Elastic Peak Intensity Research Articles

Monte Carlo Simulation of Free-standing Thin Films under Low Energy Electron Bombardment: Electron Inelastic Mean Free Path (IMFP) Determination Using Elastic Peak of the Transmitted Electrons

Abstract: The electron energy spectra of transmitted scattered electrons from free-standing films are simulated using a Monte Carlo computational approach. Elastic scattering is simulated using Mott cross-sections and inelastic scattering via discrete processes determined from dielectric function data. This allows one to simulate the secondary electrons as well as the loss peaks near the elastic (zero-loss) peak. The current study suggested a directed approach for determining the electron inelastic mean free path (IMFP) of materials at low primary electron energies. The IMFP of the reference material is not necessary for the suggested technique. The suggested technique uses the ratio between the transmitted elastic peak intensity and the background intensity of backscattered electrons. Free-standing films of Si, Cu, and Au were studied with thicknesses varying from 2 to 12 nm. Primary electron energies of 1, 3, and 5 keV were applied. The results appeared very good, with the percentage error range being between 5% and 25%. We also investigated the proportion of the first and second plasmon peak intensities to the elastic peak intensity. We believe that the latter could provide a directed method of measuring the IMFP of materials. Keywords: Inelastic mean free path, Monte Carlo simulation, Geant4, Free-standing film, Zero-loss peak.

Quantitative depth‐dependent analysis using the inelastic scattering backgrounds from X‐ray photoelectron spectroscopy and hard X‐ray photoelectron spectroscopy

The inelastic scattering contribution to an X‐ray photoelectron spectrum (XPS) reflects the distribution of elements within the sample depth. Varying the energy of the incident photons changes the inelastic scattering contribution. We present a standardless, automated method that exploits this change to enable compositional analysis within the XPS information depth (also known as amount of substance, AOS3λ). In this method, the photoelectron intensities observed using two or more X‐ray photon energies are normalised using correction factors. These correction factors are derived to determine the AOS3λ from the inelastic scattering signals collected from different samples. The AOS3λ are then compared with the ratio of the elastic XPS peak intensities calculated for different sample depths to find the layer thickness. The method has been applied to analyse spectra collected from thin amorphous carbon films to yield quantitative, standardless and automated XPS analysis.

Electron inelastic mean free path (IMFP) values of Kapton, polyethylene (PE), polymethylmethacrylate (PMMA), polystyrene (PS) and polytetrafluoroethylene (PTFE) measured with elastic peak electron spectroscopy (EPES)

Elastic peak electron spectroscopy (EPES) was employed to measure the inelastic mean free path (IMFP) for energies between 500 and 1600 eV for five insulating organic compounds: Kapton, polyethylene (PE), poly(methyl methacrylate) (PMMA), polystyrene (PS) and polytetrafluoroethylene (PTFE). A Ni and a Si sample were used as reference materials to avoid measurement of the elastic reflection coefficient in absolute units. Correction of experimental elastic peak intensities for surface excitations was performed which turned out to be essential. The results are compared with recent evaluations of optical constants to yield the IMFP in the literature giving satisfactory agreement, with deviations generally below 20%. Investigation of the kinematics in an electron reflection experiment shows that the dispersion coefficient used in REELS data analysis cannot be identified with the true plasmon dispersion.

Energy resolution and neutron flux of the 4SEASONS spectrometer revisited

The elastic energy resolution, integrated intensity, and peak intensity of the direct-geometry neutron chopper spectrometer 4SEASONS at Japan Proton Accelerator Research Complex (J-PARC) were re-investigated. This was done with respect to the incident energy and the rotation speed of the Fermi chopper using incoherent scattering of vanadium and simple analytical formulas. The model calculations reproduced the observed values satisfactorily. The present work should be useful for estimating in instrument performance in experiments.

Absolute cross section measurements for the scattering of low- and intermediate-energy electrons from PF3. II. Inelastic scattering of vibrational and electronic excitations.

