Morphology,surface roughness,electron inelastic and quasielastic scattering in elastic peak electron spectroscopy of polymers

In elastic peak electron spectroscopy (EPES), the nearest vicinity of elastic peak in the low kinetic energy region reflects electron inelastic and quasielastic processes. Incident electrons produce surface excitations, inducing surface plasmons, with the corresponding loss peaks separated by 1–20 eV energy from the elastic peak. In this work, X‐ray photoelectron spectroscopy (XPS) and helium pycnometry are applied for determining surface atomic composition and bulk density, whereas atomic force microscopy (AFM) is applied for determining surface morphology and roughness. The component due to electron recoil on hydrogen atoms can be observed in EPES spectra for selected primary electron energies. Simulations of EPES predict a larger contribution of the hydrogen component than observed experimentally, where hydrogen deficiency is observed. Elastic peak intensity is influenced more strongly by surface morphology (roughness and porosity) than by surface excitations and quasielastic scattering of electrons by hydrogen atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Obtaining quantitative information on surface excitations from reflection electron energy‐loss spectroscopy (REELS)

A new analysis of reflection electron energy‐loss spectroscopy (REELS) spectra is presented. Assuming inelastic scattering in the bulk to be quantitatively understood, this method provides the distribution of energy losses in a single surface excitation in absolute units without the use of any fitting parameters. For this purpose, REELS spectra are decomposed into contributions corresponding to surface and volume excitations in two steps: first the contribution of multiple volume excitations is eliminated from the spectra and subsequently the distribution of energy losses in a single surface scattering event is retrieved. This decomposition is possible if surface and bulk excitations are uncorrelated, a condition that is fulfilled for medium‐energy electrons because the thickness of the surface scattering layer is small compared with the electron elastic mean free path. The developed method is successfully applied to REELS spectra of several materials. The resulting distributions of energy losses in an individual surface excitation are in good agreement with theory. In particular, the so‐called begrenzungs effect, i.e. the reduction of the intensity of bulk losses due to coupling with surface excitations near the boundary of a solid‐state plasma, becomes clearly observable in this way. Copyright © 2003 John Wiley & Sons, Ltd.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of Fe-Si layered structures by reflected electron energy loss spectroscopy and inelastic scattering cross-section

This paper reports on our study of the formation of an interface of layered structures in the Fe-Si system by reflected electron energy loss spectroscopy (REELS). Quantitative element analysis was performed using the product of the mean length of the inelastic free path by the inelastic scattering cross-section of electrons. It is shown that the Fe-Si interface is quite uniform.     

Studies of AuNi alloys by electron spectroscopies with the aid of the line shape analysis by the pattern recognition method

The spectrum of electrons elastically backscattered from the surface and within its vicinity reflects the probability of electron elastic backscattering on the surface atoms, quasi‐elastic scattering and the inelastic scattering visible in the low energy side of the elastic peak. The method for investigating the processes of electron elastic backscattering on the surface atom is called the elastic peak electron spectroscopy (EPES). In the present work, AuNi alloys of different compositions are investigated using X‐ray photoelectron spectroscopy (XPS) and the EPES method with the aid of the line shape analysis called the fuzzy k‐nearest neighbour (fkNN) rule. The line shape analysis was found to be applicable for EPES spectroscopy. It allows distinguishing the surfaces exhibiting various surface roughness, texture and grain size, and quantifying the selected information depths. The quantitative results obtained from the XPS analysis and the EPES spectra line shape analysis indicated Au surface segregation with Au surface enriched profile. Quantitative discrepancies are discussed within the non‐uniform concentration profiles of constituents due to sputter cleaning and annealing, different diffusion coefficients for Au and Ni, differences in the information depths sampled by XPS and EPES methods and differences in electron elastic backscattering cross‐sections for Ni and Au. Copyright © 2006 John Wiley & Sons, Ltd.     

