Surface and Interface Analysis

A compilation is presented of all published measurements of electron inelastic mean free path lengths in solids for energies in the range 0–10 000 eV above the Fermi level. For analysis, the materials are grouped under one of the headings: element, inorganic compound, organic compound and adsorbed gas, with the path lengths each time expressed in nanometers, monolayers and milligrams per square metre. The path lengths are vary high at low energies, fall to 0.1–0.8 nm for energies in the range 30–100 eV and then rise again as the energy increases further. For elements and inorganic compounds the scatter about a ‘universal curve’ is least when the path lengths are expressed in monolayers, λ_m. Analysis of the inter‐element and inter‐compound effects shows that λ_m is related to atom size and the most accuratae relations are λ_m = 538E⁻²+0.41(aE)^1/2 for elements and λ_m=2170E⁻²+0.72(aE)^1/2 for inorganic compounds, where a is the monolayer thickness (nm) and E is the electron energy above the Fermi level in eV. For organic compounds λ_d=49E⁻²+0.11E^1/2 mgm⁻². Published general theoretical predictions for λ, valid above 150 eV, do not show as good correlations with the experimental data as the above relations.

Ferrous (Fe²⁺) and ferric (Fe³⁺) compounds were investigated by XPS to determine the usefulness of calculated multiplet peaks to fit high‐resolution iron 2p_3/2 spectra from high‐spin compounds. The multiplets were found to fit most spectra well, particularly when contributions attributed to surface peaks and shake‐up satellites were included. This information was useful for fitting of the complex Fe 2p_3/2 spectra for Fe₃O₄ where both Fe²⁺ and Fe³⁺ species are present. It was found that as the ionic bond character of the iron —ligand bond increased, the binding energy associated with either the ferrous or ferric 2p_3/2 photoelectron peak also increased. This was determined to be due to the decrease in shielding of the iron cation by the more increasingly electronegative ligands. It was also observed that the difference in energy between a high‐spin iron 2p_3/2 peak and its corresponding shake‐up satellite peak increased as the electronegativity of the ligand increased. The extrinsic loss spectra for ion oxides are also reported; these are as characteristic of each species as are the photoelectron peaks. Copyright © 2004 John Wiley & Sons, Ltd.

We report calculations of electron inelastic mean free paths (IMFPs) of 50–2000 eV electrons for a group of 14 organic compounds: 26‐n‐paraffin, adenine, β‐carotene, bovine plasma albumin, deoxyribonucleic acid, diphenylhexatriene, guanine, kapton, polyacetylene, poly(butene‐1‐sulfone), polyethylene, polymethylmethacrylate, polystyrene and poly(2‐vinylpyridine). The computed IMFPs for these compounds showed greater similarities in magnitude and in the dependences on electron energy than was found in our previous calculations for groups of elements and inorganic compounds (Papers II and III in this series). Comparison of the IMFPs for the organic compounds with values obtained from our predictive IMFP formula TPP‐2 showed systematic differences of ∼40%. These differences are due to the extrapolation of TPP‐2 from the regime of mainly high‐density elements (from which it had been developed and tested) to the low‐density materials such as the organic compounds. We analyzed the IMFP data for the groups of elements and organic compounds together and derived a modified empirical expression for one of the parameters in our predictive IMFP equation. The modified equation, denoted TPP‐2M, is believed to be satisfactory for estimating IMFPs in elements, inorganic compounds and organic compounds.

Quantitative information from electron spectroscopy for chemical analysis requires the use of suitable atomic sensitivity factors. An empirical set has been developed, based upon data from 135 compounds of 62 elements. Data upon which the factors are based are intensity ratios of spectral lines with F1s as a primary standard, value unity, and K2p_3/2 as a secondary standard. The data were obtained on two instruments, the Physical Electronics 550 and the Varian IEE‐15, two instruments that use electron retardation for scanning, with constant pass energy. The agreement in data from the two instruments on the same compounds is good. How closely the data can apply to instruments with input lens systems is not known. Calculated cross‐section data plotted against binding energy on a log‐log plot provide curves composed of simple linear segments for the strong lines: 1s, 2p_3/2, 3d_5/2 and 4f_7/2. Similarly, the plots for the secondary lines, 2s, 3p_3/2, 4d_5/2 and 5d_5/2, are shown to be composed of linear segments. Theoretical sensitivity factors relative to F1s should fall on similar curves, with minor correction for the combined energy dependence of instrumental transmission and mean free path. Experimental intensity ratios relative to F1s were plotted similarly, and best fit curves were calculated using the shapes of the theoretical curves as a guide. The intercepts of these best fit curves with appropriate binding energies provide sensitivity factors for the strong lines and the secondary lines for all of the elements except the rare earths and the first series of transition metals. For these elements the sensitivity factors are lower than expected, and variable, because of multi‐electron processes that vary with chemical state. From the data it can be shown that many of the commonly‐accepted calculated cross‐section data must be significantly in error—as much as 40% in some cases for the strong lines, and far more than that for some of the secondary lines.