As a sequel paper to our study of the elastic scattering for electron collisions with phosphorus trifluoride, PF3 molecules, we report absolute inelastic differential and integral cross sections (DCS and ICS) of vibrational excitations for the compound fundamental vibrational modes v13 (v1 + v3), v24 (v2 + v4), and their sum in the impact energy range of 2.0-10 eV and over a scattering angle range of 20°-130°. The measured angular distributions of scattered electron intensities for the present inelastic scattering are normalized to the elastic peak intensity corresponding to the DCSs of He. These vibrational excitation measurements demonstrate the presence of resonances around 2 eV and also around 6-10 eV. In addition, a generalized oscillator strength analysis is applied to derive oscillator strength f0-values and (unscaled Born) ICSs from the corresponding DCSs measured for the low-lying optically allowed 8a1-1 → 7e (σ*) excitation band, which is assigned as the Jahn-Teller splitting and 8a1-1 → 4s Rydberg transition at impact energies of 100, 200, and 300 eV, over a scattering angle range of 1.0°-15°. The f0-values obtained in the present study are compared with the results of previous photoabsorption and pseudo-optical measurements. The unscaled Born ICSs are compared with the binary-encounter f-scaled Born ICSs estimated over a wide impact energy region from the excitation thresholds.

Strong O 2p–Fe 3d Hybridization Observed in Solution-Grown Hematite Films by Soft X-ray Spectroscopies

Photoelectrochemical (PEC) water splitting holds the potential as a direct route for solar energy conversation and storage. The performance of a PEC device is strongly influenced by the electronic properties of the photonanode surface. It has been shown that the synthesis methods can have a profound impact on the electronic properties and PEC performance of various photoelectrode materials such as hematite. Soft X-ray spectroscopic techniques, including O K-edge XAS and Fe L-edge XAS/XES, have been employed to investigate how the synthesis methods impact the electronic structure of resulting hematite materials. It is found that the hematite samples via solution regrowth methods show dramatically increased 3d-4sp band ratios in O K-edge XAS spectra and decreased relative elastic peak intensities in Fe L-edge RIXS spectra compared with samples synthesized via ALD or solution grown. The difference observed in O-K and Fe-L spectra indicated that solution regrowth strategy alters the O 2p-Fe 3d hybridization and hence the electronic structure of the hematite films, which proves to be beneficial for PEC performance of the hematite photoanode. Our findings provided new insights and potentially useful strategies for enhancing the PEC performance of photoanode materials.

Absolute determination of optical constants by reflection electron energy loss spectroscopy

We present an absolute extraction method of optical constants of metal from the measured reflection electron energy loss (REELS) spectra by using the recently developed reverse Monte Carlo (RMC) technique. The method is based on a direct physical modeling of electron elastic and electron inelastic scattering near the surface region where the surface excitation becomes important to fully describe the spectrum loss feature intensity in relative to the elastic peak intensity. An optimization procedure of oscillator parameters appeared in the energy loss function (ELF) for describing electron inelastic scattering due to the bulk- and surface-excitations was performed with the simulated annealing method by a successive comparison between the measured and Monte Carlo simulated REELS spectra. The ELF and corresponding optical constants of Fe were obtained from the REELS spectra measured at incident energies of 1000, 2000 and 3000 eV. The validity of the present optical data has been verified with the f- and ps-sum rules showing the accuracy and applicability of the present approach. Our data are also compared with previous optical data from other sources.

Investigation of activation kinetics in geopolymer paste using quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been used to investigate the binding process of water molecules in pastes of calcium geopolymer prepared with low and high calcium fly ash contents, and at two different NaOH molarities, 10 and 14M. The in situ measurements were carried at ambient and elevated curing temperatures (60°C). By carefully monitoring the time evolution of the elastic peak intensity, we infer a gelation process, followed by polymerization and a hardening in the high calcium geopolymer paste at 60°C, in agreement with previously proposed geopolymerization model. This behavior was neither observed at ambient temperature in both low and high calcium geopolymer cement paste, within the precision of the neutron instrument. Our study clearly shows that a minimal amount of heat is necessary to form gelation and polymerization during the activation process. The activation of geopolymer paste with high NaOH molarity involves more chemically bound water molecules than that at lower activator concentration. This work shows that the QENS technique can be effectively used to characterize the alkali-activation kinetics in certain geopolymer pastes, and that temperature and molarity of the activator play a vital role in controlling the gel mechanism.

