Deconvolution of core electron energy loss spectra

Different deconvolution methods for removing multiple scattering and instrumental broadening from core loss electron energy loss spectra are compared with special attention to the artefacts they introduce. The Gaussian modifier method, Wiener filter, maximum entropy, and model based methods are described. Their performance is compared on virtual spectra where the true single scattering distribution is known. A test on experimental spectra confirms the good performance of model based deconvolution in comparison to maximum entropy methods and shows the advantage of knowing the estimated error bars from a single spectrum acquisition.     

The quantitation of electron energy loss spectra

An interactive computer program which allows the quantitation of electron energy loss spectra is described. The program performs a standardless analysis using hydrogenic cross-section models. The accuracy of the results obtained for a wide range of experimental conditions is typically within 20% of expected values for well characterized samples. The lack of agreed thin film standards is a limit to the improvement of the cross-section models, but under most practical operating conditions the largest errors arise from instrumental factors such as the effect of finite beam convergence and chromatic errors in the coupling optics.     

Extended fine structure in electron energy loss spectra of MgO crystallites

EXAFS-like features in energy loss spectra (known as EXELFS) were observed by using an imaging energy filter built into an electron microscope. Single crystal micro-platelets of MgO were used as test samples. Extended fine structures were obtained reproducibly in the energy range up to 150 eV beyond the OK, MgK and MgL edges. Comparison with single scattering calculations for the K edges showed a fair agreement, indicating the feasibility of this type of analysis.     

Separation of overlapping core edges in electron energy loss spectra by multiple-least-squares fitting

A multiple-least-squares fitting procedure for quantitating electron energy loss spectra is demonstrated on some strongly overlapping core edges. The method, first applied by Shuman and Somlyo [Ultramicroscopy 21 (1987) 23], takes into account plural inelastic scattering and can be applied under conditions of non-uniform sample thickness where Fourier deconvolution techniques are invalid. By using appropriate reference spectra generated from pure compounds, quantitation of potassium and calcium (L23 edges) is possible in the presence of carbon (K edge), and sulfur in the presence of phosphorus (L23 edges). Some of the advantages and limitations of the multiple-least-squares approach are discussed.     

Experimental and theoretical determination of low electron energy loss spectra of Ag and Ru

We show the experimental and calculated q-dependent low energy loss electron energy loss spectrum of Ru and Ag. The spectra were calculated within the time-dependent density-functional theory including local-field effects. For Ag, the momentum transfer was parallel to the (110) direction. For Ru the three main directions (010), (110) and (001) were investigated. The agreement between theory and experiment is very good for Ag and for momentum transfers parallel to the (001) direction of Ru. For momentum transfers parallel to the in-plane directions (110) and (010) the agreement for Ru is not satisfactory, which could be attributed to relativistic effects or to strong localization of the 4d states of Ru.     

Optimizing the window size for multiple least squares fitting of electron energy loss spectra

The purpose of this study was to determine the effect of the fitting window size used with the linear least squares fit method when quantitating trace elements with electron energy loss spectroscopy. Theory and computer simulation with a simple model of two 'signals' show that, when the background underlying the signal is slowly varying and the signal is localized, there exists a minimum window optimal for fitting raw spectra. The width of the minimum fitting window can be determined directly from the reference standard spectrum of the signal alone and the estimates of signal and background are related. Use of narrower than the minimum window will increase the fitting uncertainty of the signal and yield less reliable results. More complicated experimental spectra must be fitted to more than two standards and a simple analytical expression of the minimum fitting window cannot be derived, but can be determined empirically. Our study shows that the empirical value obtained from experimental spectra is only slightly larger than the theoretical value derived from the simple model, indicating that this conclusion is still valid. When fitting difference spectra with a slowly varying background, the estimates of signal and background are independent and windows wider than the size of the signal will yield the same fitting uncertainty. In the presence of a non-slowly varying background, common in difference spectra, the minimum window size depends on the fine structure of the signal and the background.     

Correcting electron energy loss spectra for artefacts introduced by a serial data collection system

The large range of signal intensities found in electron energy loss spectroscopy causes difficulty when trying to record the spectrum accurately. This paper describes a system which records the data serially as simultaneous pulse and analogue signals without the need for changing the gain of the photomultiplier tube during the scan. The effect of deadtime and dark current on the recorded signals is discussed and procedures for correcting the data for these effects are described. At the same time, it is possible to correct the data for stray signal in the spectrometer and non-linearity in the photomultiplier head-amplifier at high output signal levels. In addition to its use for electron energy loss spectroscopy, this system is ideal for recording other signals which have a wide dynamic range and change rapidly from point to point, e.g. energy filtered diffraction patterns from thin amorphous films.     