Quantitative chemical state X‐ray photoelectron spectroscopic analysis of mixed nickel metal, oxide, hydroxide and oxyhydroxide systems is challenging due to the complexity of the Ni 2p peak shapes resulting from multiplet splitting, shake‐up and plasmon loss structures. Quantification of mixed nickel chemical states and the qualitative determination of low concentrations of Ni(III) species are demonstrated via an approach based on standard spectra from quality reference samples (Ni, NiO, Ni(OH)₂, NiOOH), subtraction of these spectra, and data analysis that integrates information from the Ni 2p spectrum and the O 1s spectra. Quantification of a commercial nickel powder and a thin nickel oxide film grown at 1‐Torr O₂ and 300 °C for 20 min is demonstrated. The effect of uncertain relative sensitivity factors (e.g. Ni 2.67 ± 0.54) is discussed, as is the depth of measurement for thin film analysis based on calculated inelastic mean free paths. Copyright © 2009 John Wiley & Sons, Ltd.

We report calculations of electron inelastic mean free paths (IMFPs) for 50–2000 eV electrons in a group of 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Ta, W, Re, Os, Ir, Pt, Au and Bi). This work extends our previous calculations (Surf. Interface Anal. 11, 57 (1988)) for the 200–2000 eV range. Substantial variations were found in the shapes of the IMFP versus energy curves from element to element over the 50–2000 eV range and we attribute these variations to the different inelastic scattering properties of each material. Our calculated IMFPs wee fitted to a modified form of the Bethe equation for inelastic electron scattering in matter; this equation has four parameters. These four parameters could be empirically related to several material parameters for our group of elements (atomic weight, bulk density and number of valence electron per atom). IMFPs and those initially calculated was 13%. The modified Bethe equation and our expressions for the four parameters can therefore be used to estimate IMFPs in other materials. The uncertainties in the algorithm used for our IMFP calculation are difficult to estimate but are believed to be largely systematic. Since the same algorithm has been used for calculating IMFPs, our predictive IMFP formula is considered to be particularly useful for predicting the IMFP dependence on energy in the 50–2000 eV range and the material dependence for a given energy.

This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d_5/2 and Ce 3d_3/2 levels in Ce⁴⁺ and Ce³⁺ states. The well‐resolved components of Ce 3d_5/2 and Ce 3d_3/2 spin‐orbit components, due to various final states (4f⁰, 4f¹, 4f² configurations), were determined on 3d XPS spectra from commercial powders (CeO₂, CePO₄).

These results were used to study the 3d spin‐orbit component of mixed cerium‐titanium oxide.

This compound was prepared by co‐melting commercial powders of CeO₂ and TiO₂ at 1800 K under air using a solar furnace with a flux density of 16 MW.m⁻² at the focal point of the parabolic concentrator. The mixed oxide Ce₂Ti₂O₇ was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil.

The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright © 2008 John Wiley & Sons, Ltd.

We present new calculations of electron inelastic mean free paths (IMFPs) for 200–2000 eV electrons in 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au and Bi) and four compounds (LiF, SiO₂, ZnS and Al₂O₃). These calculations are based on an algorithm due to Penn which makes use of experimental optical data (to represent the dependence of the inelastic scattering probability on energy loss) and the theoretical Lindhard dielectric function (to represent the dependence of the scattering probability on momentum transfer). Our calculated IMFPs were fitted to the Bethe equation for inelastic electron scattering in matter; the two parameters in the Bethe equation were then empirically related to several material constants. The resulting general IMFP formula is believed to be useful for predicting the IMFP dependence on electron energy for a given material and the material‐dependence for a given energy. The new formula also appears to be a reasonable but more approximate guide to electron attenuation lengths.

X‐ray photoelectron spectroscopy (XPS) utilising monochromatic Al Kα radiation has been employed to study metallic ruthenium and the catalytically and technologically important ruthenium compounds RuO₂, RuCl₃, Ru(NO)(NO₃)₃ and Ru(AcAc)₃. The results improve on the accuracy of already published Ru(3d) binding energies, expand known Ru(3p) binding energies and also report spin‐orbit splitting for the core levels. For RuO₂, the difference between anhydrous and hydrated samples is explored, and the effect on curve fitting such spectra is discussed. Analysis of RuCl₃ has allowed, for the first time, the positive identification of Ru(OH)₃ by XPS. Copyright © 2015 John Wiley & Sons, Ltd.

X‐ray photoelectron spectroscopy of cerium oxides is discussed. The well‐resolved 3d_3/2 (5d 6s)⁰ 4f⁰ 2p⁶ peak at 916.70 eV cannot be used for calculating the amount of reduction because the correlation between its intensity and the concentration of Ce(IV) and CE(III) species is not liner. We have therefore develped a complete anlaysis of the whole spectrum. The deconvolution procedure is explained in detail and the method is applied to two examples: a CeO₂/Al₂O₃ sample and a Pd/CeO₂ catalyst reduced under hydrogen.