Analytical theory of elastic electron backscattering from elements, alloys and compounds: Comparison with experimental data

Probability of elastic electron backscattering from surfaces is typically calculated from theoretical models implemented in Monte Carlo simulation strategies since this approach is considered to be the most accurate. However, an analytical model proposed by Oswald et al. [J. Electron Spectrosc. Relat. Phenom. 61 (1993) 251], after later modifications, has been found to be of similar accuracy. The relevant analysis was performed for selected elemental solids. In the present work, a possibility to use the analytical formalism in elastic peak electron spectroscopy for determination of the IMFP has been presently studied. For this purpose, two further modifications of the analytical theory were made: (i) creation of a database of parameters facilitating calculations of the angular distribution of electrons after multiple collisions, and (ii) extension of the formalism to solids constituted of different atomic species, i.e. alloys and compounds. Comparison of experimental data (angular distribution of backscattered electrons, ratios of elastic peak intensities) with predictions of both theoretical models proves that these models are of similar accuracy. The IMFPs for elemental solids derived from experimentally measured ratios using the analytical formalism and the Monte Carlo simulations are practically identical. On analysis of IMFPs obtained for 13 elemental solids in the energy range from 200eV to 5000eV, it has been estimated that the IMFPs obtained from the analytical formalism deviate on average from IMFPs from Monte Carlo calculations by 3.09%. Similar agreement was found for an alloy (Al0.48Ni0.52) and compounds (GaSb, InSb, GaN and SiC) considered here.

Surface excitation parameter for rough surfaces

In order to assess quantitatively the importance of surface excitation effect in surface electron spectroscopy measurement, surface excitation parameter (SEP) has been introduced to describe the surface excitation probability as an average number of surface excitations that electrons can undergo when they move through solid surface either in incoming or outgoing directions. Meanwhile, surface roughness is an inevitable issue in experiments particularly when the sample surface is cleaned with ion beam bombardment. Surface roughness alters not only the electron elastic peak intensity but also the surface excitation intensity. However, almost all of the popular theoretical models for determining SEP are based on ideal plane surface approximation. In order to figure out whether this approximation is efficient or not for SEP calculation and the scope of this assumption, we proposed a new way to determine the SEP for a rough surface by a Monte Carlo simulation of electron scattering process near to a realistic rough surface, which is modeled by a finite element analysis method according to AFM image. The elastic peak intensity is calculated for different electron incident and emission angles. Assuming surface excitations obey the Poisson distribution the SEPs corrected for surface roughness are then obtained by analyzing the elastic peak intensity for several materials and for different incident and emission angles. It is found that the surface roughness only plays an important role to SEP only for large interaction angle cases (larger than 60°) and large surface roughness (root-mean-square value lager than 15nm). This result is a clear evidence that the SEP database calculated based on ideal plane surface model are still efficient for realistic sample surface with common surface roughness.

Microscopic damage and dynamic mechanical properties of rock under freeze–thaw environment

For understanding the rock microscopic damage and dynamic mechanical properties subjected to recurrent freeze–thaw cycles, experiments for five groups of homogeneous sandstone under different freeze–thaw cycles were conducted. After freeze– thaw, nuclear magnetic resonance (NMR) tests and impact loading tests were carried out, from which microscopic damage characteristics of sandstone and dynamic mechanical parameters were obtained. The results indicate that the porosity increases with the increase of cycle number, the rate of porosity growth descends at the beginning of freeze–thaw, yet accelerates after a certain number of cycles. The proportion of pores with different sizes changes dynamically and the multi-scale distribution of pores tends to develop on pore structure with the continuing impact of freeze–thaw and thawing. Dynamic compressive stress–strain curve of sandstone undergoing freeze–thaw can be divided into four phases, and the phase of compaction is inconspicuous compared with the static curve. Elastic modulus and dynamic peak intensity of sandstone gradually decrease with freeze–thaw cycles, while peak strain increases. The higher the porosity is, the more serious the degradation of dynamic intensity is. The porosity is of a polynomial relationship with the dynamic peak intensity.

