Primary Excitation Spectra Research Articles

XPS quantification with universal inelastic electron scattering cross section including intrinsic excitations

In X‐ray excited photoelectron emission (XPS), the shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface. It is also influenced by intrinsic excitations due to the sudden creation of the static core hole. In order to approximately determine the primary excitation spectrum of the considered transition corrected for both extrinsic and intrinsic excitations, we developed in a previous work [E. Gnacadja, N. Pauly, S. Tougaard, Surf. Interface Anal. 52 (2020) 413] a universal analytical expression for the energy loss cross section including extrinsic and intrinsic excitations. We apply the present universal cross section to test to what extent these primary excitations spectra can be used for XPS quantification based on peak area ratios. The procedure is applied to the study of three sets of polycrystalline alloys (Cu0.75Au0.25, Cu0.50Au0.50, and Cu0.25Au0.75) and to three metal oxides (HfO2, ZrO2, and Cu2O). We show that although the individual peaks are very different from those obtained with the classical universal Tougaard cross section, the determined quantitative compositions are equivalent (but not better). This implies that the relative contribution from intrinsic excitations is roughly the same for all peaks for a given sample and they therefore cancel out when peak area ratios are considered.

Universal inelastic electron scattering cross‐section including extrinsic and intrinsic excitations in XPS

AbstractThe high importance of X‐ray photoelectron spectroscopy (XPS) in surface analysis is well established. In XPS, the shape of the measured peaks is affected by two classes of energy loss: extrinsic losses because of the transport of photoelectrons in the matter and intrinsic losses because of the sudden creation of the static core hole. In order to perform a quantitative, comprehensive determination of the zero‐energy loss spectrum, a systematic and physically meaningful background subtraction method must be used. In this paper, we propose a universal analytical expression to model the energy loss cross section of the emitted photoelectrons for transition metals and their oxides. The proposed expression is a generalization of the well‐known Tougaard's universal inelastic scattering cross section to also account for the intrinsic losses. We demonstrate the use of this to determine the primary excitation spectra of several transition metals and their oxides, and we compare the results with a more accurate calculation based on the dielectric response model for XPS.

XPS primary excitation spectra of Zn 2p, Fe 2p, and Ce 3d from ZnO, α‐Fe2O3, and CeO2

Metal oxides are important for current development in nanotechnology. X‐ray photoelectron spectroscopy(XPS) is a widely used technique to study the oxidation states of metals, and a basic understanding of the photoexcitation process is important to obtain the full information from XPS. We have studied core level excitations of Zn 2p, Fe 2p, and Ce 3d photoelectron emissions from ZnO, α‐Fe2O3, and CeO2. Using an effective energy‐differential XPS inelastic‐scattering cross section evaluated within the semiclassical dielectric response model for XPS, we analysed the experimental spectra to determine the corresponding primary excitation spectra, ie, the initial excitation processes. We find that simple emission (Zn 2p) as well as complex multiplet photoemission spectra (Fe 2p and Ce 3d) can be quantitatively analysed with our procedure. Moreover, for α‐Fe2O3, it is possible to use the software package CTM4XAS (Charge Transfer Multiplet program for X‐ray Absorption Spectroscopy) to calculate its primary excitation spectrum within a quantum mechanical model, and it was found to be in good agreement with the spectrum determined by analysis of the experiment.

Quantitative analysis of Yb 4d photoelectron spectrum of metallic Yb

The measured Yb 4d3/2 intensity is larger than the Yb 4d5/2 in X‐ray photoelectron (XPS) emission of metallic Yb, which is unexpected. The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To quantitatively extract from experimental XPS the primary excitation spectrum (ie, the initial excitation process) of the considered transition, these effects must be included within the theoretical description. The combined effect of both extrinsic and intrinsic excitations can be described by an effective energy‐differential inelastic electron scattering cross section for XPS evaluated by a dielectric response model with the dielectric function as only input. Then, using this cross section, a direct evaluation of the primary excitation spectrum is performed by standard peak shape analysis for thick homogeneous samples. We use this approach in the present paper to determine the Yb 4d photoemission spectrum for metallic Yb. We show that the unexpected larger intensity of Yb 4d3/2 compared to 4d5/2 can be fully accounted for by our model and that the total spectrum consists of a sum of symmetric primary excitation peaks.