Analytical applications of elastic electron backscattering from surfaces

Measurements of probability of elastic electron backscattering from surfaces can provide information on physical properties of the surface region with thickness comparable to the inelastic mean free path (IMFP) of electrons. The analytical technique, based on such measurements, is known as elastic peak electron spectroscopy (EPES). The most frequent application of EPES is the determination of the IMFP in solids. However, this technique can also be used to measure overlayer thickness, or to determine surface composition.

Quantitative applications of EPES, addressed here, require a reliable theoretical model describing the elastic backscattering probability from surfaces with a given structure and composition. Unfortunately, there is no simple analytical model which describes the elastic backscattering probability with an acceptable accuracy. Values of the elastic backscattering probability are usually estimated from Monte Carlo (MC) simulations of elastic backscattering events, since the theoretical model implemented in the MC scheme seems closest to reality, as compared with models leading to different analytical expressions. It is shown that the reliability of the theory is associated with accuracy of the parameters needed in the calculations. The most important parameters are the differential elastic scattering cross-sections which are presently known, especially in some angular regions, with limited accuracy. The IMFP values, determined in different laboratories via EPES, exhibit a considerable scatter, which may be due to the fact that different experimental geometries are used in measurements. Other sources of errors are briefly discussed.     

Report on the 47th IUVSTA Workshop ‘Angle‐Resolved XPS: the current status and future prospects for angle‐resolved XPS of nano and subnano films’

A summary of the workshop entitled ‘Angle‐Resolved XPS: The Current Status and Future Prospects for Angle‐resolved XPS of Nano and Subnano Films’ is given, which was held at the Riviera Maya, Mexico, 26–30 March 2007, under the main sponsorship of the International Union for Vacuum Science, Technique and Applications (IUVSTA). Angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) can provide detailed chemical as well as depth profile information about the near‐surface composition of materials and thin films. This workshop was held to review the present status and level of understanding of Angle‐resolved XPS, and to stimulate discussions leading to a deeper understanding of current problems and new solutions. The main goal of the workshop was to find better ways to perform experiments and, very importantly, better ways to extract information from the experimental data. This report contains summaries of presentations and discussions that were held in sessions entitled ‘Basics and Present Limits of ARXPS’, the Analysis of ARXPS Data, Applications of ARXPS, Equipment for ARXPS, and Future Developments in ARXPS'. There were 33 participants at the workshop. Copyright © 2009 John Wiley & Sons, Ltd.     

Monte Carlo Simulation of Electron Transport and X-Ray Generation. I. Electron Elastic and Inelastic Scattering

We present different theoretical approaches to determine differential cross sections for elastic and inelastic interactions of electrons. These cross sections are the basic ingredients for accurate Monte Carlo simulation of electron transport in matter. The considered models range from simple analytical approximations employed in early calculations to purely numerical differential cross sections described by large databases calculated with state-of-the-art theory.     

Comparison of the Non‐linear Spin Dynamics in Antiferromagnetic Chain Compounds Na2MnF5 and (ND4)2MnF5

Na₂Mn_0.98Fe_0.02F₅ and (ND₄)₂Mn_0.98Fe_0.02F₅ was studied with the aid of Mössbauer spectroscopy. These results were interpreted on the basis of classical soliton theory. In order to confirm this concept, we have performed neutron scattering experiments on large single crystals of the pure compounds. We discuss the results obtained on a thermal and cold three‐axis spectrometer, which probe the magnon spin wave excitations and the existence of the nonlinear excitations in the quasi 1‐d antiferromagnetic chains of Na₂MnF₅ and (ND₄)₂MnF₅, respectively. Additionally, we include elastic neutron diffraction and dc. single crystal susceptibility measurements to determine the magnetic structure. From the width of the quasielastic scattering signal the temperature dependence of the inverse magnetic correlation lengths was derived resulting in a soliton activation energy of E_s/k = 65(3) K and E_s/k = 81(3) K, respectively, which are in good agreement with the soliton energies obtained by our high resolution inelastic neutron scattering experiment. In contrast to these results the Mössbauer spectroscopy gives twice the value of the soliton energy caused by soliton pair or inter‐band excitations.     

