A. Jablonski

Experimental determination of the inelastic mean free path of electrons in solids

Abstract The method based on the elastic peak electron spectroscopy was used to determine the energy dependence of the IMFP for different carbon samples and selected metals. This method provides the values of the IMFP corresponding to the definition of the E-42 Committee of the ASTM. The measurements were made with respect to the aluminium standard. The main features of the method were extensively discussed: the choice of the aluminium standard, quantification of the elastic peak intensity, and the validity of the theoretical model relating the elastic peak intensity to the IMFP. The values of the IMFP were obtained for energies ranging from 270 to 2350 eV. The results are in good agreement with the theoretical data available in the literature. Attention is drawn to the fact that the density of the sample should be known, since this parameter influences greatly the resulting values of the IMFP.

Quantitative surface analysis by Auger and x-ray photoelectron spectroscopy

Abstract Recent developments in quantitative surface analysis by Auger (AES) and x-ray photoelectron (XPS) spectroscopies are reviewed and problems relating to a more accurate quantitative interpretation of AES XPS experimental data are discussed. Special attention is paid to consideration of elementary physical processes involved and influence of multiple scattering effects on signal line intensities. In particular, the major features of core-shell ionization by electron impact, Auger transitions and photoionization are considered qualitatively and rigorous approaches used to calculate the respective transition probabilities are analysed. It is shown that, in amorphous and polycrystalline targets, incoherent scattering of primary and signal Auger and photoelectrons can be described by solving analytically a kinetic equation with appropriate boundary conditions. The analytical results for the angular and energy distribution, the mean escape depth, and the escape probability as a function of depth of origin of signal electrons as well as that for the backscattering factor in AES are in good agreement with the corresponding Mote Carlo simulation data. Methods for inelastic background subtraction, surface composition determination and depth-profile reconstructions by angle-resolved AES XPS are discussed. Examples of novel techniques based on x-ray induced photoemission are considered.

Escape probability of signal photoelectrons from non-crystalline solids: influence of anisotropy of photoemission

Abstract The escape probability of photoelectrons as a function of depth of orgin haa been studied experimentally, analytically and by the Monte Carlo (MS) technique. The depth distribution function (DDF) describing the probability for an electron emitted at a certain depth to leave a surface without being scattered inelastically has been obtained by solving a kinetic equation in the transport approximation. The analytically derived DDF is a universal function of the ratio of the inealstic to the transport mean free paths and the asymmetry parameter. In the directions of minima of the angular distribution, this function is no longer exponential, but it may be essentially nonmonotonic, reaching its maximum value at the depth comparable with the inelastic mean free path. The maximum value of the DDF exceeds its surface value by about 50% for the asymmetry parameter being equal to 2 in the emission directions close to that of X-ray propagation. Under the same conditions, the mean escape depth of electrons may be several times larger than the value predicted by the usual XPS formalism. Such behaviour of the escape probability is explained by elastic scattering of photoelectrons. The solution to the kinetic equation for a uniform target is generalized for a sample with an arbitrary depth profile and depth-dependent elastic and inelastic scattering cross-sections under the condition of the ratio of the inelastic to the transport mean free paths being independent of depth. Analytical formulas for the photoelectron yield from overlayer/substrate structure have been derived and studied in detail. The analytical predictions are compared with the experimental and Monte Carlo simulation data obtained for aluminium oxide/aluminium specimen. A satisfactory agreement is observed between the experimental and theoretical results.

Study on lead additives in modified palladium catalysts

Abstract The characterization of palladium-lead on calcium carbonate catalysts has been carried out. Although widely utilized in laboratory and industrial syntheses, as e.g. the Lindlar catalyst, they are not sufficiently understood. Hydrogen sorption studies by catalysts of this type, their X-ray and XPS characteristics, and the results of kinetic tests as catalysts in the semihydrogenation of acetylene indicated the nature of the metallic component of the Lindlar catalyst as palladium crystallites partially covered with a dispersed lead oxide deposit. The second catalyst characterized was a Pd-Pb alloy supported on CaCO 3 system, and was identified as a Pd 3 Pb intermetallic compound. Its selectivity, higher than that of the commercial Lindlar catalyst, was attributed to the ordered structure of the Pd-Pb alloy.