An almost knowledge-free approach to XPS intensity evaluation where use of atomic photoemission cross sections suffices for yielding material-specific inelastic background

A general procedure for peak intensity evaluation on the inelastic background of X-ray photoelectron spectra (XPS) has been developed, which is made up of tasks in three nested levels, level I, II and III. The task in level I is a simple background optimization for the case when the inelastic mean free path (IMFP, denoted λ(E)) and other constraints are given in advance [M. Jo, Surf. Interface Anal., 2003, 35, 729–737]. In order to find appropriate constraints, a batch job, which is a series of single jobs with systematically varied constraints, is constructed and iteratively solved in level II by improving the constraints from the viewpoint of how successfully the background is subtracted. It is found that, by the task in level II, for polycrystal Cu metal, the spectrum after inelastic background subtraction still contains a baseline component proportional to E−k, where E (>50eV) is the electron kinetic energy and the exponent k around 2.4. This baseline is attributed to the high-energy tail of primary excitation spectrum of true secondary electron emission. The relative core intensities thus determined are in good agreement with the Scofield’s photoexcitation cross sections. The relation of core and Auger peak intensities is consistent as demonstrated by the fact that the sum of Cu 2p and 2s equals that of all Cu LMM peaks. The level III calculation is designed for the general case where λ is not available before the analysis. The relative energy dependence of λ except for the absolute value can be estimated by iterating the level II tasks with updated λ(E), based on the assumption that the peak intensities should be proportional to Scofield’s values. The relative deviation of thus determined λ(E) in level III from those by the well-known calculation (Tanuma, Powell, and Penn, TPP) is less than 1.7% between E=600 and 2000eV if their values at 800eV are assumed to coincide. The normalized peak shapes and relative intensities after background subtraction in this level are also very similar to those obtained in level II starting with λ(E) by TPP that is currently known to be reliable. These findings indicate that an almost knowledge-free analysis, which is free from the detail of the solid’s properties such as chemical composition and nevertheless retains the material-dependent results, became possible for the first time, as long as one keeps away from the discussion of absolute intensity.

Quantitative analysis of satellite structures in XPS spectra of gold and silver

Identification of specific chemical states and local electronic environments at surfaces by X-ray photoelectron spectroscopy (XPS) is often difficult because it is not straightforward to quantitatively interpret the shape and intensity of shake-up structures that originate from the photoexcitation process. Indeed the shape and intensity of measured XPS structures are strongly affected by both extrinsic excitations due to electron transport out of the surface and intrinsic excitations induced by the sudden creation of the static core hole. These processes must be taken into account to quantitatively extract, from experimental XPS, the primary excitation spectrum of the considered transition which includes all effects that are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. It was previously shown [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17] that both extrinsic and intrinsic excitations could be included in an effective energy-differential inelastic electron scattering cross section for XPS which is then convoluted with the primary excitation spectrum to model the full XPS spectrum. This method can thus be applied to determine the primary excitation spectrum from any XPS spectrum. We use this approach in the present paper to determine the Au 4f and Ag 3d photoemission spectra from pure metals. We observe that characteristic energy loss features of the XPS spectra are not only due to photoelectron energy losses. We thus prove the existence of a double shake-up process characterized by a 4d→5s/5p transition for Ag and a 5d→6s/6p transition for Au. We finally accurately quantify the energy position and intensity of these shake-up peaks.

Quantitative analysis of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix

In X-ray excited photoelectron emission (XPS), besides the initial excitation process, the shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To make an accurate quantitative interpretation of features observed in XPS, these effects must be included in the theoretical description of the emitted photoelectron spectra. It was previously shown [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17] that these three effects can be calculated by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS) in terms of effective energy-differential inelastic electron scattering cross-sections. The only input needed to calculate these cross-sections is the energy loss function of the media which is determined from analysis of Reflection Electron Energy Loss Spectra (REELS). The full XPS spectrum is then modeled by convoluting this energy loss cross-section with the primary excitation spectrum that accounts for all effects which are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. In this paper we apply the previously presented procedure to the study of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix (Ni:SiO2), samples being prepared by reactive magnetron sputtering at room temperature. We observe a significant difference between the corresponding Ni 2p primary excitation spectra. The procedure allows quantifying the relative intensity of the c3d9L, c3d10L2, and c3d8 final states contributing to the Ni 2p photoemission spectra of the Ni2+ species in the oxide matrices. Especially, the intensity ratio in NiO between the non-local and local contributions to the 3d9L configuration is determined to be 2.5. Moreover the relative intensity ratio of the c3d9L/c3d10L2/c3d8 configurations is found to be 1.0/0.83/0.11 for both the NiO and Ni:SiO2 samples.

