A new asymmetric Pseudo‐Voigt function for more efficient fitting of XPS lines

Asymmetric peak profiles for the application in spectroscopy can be obtained in a simple way by substituting the usually constant full width at half maximum parameter in Pseudo‐Voigt functions with an energy‐dependent expression, for instance of sigmoidal shape. While this approach has been successfully applied to vibrational spectra, we find that the resulting curves are less suitable for least‐squares fits of X‐ray photoelectron spectroscopy (XPS) data. However, if one additionally allows a variable displacement of the sigmoidal step relative to the peak, excellent fitting results can be obtained. We demonstrate the applicability of our extended approach on several inherently asymmetric XPS lines, i.e. the C 1s signal of graphite and C₂H₂/Pd(100), the 3d_5/2–3d_3/2 doublet of palladium, and the 4f_7/2–4f_5/2 doublet of platinum. Comparison of the corresponding fit results with the results obtained by the application of more elaborate, theory‐based line profiles (Doniach‐?unji? and Mahan functions) shows that the modified Pseudo‐Voigt function gives practically identical results in terms of peak shape and area, while requiring much less computational effort since no convolution procedures are required for its calculation. Thus, this function is most suitable for application in one of the following situations: (i) the peak shape of a given signal is known but cannot be calculated with ease, and (ii) the theoretical peak shape is not (yet) known, however, one wants to perform a first quantitative screening of the data at issue. Copyright © 2014 John Wiley & Sons, Ltd.     

Fast and accurate expression for the Voigt function. Application to the determination of uranium M linewidths

The Voigt function is the convolution between a Gaussian and a Lorentzian distribution. The numerical implementation of this function is required in diverse areas of physics and applied mathematics. An explicit representation for the Voigt function is developed in terms of series of trigonometric and hyperbolic functions. The obtained expression permits a very fast evaluation of Voigt profiles with a degree of accuracy higher than the one required for spectroscopy applications. In addition, this expression is implemented in a numerical algorithm of parameter optimization in electron probe microanalysis, and applied to determine natural linewidths for several transitions to the uranium M levels.     

Photoelectron spectroscopic studies of the interfacial reaction between glass and commercial adhesive

The valence band and core‐level photoelectron spectroscopy [using X‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)] were used to probe the interfacial reaction between glass and a commercial adhesive (Loctite). The interaction was investigated by comparing experimental valence band spectra with spectra calculated for various possible interaction schemes. The valence band spectrum for the interfacial region between the glass and the adhesive was obtained using difference spectra on a thin film of adhesive on glass. This film was sufficiently thin that the adhesion interphase could be directly probed. Chemical interaction occurs at the interface as evidenced by the fact that the spectrum for the interfacial region could not be represented by the addition of the spectrum of the glass alone and the adhesive alone. The XPS valence band spectrum and the UPS spectrum showed that the shallow top surface layer is very much enriched in acrylic acid, which is a minor component in the adhesive. When the Loctite adhesive was coated on glass, the C1s and O1s regions of the adhesion interface region showed evidences of new chemistry at the adhesive–glass interface. The possible reactions were evaluated by comparison of the experimental spectra with calculated ones based on different models using ab initio molecular orbital calculations. The experimental spectra are well represented by models where the acrylic acid of the surface region of the adhesive reacts with the glass, suggesting chemical interaction occurred at the adhesion interphase. Copyright © 2009 John Wiley & Sons, Ltd.     

Electroreduction of Aryldiazonium Ion at the Polar and Non-Polar Faces of ZnO: Characterisation of the Grafted Films and Their Influence on Near-Surface Band Bending

ZnO is a strong candidate for transparent electronic devices due to its wide band gap and earth-abundance, yet its practical use is limited by its surface metallicity arising from a surface electron accumulation layer (SEAL). The SEAL forms by hydroxylation of the surface under normal atmospheric conditions, and is present at all crystal faces of ZnO, although with differing hydroxyl structures. Multilayer aryl films grafted from aryldiazonium salts have previously been shown to decrease the downward bending at O-polar ZnO thin films, with Zn−O−C bonds anchoring the aryl films to the substrate. Herein we show that the Zn-polar (0001), O-polar (000 ), and non-polar m-plane (10 0) faces of ZnO single crystals, can also be successfully electrografted with nitrophenyl (NP) films. In all cases, X-ray photoelectron spectroscopy (XPS) measurements reveal that the downward surface band bending decreases after modification. XPS provides strong evidence for Zn−O−C bonding at each face. Electrochemical reduction of NP films on O-polar ZnO single crystals converts the film to a mainly aminophenyl layer, although with negligible further change in band bending. This contrasts with the large upward shifts in band bending caused by X-ray induced reduction.     