Calculation of Surface Excitation Parameters by a Monte Carlo Method

Electron inelastic mean free path (IMFP) is an important parameter for surface chemical quantification by surface electron spectroscopy techniques. It can be obtained from analysis of elastic peak electron spectroscopy (EPES) spectra measured on samples and a Monte Carlo simulation method. To obtain IMFP parameters with high accuracy, the surface excitation effect on the measured EPES spectra has to be quantified as a surface excitation parameter (SEP), which can be calculated via a dielectric response theory. However, such calculated SEP does not include influence of elastic scattering of electrons inside samples during their incidence and emission processes, which should not be neglected simply in determining IMFP by an EPES method. In this work a Monte Carlo simulation method is employed to determine surface excitation parameter by taking account of the elastic scattering effect. The simulated SEPs for different primary energies are found to be in good agreement with the experiments particularly for larger incident or emission angles above 60° where the elastic scattering effect plays a more important role than those in smaller incident or emission angles. Based on these new SEPs, the IMFP measurement by EPES technique can provide more accurate data.     

Photoelectron angular distributions of Cu,Ag, Pt and Au samples: experiments and simulations

Angular distributions of photoelectrons emitted from semi‐infinite Cu, Ag, Pt and Au specimens have been measured for off‐normal emission angles in the range between 20 and 70° with a Thermo Theta Probe electron spectrometer. Experimental peak intensities for peaks of atomic subshells observable in the spectra were compared with results of simulations using the NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) that takes into account the effects of (i) anisotropy of the photoelectric cross‐section; (ii) elastic scattering of the photoelectrons; and (iii) the finite solid angle of the detector. In addition, a separate correction was made to the simulated intensities for the effects of surface excitations. The combined influence of these effects was found to significantly affect the angular distributions. Furthermore, it was found that ratios of the calculated peak intensities of the observed subshells for a particular material to the measured intensities deviate from unity by typically less than 1% after corrections for multiple inelastic scattering and surface excitations. Copyright © 2010 John Wiley & Sons, Ltd.     

Calculations of electron inelastic mean free paths

We report calculations of electron inelastic mean free paths (IMFPs) for 50–2000 eV electrons in 14 elemental solids (Li, Be, diamond, graphite, Na, K, Sc, Ge, In, Sn, Cs, Gd, Tb, and Dy) and for one solid (Al) using better optical data than in our previous work. The new IMFPs have also been used to test our TPP‐2M equation for estimating IMFPs in these materials. We found surprisingly large root‐mean‐square (RMS) deviations (39.3–71.8%) between IMFPs calculated from TPP‐2M and those calculated here from optical data for diamond, graphite and cesium; previously we had found an average RMS deviation of 10.2% for a group of 27 elemental solids. An analysis showed that the large deviations occurred for relatively small computed values of the parameter β in the TPP‐2M equation (β ～ 0.01 for diamond and graphite) and also for relatively large values of β (β ～ 0.25 for Cs). Although such extreme values of β are unlikely to be encountered for many other materials, the present results indicate an additional limitation in the reliability of the TPP‐2M equation. We also show that the parameter N_v in the TPP‐2M equation should be computed for the rare‐earth elements from the number of valence electrons and the six 5p electrons. Copyright © 2004 John Wiley & Sons, Ltd.     

New developments in theory of fast electron scattering in solids: Applications to microbeam analysis

The study of fast electron interaction with solids in the energy range from 100 eV to several tens of keV is prompted by quickly developing microbeam analysis techniques such as electron probe microanalysis, scanning electron microscopy, electron energy loss spectroscopy and so on. It turned out that for random solids the electron transport problem might be solved on the basis of the generalized radiative field similarity principle. The latter states that the exact differential elastic cross section in the kinetic equation may be replaced by an approximate one provided the conditions of radiative field similarity are fulfilled. Application of the generalized similarity principle to electron scattering in solids has revealed many interesting features of electron transport. Easy to use and effective formulae have been obtained for the angular and energy distribution of electrons leaving a target, total yields of characteristic photons and slow electrons escaping from a sample bombarded by fast primaries, escape probability of Auger electrons as a function of depth etc. The analytical results have been compared with Monte Carlo calculations and experiments in a broad range of electron energies and scattering properties of solids and good agreement has been observed.     