Primary excitation spectra in XPS and AES of Cu, CuO: Relative importance of surface and core hole effects

Quantitative interpretation of structures observed in XPS and AES requires models to correct for various physical processes involved. Besides the initial excitation process in XPS and AES, the measured spectrum is affected by three additional effects: the corehole(s), transport to the surface region and passage through the surface and vacuum regions. These three effects can be calculated by the QUEELS-XPS software (Quantitative analysis of Electron Energy Losses at Surfaces) in terms of energy-differential inelastic electron scattering cross sections. From this and the QUASES software (Quantitative Analysis of Surfaces by Electron Spectroscopy), background contributions and primary excitation spectra are obtained for various transitions (Cu 2p from Cu or CuO and Cu L3M23M23) and we investigate the separate effect of bulk, surface, and core hole(s) excitations. We show that the shape of the XPS and AES primary spectra and background contributions are modified slightly by surface effects and very strongly by core hole(s) effects. For metals, the intrinsic excitations give rise to a prominent spike in the background close to the XPS-peak energy. This spike will be much reduced for wide band gap insulators. Moreover our method gives an easy procedure to obtain the true primary excitation spectra for XPS and AES.

LMM Auger primary excitation spectra of copper

The shape and intensity of measured Auger peaks are strongly affected by extrinsic excitations due to electron transport out of the surface and to intrinsic excitations induced by the sudden creation of the two static core holes. Following a method developed for XPS in a previous work [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17], we have calculated the effective energy-differential inelastic electron scattering cross-sections, including the effects of the surface and of the two core holes, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The Auger spectra are then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial Auger process, i.e. L–S coupling and vacancy satellite effects. The shape of this primary excitation spectrum is fitted to get close agreement between the theoretical and the experimental spectra obtained from X-ray excited Auger electron spectroscopy (XAES). We have performed these calculations of XAES spectra for various LMM Auger transitions of pure Cu (L3M45M45, L3M23M45, L3M23M23 and L2M45M45 transitions). We compare the resulting primary excitation spectra with theoretical results published in the literature and obtain reasonable quantitative agreement. In particular, we extract from experimental spectra quantitative intensities due to Coster–Kronig, shake-off and shake-up processes relative to the intensity from the “normal” Auger process.

Determination of the Cu 2p primary excitation spectra for Cu, Cu2O and CuO

The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. These effects must be included in the theoretical description of the emitted photoelectron spectra. We have calculated the effective energy-differential inelastic electron scattering cross section for XPS, including both surface and core hole effects, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The full XPS spectrum is then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. The shape of this primary excitation spectrum is determined by requiring close agreement between the resulting theoretical spectrum and the experimental XPS spectrum. These calculations were performed for Cu 2p peaks of Cu, Cu2O, and CuO. For CuO, we compare the obtained primary excitation spectra with first principle calculations performed with the CTM4XAS software (Charge Transfer Multiplet program for X-ray Absorption Spectroscopy) for the corresponding emissions and we find good quantitative agreement.

Abstract: P931 CONSUMPTION OF DIETS WITH DIFFERENT TYPE OF FAT INFLUENCES TRIACYLGLYCEROLS-RICH LIPOPROTEINS PARTICLE NUMBER AND SIZE DURING THE POSTPRANDIAL STATE