Evaluation of theoretical models for elastic electron backscattering from surfaces

In recent years, there has been an increasing interest in the elastic electron backscattering phenomenon partly due to its use in experimental determination of inelastic electron mean-free-paths. A critical review is presented of the current status of theoretical models to describe this phenomenon. Particular stress is put on determination of the accuracy and range of validity of various analytical models by comparison to extensive Monte Carlo simulations.

Towards a universal description of elastic scattering effects in X-ray photoelectron spectroscopy

Abstract Expressions predicting photoelectron intensities, within the common formalism of X-ray photoelectron spectroscopy (XPS), are derived on the assumption that the effects of elastic electron scattering are negligible. In recent years, this assumption has been frequently proved to be invalid. In the present work, a general analytical formalism for quantitative XPS analysis is proposed. This formalism accounts properly for both elastic and inelastic interactions of the signal electrons with one-element solids, and is applicable to any experimental configuuration of XPS. The problem of elastic scattering is treated within the framework of the generalized radiative field similarity principle implying the validity of the so-called transport approximation. The dependence of the photoelectron yield on the X-ray incidence angle and the analyser position, derived within the analytical theory, is compared with the results of Monte Carlo simulations. Very good agreement is observed which proves the reliability of the proposed analytical model. The computational procedure, making use of the developed formalism, is described in detail. Extensive tabulation of parameters necessary for these calculations is provided.

Escape probability of electrons from solids. influence of elastic electron scattering

The neglect of elastic electron scattering effects in the formalism of Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) may have substantial influence on results of quantitative applications, in particular for large emission angles. This may lead to considerable errors in concentration depth profiles derived from the angle-resolved AES and XPS techniques where these effects are usually not taken into account. Recently, we proposed to describe the effects of elastic electron scattering on AES and XPS peak intensities by a function CF defined as the ratio of the depth distribution functions derived from models that include and neglect elastic collisions. If the function CF is known, a very simple formalism is obtained. In the present work we have found that an analytical expression resulting from solution of the kinetic Boltzmann equation within the transport approximation can effectively be used to calculate the CF function for any solid, electron energy, and experimental configuration. It has been found that the CF function may differ from unity (which corresponds to negligible elastic scattering) by up to more than one order of magnitude. The accuracy of the analytical formalism has been determined by comparison to an extensive database of the CF values obtained from Monte Carlo simulations of electron transport in solids. The analytical expression was found to give CF values that agree with the Monte Carlo results within a mean percentage deviation ≤10%.

Quantitative XPS — multiline approach

Abstract The algorithm for quantitative XPS analysis developed some years ago has been extended by a new presentation of the fundamental parameters and by using as many lines as possible from each element under investigation. It now gives the opportunity to reduce the influences of statistical (from measurement) and systematic errors (from fundamental parameters) on quantitative analysis results.

Derivation of the electron inelastic mean free path from the elastic peak intensity

Measurements of the elastic peak intensity make possible estimation of the true inelastic mean free path (IMFP). Corresponding experiments are relatively simple and can be performed for any sample. The theoretical models can be used to derive the relation between elastic peak intensity and IMFP for single and multiple elastic scattering. In the present work both models were based on the differential scattering cross-sections calculated within the partial wave expansion method. Examples of calculations of the IMFP using both theoretical models are presented.

Effects of elastic and inelastic electron scattering on quantitative surface analyses by AES and XPS

Abstract A review is given that describes the complications due to elastic and inelastic electron scattering in quantitative surface analyses by Auger-electron spectroscopy and x-ray photoelectron spectroscopy. Four principal topics are addressed. First, the simple formulae for surface analyses are based on a model that ignores elastic scattering. Recent work assessing the effects of elastic scattering is summarized which shows that the simple formulae are valid in certain analytical situations but with an appropriate choice of the parameter describing inelastic scattering. Second, we review measurements of effective attenuation lengths and point out many sources of significant systematic error in these measurements. Third, we describe recent calculations of inelastic mean free paths (IMFPs) in over fifty materials that have been utilized to develop a predictive IMFP formula. Finally, we discuss the complicating effects of inelastic scattering on reliable measurements of AES and XPS intensities.