A comparative study of the He–Sb(111) interaction potential from close-coupling calculations and helium atom scattering experiments

The exact elastic close-coupling formalism is used to compare the performance of several interaction potentials suggested in literature for describing the measured elastic diffraction peak intensities in helium scattering experiments. The coupling parameters have been analytically calculated for the corrugated Morse potential on a hexagonal surface structure and adapted for usage with similar interaction potentials. The potentials used have been fitted to previously known bound state energies complemented by two additional levels which are found by improving energy resolution. It is established that the shifted Morse potential reproduces the experimental He–Sb(111) bound state more closely than the other considered potential shapes. The performance of several interaction potentials in describing the elastic scattering intensities is presented and discussed. Morse and Morse-related potentials provide the best compromise for the description of elastic scattering intensities. The different effects of the potential shape were determined by comparing the calculated scattering intensities.

Quantification of surface roughness effect on elastically backscattered electrons

Electron inelastic mean free path can be obtained from a measured elastic peak electron spectroscopy spectrum combined with a Monte Carlo simulation. It is thus necessary to know the influence of various experimental factors to the measured and calculated results. This work investigates the effect of the surface roughness or the surface topography on the intensity of the elastic peak. A Monte Carlo simulation, by taking into account of realistic surface roughness for both Gaussian and non‐Gaussian type rough surfaces experimentally prepared, has been employed to study the surface topography effect. The simulations of elastic peak electron spectroscopy were performed for both planar and rough Al and Cu surfaces and for varied primary energies ranging from 200 to 2000 eV. To quantify the surface roughness effect, the surface roughness parameter is introduced according to the ratio of elastic peak intensities between a rough surface and an ideal planar surface. Simulation results have shown that surface roughness parameter is important in a certain range of emission angle and particularly for large emission angles. For grazing emission, the elastic peak intensity can be largely enhanced by roughness even at nanometer scale. Copyright © 2014 John Wiley & Sons, Ltd.

Influence of surface topography on elastically backscattered electrons

A Monte Carlo simulation, taking into account of the detailed surface roughness of a realistic solid sample, has been performed to study the surface topography influence on elastic peak intensity. To describe quantitatively the surface topography effect, here we introduce surface roughness parameter (SRP) according to the ratio of elastic peak intensities between a rough surface and an ideal planar surface. Simulation results for Al sample have shown that SRP varies with surface roughness particularly at large incidence/emission angles.

Influence of surface roughness on elastically backscattered electrons

The electron inelastic mean free path can be obtained from the elastic peak electron spectroscopy (EPES) combined with a Monte Carlo simulation. It is thus necessary to know the influence of various experimental factors on the measured and calculated results. In this work, we consider an important factor, i.e. the surface roughness or the surface topography. A Monte Carlo model, taking into account the details of the surface excitation and realistic sample surface roughness, is built to study how the surface excitation and surface roughness affect elastic peak intensity. The simulations of EPES were performed for both planar and rough Ni surfaces and for primary energies from 100 to 5000 eV. The simulation results are found to be in good agreement with the experiments; the quantitative information of elastic peak intensity influenced by surface roughness is obtained for both cylindrical mirror analyzer and retarding field analyzer. It is found that the elastically backscattered electron intensity measured by the two analyzers is only weakly influenced by surface roughness in the size range below 30 nm. The angular resolved elastic peak intensity varies with surface roughness significantly only at the large emission angles above 70o. The present simulation results then suggest that for the inelastic mean free path measurements the emission angle should be below 70o in an EPES experiment to reduce the roughness effect. Copyright © 2012 John Wiley & Sons, Ltd.

A study of the influence of surface layers’ disordering on the spectrum of energy losses of electrons passed through copper thin single crystal films

Results of the experimental investigation of energy losses of electron passed through thin films Cu (100) with a thickness of d ≈ 420 A at E p = 3 keV are presented. It is revealed that the surface plasmon peak disappears at a primary electron energy of ≥6 keV. For the first time, the degree of surface amorphization and area of the disordered layer are estimated by the change of elastic peak intensity during the passage of electrons through thin films of Cu (100). There is an additional energy loss of electrons peak caused by bulk plasmon, its intensity depending on the irradiation dose under ionic bombardment.