The shape and intensity of measured Auger peaks are strongly affected by extrinsic excitations due to electron transport out of the surface and to intrinsic excitations induced by the sudden creation of the two static core holes. Following a method developed for XPS in a previous work [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17], we have calculated the effective energy-differential inelastic electron scattering cross-sections, including the effects of the surface and of the two core holes, within the dielectric response theory by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS). The Auger spectra are then modeled by convoluting this energy loss cross section with the primary excitation spectrum that accounts for all effects which are part of the initial Auger process, i.e. L–S coupling and vacancy satellite effects. The shape of this primary excitation spectrum is fitted to get close agreement between the theoretical and the experimental spectra obtained from X-ray excited Auger electron spectroscopy (XAES). We have performed these calculations of XAES spectra for various LMM Auger transitions of pure Cu (L3M45M45, L3M23M45, L3M23M23 and L2M45M45 transitions). We compare the resulting primary excitation spectra with theoretical results published in the literature and obtain reasonable quantitative agreement. In particular, we extract from experimental spectra quantitative intensities due to Coster–Kronig, shake-off and shake-up processes relative to the intensity from the “normal” Auger process.

X-ray Fluorescence Analysis of Flux-Cored Wire Burden

Interelement effects were estimated for the burden components of the PP-AN4 flux-cored wire, and the predominance of absorption effects was demonstrated. Reasons for the variation of the effective wavelengths in the primary excitation spectrum were revealed for some analytes. An iteration Δμ-correction method was proposed for calculating the weight portions of elements. This method is based on the regression analysis of the absorption parameters of test samples; it takes into account the variation of the effective wavelength of the primary radiation. The convergence of the iteration procedure was shown in the calculations of the weight portions of burden elements under real conditions. A method was proposed for the preparation of burden samples for analysis taking into account the peculiar features of the technological process. Metrological certification was performed in accordance with the industry-specific regulations. The results of X-ray fluorescence analysis of the burden were used for the quality control of the produced flux-cored wire.

Fundamental quantification procedure for total reflection X-ray fluorescence spectra analysis and elements determination

A method for the determination of the concentrations of elements in particulate-like samples measured in total reflection geometry is proposed. In the proposed method the fundamental parameters are utilized for calculating the sensitivities of elements and an internal standard is used to account for the unknown mass per unit area of a sample and geometrical constant of the spectrometer. The modification of the primary excitation spectrum on its way to a sample has been taken into consideration. The concentrations of the elements to be determined are calculated simultaneously with the spectra deconvolution procedure. In the process of quantitative analysis the intensities of all X-ray peaks corresponding to K and L-series lines present in the analyzed spectrum are taken into account. © 1997 Elsevier Science B.V.

Background subtraction from transition metal 2p XPS by deconvolution using ligand atom XPS: study on first transition metal cyanide complexes

The primary excitation spectra of the first transition metal series 2p X-ray photoelectron spectra for the metal cyanide ions are obtained by a deconvolution method. As the CN− ions coordinate to the transition metal, N1s spectra are used as a response function for the deconvolution using a Fourier transform method. The obtained spectra have flat zero level backgrounds, whereas the universal curve of the Tougaard method cannot remove the full contribution of energy loss. Among some background subtraction methods, the intensity ratios of 2p32 to 2p12 calculated by the present method are closest to the theoretical photoionization cross section ratios.

Role of Surface Effects in the Inelastic Background of X-ray Photoelectron Spectroscopy

We have derived a new deconvolution formula to obtain the original no-loss XPS spectra, the so-called source function, by taking surface effects into account. With this formula the primary XPS spectra of Au 4d and Au 4f are carried out from the experimental data. The primary excitation spectra are compared to the results derived by Tougaard's method in which surface effects were neglected. The present result is markedly different from Tougaard's result, which consists of a tail extending ∼50 eV below the peak. The result reveals that the influence of surface effects on the background removal of the spectra is considerably significant for the energy range ∼50 eV below the peak energy. It is also found that the large tail in Tougaard's results can be essentially removed when surface effects are considered.

Generalization of the tougaard method for inelastic‐background estimation in electron spectroscopy: Incorporation of a depth‐dependent inelastic mean free path

AbstractA generalization of the Tougaard method for subtraction of the background of inelastically scattered electrons from an electron‐energy spectrum is presented. Firstly, a depth dependence of the inelastic mean free path is introduced, which enables the analysis of electron‐energy spectra from elements in inhomogeneous samples. The incorporation of the depth dependence of the inelastic mean free path also provides an equation for the calculation of peak intensities of electron‐energy spectra from elements in inhomogeneous samples and the possibility of estimating the effect of elastic scattering of signal electrons. Secondly, it is shown that the Tougaard method allows the direct calculation of the background of inelastically scattered electrons from a known primary excitation spectrum for an arbitrary in‐depth distribution of electron emitters. The validity of these generalizations is demon‐strated for the case of an XPS model spectrum originating from a substrate covered with an overlayer, by comparing the results obtained in the present study with those reported previously.

