiO 2 interface by internal electron photoemission scattering spectroscopy

The characteristic energy loss corresponding to band-to-band electron excitation at the SiC surface is observed in the internal photoemission (IPE) spectra of SiC–SiO 2 interfaces for three polytypes of SiC (4H, 6H, and 15R). From this feature the SiC bandgap width at the interface with thermal oxide was found to be the same as in the bulk of SiC. This importantly indicates that oxidation in dry O 2 at temperatures as high as 1300 °C does not lead to polytypic transition in SiC.

Cross sections for low energy (1–12 eV) inelastic electron scattering from condensed thymine

We describe a new method to obtain the absolute integral cross sections per scatterer for inelastic electron scattering from sub- and monolayer film of molecules condensed on an inert surface. These cross sections are determined by means of a high-resolution electron-energy loss (HREEL) spectrometer for electrons of incident energies similar to those of secondary electrons generated by high energy radiation, i.e. 1–12 eV. Using the linear relationship between the reflected and transmitted current, a HREEL spectrum can be normalized in such a way that the energy integral performed over an energy loss feature can be expressed in terms of an absolute reflectivity value. Given the thickness of the film and assuming thin film and single collision conditions, those reflectivity values can then be converted into absolute integral cross sections for inelastic electron scattering. As an example, we report the energy dependence of the cross sections for the vibrational excitations of thymine. The maximum of 1.6×10 −16 cm 2 at about 4 eV in the cross section for an energy loss of 95 meV is attributed to the formation of a transient anion state that is decaying into vibrational excitation channels.

ELS and XPS study of Pd/PdO methane oxidation catalysts

Electron energy loss spectra (ELS) and X-ray photoelectron spectroscopy (XPS) data obtained from palladium powder catalysts used for complete methane oxidation have been obtained in this study in order to gain information about the Pd chemical state. Metallic Pd and PdO yield significantly different ELS spectra making ELS useful for chemical-state determination. Palladium(II) oxide is readily detected by the presence of an energy loss feature at 3.7eV. Species distribution in the direction normal to the surface can be determined using ELS by varying the primary beam energy.Both XPS and ELS data indicate that PdO forms during reaction, and the ELS data demonstrate that PdO forms as a film over the Pd metal. If any Pd metal is present in the PdO film, it is there at levels below the detection limit of ELS. These observations have important consequences with regard to the mechanism of methane oxidation over Pd-containing catalysts.Hydrous and anhydrous palladium(II) oxides have significantly different energy loss features in the low loss-energy region of the ELS spectra. ELS features attributed to water are observed at 5.5–6.5 and 8eV for hydrous palladium(II) oxide.

Characteristic energy losses of channeled electrons; reciprocity and double alignment

Characteristic energy losses of channeled electrons; reciprocity and double alignment

Non-dipole electron impact spectroscopy of a common local anesthetic: Generalized oscillator strengths of valence, Cl 2p and C 1s pre-edge transitions of chloroethane

Angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy has been used to determine absolute generalized oscillator strengths (GOSs) of discrete transitions in the pre-edge regions of the valence, Cl 2p and C 1s shells of chloroethane as functions of energy loss and momentum transfer. A new lowest-lying preionization-edge energy-loss feature at 7.2 eV has been clearly identified at a non-zero momentum transfer. The corresponding GOS profile is found to have a shape characteristic of non-dipole transitions that can be attributed to electronic excitations from nonbonding Cl 3p initial-state orbitals (4a″ and 13a′) to antibonding σ* C–Cl final-state orbital (14a′). The GOS profiles of the other low-lying valence-shell and Cl 2p and C 1s pre-edge features are found to have maximum intensity at zero momentum transfer, generally consistent with the spectral assignments to dipole-allowed transitions involving final-state Rydberg orbitals. Differences in and between the GOS profiles for the valence-shell and these inner-shell pre-edge features in C 2H 5Cl are identified. Although no discernible secondary extrema can be found in any of the inner-shell GOS profiles, subtle differences can be observed and are consistent with the proposed assignments. The present GOS results for C 2H 5Cl are compared with the available GOS data reported for other mono-chlorides, including CF 3Cl and CHF 2Cl.