Experimental determination of the electron elastic backscattering probability and the surface excitation parameter for Si, Ni, Cu and Ag at 0.5 and 1 keV energies

AbstractElectron spectra are generally presented in arbitrary units. The experimental elastic peak intensity Iespec(E) is determined by the elastic backscattering probability Ie(E) of electrons backscattered elastically within the solid angle of the spectrometer. The experimental elastic peak Iespec(E) is converted to Ie(E) backscattering probability using our new procedure based on the Goto ie(E) elastic backscattering current database. The elastic backscattering probability Ic(E) was calculated applying the EPESWIN software of Jablonski. Ie(E) < Ic(E) due to the surface losses of electrons, characterized by the surface excitation parameter Pse (SEP). Pse(E) was determined experimentally using the Goto database and the relationship of Tanuma. Our new procedure is applied to angular‐resolved (AREPES) spectra of Jablonski and Zemek presented in arbitrary units. In their AREPES experiments, the experimental elastic peak intensity Iespec = Ie(E, αd, ΔΩ) was measured at αd angle of detection (35–74°) with a small HSA, with ΔΩ solid angle. The experimental value at 42° $I_{e}(E, {\it{42}}\deg{\hbox{}}, {\Delta}\Omega)$ was converted to probability with the Goto database. It was corrected with a SEP parameter Pse, determined by trial and error method for Si, Ni, Cu and Ag for E = 0.5 and 1 keV primary energies. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental and theoretical study of rotationally inelastic diffraction of D2 from NiAl(110)

We present a detailed experimental and theoretical study of elastic and rotationally inelastic diffraction of D(2) from NiAl(110) in the energy range 85-150 meV. The experiments were performed using a high-resolution, fixed angle geometry apparatus. Quantum and classical dynamical calculations were performed by using a six-dimensional potential energy surface constructed upon interpolation of a set of DFT (density functional theory) data. We show that, although elastic diffraction peak intensities are accurately described by theory in the whole range of incidence energies and angles explored, significant discrepancies are obtained for RID peaks, especially for those involving rotational initial states with j(i) > 0. Possible reasons for this discrepancy are discussed.

The inelastic mean free path of electrons. Past and present research

The inelastic mean free path (IMFP) is a fundamental material parameter. Presently the IMFPs calculated by the TPP-2M predictive formula (NIST SRD 71) are generally used. Elastic peak electron spectroscopy (EPES) is proved to be adequate for experimental determination of the IMFP denoted by λ e . λ e is smaller then λ i (TPP-2M) values, due to surface losses, characterized by the SEP (surface excitation) P se material parameter. The present research is focused on the experimental determination of P se based on Tanuma's work. The Tanuma factor f sT is the ratio of experimental I e and calculated I c elastic peak intensities I e ( E)/ I c ( E). The detection angle dependent I c is proportional to Δ Ω, the solid angle of detection. The angular I c ( E,Δ Ω, α d ) was calculated applying the EPESWIN software of Jablonski. In our work, experimental data of Goto were analysed for Si and Ni. Recent angularly resolved AREPES results of Jablonski and Zemek were quantified by fitting them at the 42° CMA (cylindrical mirror analyser) angle to absolute data of Goto and applying f sT. The models and SEP material parameters published by Werner et al., Ding et al., Kwei et al., Jablonski–Zemek and Nagatomi–Goto (Ni) and our data obtained by modifying Chen's data (Si) have been tested. The best approach was obtained using data of Werner for Si, and data of Nagatomi for Ni. The SEP corrected IMFPs λ eco were deduced. The EPES SEP parameters were valid for AREPES by averaging over α d = 35–70°. EPES spectra are quantified by applying the backscattering yield.

弾性散乱ピーク強度比を用いた金属元素における 200 - 5000 eVのIMFPと SEPの同時推定

弾性散乱ピーク強度比を用いた金属元素における 200 - 5000 eVのIMFPと SEPの同時推定