Deexcitation of hollow atoms: autoionization of multiply excited N, O and Ne atoms at a Pt(110) surface

The K-Auger electron emission during the interaction of 60 keV N 6+, 70 keV O 7+ and 90 keV Ne 9+ with a Pt(110) surface occurs below the target surface for glancing angles in the range 10–90°. Therefore the projectile K-Auger electron spectra are distorted by solid state effects. These effects can be removed from the measured spectra by using a procedure originally developped for photoelectron spectroscopy. The resulting primary excitation spectra are compared with results of atomic structure calculations. The analysis reveals that at the instant of K-Auger electron emission on the average five electrons are present in the L-shells of the hydrogen-like N, O and Ne projectiles. Furthermore the ratio of the KLL- and KLM-Auger electron emission yields is determined. It is 3.4, 3.7 and 4.6 in case of the N 6+, O 7+ and Ne 9+ projectile, respectively.

Application of Tougaard background subtraction to Langmuir-Blodgett films

The inelastic background subtraction algorithm proposed by Tougaard is applied for the first time to the X-ray photoemission spectra of metal salts of arachidic acid Langmuir-Blodgett films. The background deconvolution procedure has been done using the inelastic scattering cross-section determined from reflection electron energy loss spectroscopy and using a “universal” inelastic scattering cross-section. Both approaches give physically meaningful and nearly identical primary excitation spectra. The range of validity of the deconvolution formulae is investigated by varying the electron emission angle. Possible reasons for the applicability of the Tougaard algorithm to materials containing molecules with dimensions of the order of the inelastic scattering length, as are obtained for the Langmuir-Blodgett films in this study, are discussed.

Inelastic background signal in x-ray photoelectron spectroscopy.

An inversion formula, derived previously by Tougaard and the author to extract a primary excitation spectrum from the measured photoelectron spectrum of a semi-infinite solid, has been generalized to cover the entire energy range from the highest spectral energy to zero. In the absence of elastic electron scattering, the original expression retains its form, but the full energy dependence of the mean free path and the differential energy-loss cross section need to be incorporated. Elastic scattering can be included, either qualitatively within an infinite-medium model, or rigorously on the basis of an invariant-embedding approach. Apart from measurable quantities, the elastic-scattering correction contains an integral over the primary excitation rate, i.e., the quantity looked for. Therefore, iterative solution may be required in the presence of pronounced elastic scattering.

Quantitative analysis of the inelastic background in surface electron spectroscopy

AbstractRecent progress, in the quantitative analysis of the distortion of surface electron spectra, resulting from inelastic electron scattering, is reviewed.A physical model for the problem is formulated and based on this, basic deconvolution formulae are found. In the formulae, detailed information on the differential inelastic electron scattering cross‐section K is essential. Methods to determine,K are studied and developed. One method is based on a dielectric response calculation. Furthermore, it is found that for the noble and transition metals, K is sufficiently similar that a simple ‘universal’ cross‐section can be defined. In addition, methods for experimental determination of cross‐sections are found and in particular a formula is derived which allows the extraction of direct quantitative cross‐section information through a simple analysis of REELS spectra.The importance of proper background correction is investigated for model electron spectra. Experimental XPS spectra are also studied and it is found that for both the simple and the noble metals, the primary excitation spectra extend 2–3 inelastic scattering events, i.e. ∼50 eV, below the peak energy. Beyond this energy, the spectra are of zero intensity over a wide energy range. Besides, experimental spectra are analysed on the basis of experimentally determined cross‐sections and the results are discussed, in relation to the analysis based on theoretical cross‐sections.The importance of defining simple procedures for routine analysis of spectra is stressed and it is demonstrated that the proposed ‘universal’ cross‐section provides a very simple method for approximate spectral analysis.Finally, some important issues to be considered in the near future are pointed out.