Independent component analysis: a new possibility for analysing series of electron energy loss spectra

A complementary approach is proposed for analysing series of electron energy-loss spectra that can be recorded with the spectrum-line technique, across an interface for instance. This approach, called blind source separation (BSS) or independent component analysis (ICA), complements two existing methods: the spatial difference approach and multivariate statistical analysis. The principle of the technique is presented and illustrations are given through one simulated example and one real example.     

An electron energy loss spectral library

Although the general characteristics of an energy loss spectrum are reasonably well known, tables of atomic energy levels are not always sufficient to uniquely identify all features in a given spectrum. Systematic variation on an element-to-element basis across the periodic table can be substantial and has not yet been documented in a single reference source. A wide range of specimens has been studied using EELS, resulting in a fairly extensive data base, which is now being prepared for general distribution to the scientific community in the form of a spectral library.     

Current applications of electron energy loss spectroscopy

It is undoubtedly true that the advent of efficient energy loss spectrometers for transmission microscopes over the last few years has been of considerable assistance, at least qualitatively, for the analysis of light elements and, to a more limited extent, in structure interpretation. Rather frustratingly, given the potentially better spatial resolution of EELS over EDX, realistic quantitative analysis remains difficult, and similarly - while fascinating effects are seen in, for example, the crystallographic orientation dependence of the signal - these are currently only broadly interpretable in relation to those observed in EDX. The reasons for this are discussed, as are the relative advantages of large and small collection angles for different types of experiment.     

Some effects of electron channeling on electron energy loss spectroscopy

As an electron beam (of order 100 keV) travels through a crystalline solid it can be channeled down a zone axis of the crystal to form a channeling peak centered on the atomic columns. The channeling peak can be similar in size to the outer atomic orbitals. Electron energy loss spectroscopy (EELS) measures the losses that the electron experiences as it passes through the solid yielding information about the unoccupied density of states in the solid. The interaction matrix element for this process typically produces dipole selection rules for small angle scattering. In this paper, a theoretical calculation of the EELS cross section in the presence of strong channeling is performed for the silicon L23 edge. The presence of channeling is found to alter both the intensity and selection rules for this EELS signal as a function of depth in the solid. At some depths in the specimen small but significant non-dipole transition components can be produced, which may influence measurements of the density of states in solids.     

Image-EELS: Simultaneous recording of multiple electron energy-loss spectra from series of electron spectroscopic images

A new approach for element microanalysis with energy-filtering transmission electron microscopy (EFTEM) is presented which was accomplished with the CEM 902 electron microscope (Zeiss, Germany). This method is called Image-EELS, because it is a synthesis of electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Series of energy-filtered images at increasing energy losses are recorded from one area with a TV camera. In a second step the intensity of selected regions in the image stack is measured with an image analysis system and plotted as a function of the energy loss. Thus many spectra from different objects can be calculated from one image series and compared with each other. The spatial resolution of EELS is considerably enhanced, the noise is decreased because many pixels from irregular objects are integrated, and the information from ESI can be analysed as a function of the energy loss.     

Spectroscopic imaging of electron energy loss spectra using ab initio data and function field visualization

We have devised a technique for spectral imaging using accurate ab initio electron energy loss near edge structure (ELNES) data and function field visualization. The technique is initially applied to a planar defect model in Si with different ring structures and no broken bonds where experimental probes are severely limited. The same model with B doping is also considered. It is shown that specific deviations in different energy ranges of the ELNES spectra are correlated with different structural components of the models.     

Quantitative electron energy loss spectroscopy in biology

The potential for applying electron energy loss spectroscopy (EELS) in biology is assessed. Some recent developments in instrumentation, spectrometer design, parallel detection and elemental mapping are discussed. Quantitation is demonstrated by means of the spectrum from DNA which gives an elemental ratio for N:P close to the expected value. A range of biologically important elements that can be usefully analyzed by EELS is tabulated and some possible applications for each are indicated. Detection limits and the effects of radiation damage are illustrated by spectra from the protein, insulin, and from the fluorinated amino-acid, histidine. Calcium detectability under optimum conditions may be as low as 1 mmol/kg dry weight. The application of EELS to analysis of cryosectioned adrenomedullary (chromaffin) cells is described in order to help determine the composition of the secretory granule. Water content can be determined from the amount of inelastic scattering as measured by the low-loss spectrum. The nitrogen/phosphorus ratio can be measured to provide information about the relative concentrations of ATP, chromogranin, and catecholamines. Quantitative EELS elemental maps are obtained in the STEM mode from chromaffin cells in order to measure the distribution of light elements.     

Elemental imaging by electron energy loss microscopy

Electron energy loss images obtained by parallel filtration at selected energies are used to derive qualitative and quantitative distributions of elements in specimens as diverse as low alloy steel, cartilage, and ribosomal particles. Partial ionization cross-sections for electron scattering from phosphorus L-shells are calculated.