Direct Spectroscopic Evidence of the Mechanism behind the Phase Transition of [2,2]‐Paracyclophane

[2,2]‐Paracyclophane undergoes phase transitions at 45 and 60 K. Based on simultaneous Raman spectroscopy and inelastic neutron scattering experiments (12–70 K), it was shown that a twisting motion of the ethylene bridge perpendicular to the plane of the aromatic rings drives the phase transition. The low‐temperature (<45 K) and high‐temperature (>60 K) conformers only differ by this twisting motion, which freezes out below 45 K and is thermally averaged above 60 K. Between 45 and 60 K, the system gains energy until the phase transition is complete.     

Improved Tougaard background calculation by introduction of fittable parameters for the inelastic electron scattering cross‐section in the peak fit of photoelectron spectra with UNIFIT 2011

The shape of the background in x‐ray photoemission spectra is strongly affected by scattered electrons from inelastic energy loss processes. A polynomial of low order has very often been applied to model the secondary‐electron background, giving satisfying results in some cases. An improved analysis employing the Tougaard background model has been successfully used to characterize the inelastic loss processes. However, the correct usage of the Tougaard background needs a well defined inelastic electron scattering cross‐section function λ(E) · K(E, T) (λ = inelastic mean free path, E = kinetic energy, T = energy loss). This paper presents a four‐parameter loss function λ(E) · K(E, T) = B · T/(C + C′ · T²)² + D · T² with the fitting parameters B, C, C′ and D implemented in the background function allowing the improved estimation of the λ(E) · K(E, T) function for homogenous materials. The fit of the background parameters is carried out parallel to the peak fit. The results will be compared with the parameters recommended by Tougaard. The calculation of inelastic electron scattering cross‐sections of clean surfaces from different materials using UNIFIT 2011 will be demonstrated. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Inelastic electron tunneling spectroscopy by STM of phonons at solid surfaces and interfaces

Inelastic electron tunneling spectroscopy (IETS) combined with scanning tunneling microscopy (STM) allows the acquisition of vibrational signals at surfaces. In STM-IETS, a tunneling electron may excite a vibration, and opens an inelastic channel in parallel with the elastic one, giving rise to a change in conductivity of the STM junction. Until recently, the application of STM-IETS was limited to the localized vibrations of single atoms and molecules adsorbed on surfaces. The theory of the STM-IETS spectrum in such cases has been established. For the collective lattice dynamics, i.e., phonons, however, features of STM-IETS spectrum have not been understood well, though in principle STM-IETS should also be capable of detecting phonons. In this review, we present STM-IETS investigations for surface and interface phonons and provide a theoretical analysis. We take surface phonons on Cu(1?1?0) and interfacial phonons relevant to graphene on SiC substrate as illustrative examples. In the former, we provide a theoretical formalism about the inelastic phonon excitations by tunneling electrons based on the nonequilibrium Green’s function (NEGF) technique applied to a model Hamiltonian constructed in momentum space for both electrons and phonons. In the latter case, we discuss the experimentally observed spatial dependence of the STM-IETS spectrum and link it to local excitations of interfacial phonons based on ab-initio STM-IETS simulation.     

Construction of database of effective energy‐loss functions

Effective energy‐loss functions for Al, Cu, Ag and Au were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra for 1 keV electrons using extended Landau theory. Features of the obtained effective energy‐loss functions are close to those of optical surface energy‐loss functions, revealing the significant contribution of the low energy loss below a few tens of electron‐volts in the REELS spectrum for Cu, Ag and Au. The REELS spectra were reproduced using the newly derived effective energy‐loss functions, leading to the confirmation that this type of database of the effective energy‐loss function is very useful not only for more comprehensive understanding of the measured spectrum of surface electron spectroscopies but also for practical background subtraction in surface electron spectroscopy. Copyright © 2003 John Wiley & Sons, Ltd.