Product or sum: comparative tests of Voigt,and product or sum of Gaussian and Lorentzian functions in the fitting of synthetic Voigt‐based X‐ray photoelectron spectra

A comparative study for the fitting of X‐ray photoelectron spectra (XPS) using different model functions is presented. Synthetically generated test spectra using Gaussian/Lorentzian convolution and a real measured spectrum are fitted with the three commonly used models: product, sum and Gaussian/Lorentzian convolution functions. In these limited tests, it was found that the sum function is superior to the product function, particularly for low‐noise spectra. Copyright © 2007 John Wiley & Sons, Ltd.     

XPS,AFM, and electrochemical investigation on the inner composition of insulating poly(o‐aminophenol), PoAP,deposited on platinum by CV,as a function of the number of cycles

The finite electrosynthesis, requiring 20 voltammetric cycles at neutral pH, of insulating poly(o‐aminophenol) (PoAP)and its polymerization mechanism and chemical structure up to the outermost surface were reported in previous work. By reducing the number of voltammetric cycles, XPS, atomic force microscopy, and electrochemical experiments were here confined to partially grown PoAP to better understand the role of the platinum surface on the initial adhesion mechanism and polymer growth. XPS results from reduced number and conventional 20 cycles show that PoAP composition is the same at any cycle. The main differences imaged by atomic force microscopy between 1, 5, and 20 cycles are on the amount of ‘small‐sized particles’ adsorbed on platinum prone to ‘fill’ the empty sites. The progressive filling of the platinum sites with cycles was also proved by electrochemical test with potassium‐ferricyanide. These new results add valuable information, which sustain previous hypotheses on the growth process of PoAP chains, their composition, and lateral alignment. Some questions, concerning the identification of PoAP anchoring bonds on platinum, lateral H‐bonds with water, and inter‐chains interactions, still remain unanswered and will be further addressed by the employment of new advanced means of investigation already in progress. Copyright © 2015 John Wiley & Sons, Ltd.     

A software package for the evaluation of Zeeman AAS data using a Perkin-Elmer model 3600 data station

The Perkin-Elmer Zeeman/5000 Atomic Absorption Spectrometer is only able to perform a calibration with three single standards or a one-point standard addition. The accuracy and precision of these methods are inappropriate in ETA-AAS, a computer-based data management is essential.A software package for calibration and evaluation was developed using curve fitting by linear regression based on a leastsquares fit when absorbances were transformed by the Baule-Mitscherlich function. The total analytical range could be covered by this method whether peak area or peak height values were fitted. The maximum absorbance levels could be calculated, as well.Absorbance signals were collected and stored by the HGA-graphics software (modified by Perkin-Elmer Nederland B. V.). The Pecowriter software was used for identifying data and file manipulations. The results of the calibration and evaluation can be stored on disk and/or printed. Calibration curves can be plotted on hardcopy output. The Limit of Detection and the Characteristic Mass can be calculated.The use of the CALIBRATION program is demonstrated by the calibration curves for the determination of Pb, Cd, As, Se, Cr, Al, Cu and Ni using mostly STPF-conditions. The performance of the method evaluated by the root mean square percent deviation of the fit is equal to the traditional curve-fitting function as well as to rational or quadratic functions.The use of the EVALUATION program is demonstrated by the results of the direct determination of lead in mineral waters.     

Theoretical investigations on analytical potential energy function and spectroscopic parameters for the state b3Πu of dimer 7Li2

The SAC‐CI (symmetry‐adapted‐cluster configuration‐interaction) method presented in Gaussian 03 program package is applied to investigate the adiabatic potential energy curves (PECs) of ⁷Li₂(b³Π_u). These calculations are performed at numbers of basis sets, such as 6‐311++G(3df,3pd), 6‐311++G(2df,2pd), 6‐311++G(df,pd), D95V++, D95(3df,3pd), D95(d,p), cc‐PVTZ, 6‐311++G and 6‐311++G(d,p). All the ab initio calculated points are fitted to the analytic Murrell‐Sorbie functions and then used to compute the spectroscopic parameters. The analytic potential energy function (APEF) for this b³Π_u state is reported. By comparison, the spectroscopic parameters reproduced by the APEF attained at 6‐311++G(2df,2pd) are found to be very close to the latest experimental findings. With the APEF obtained at the SAC‐CI/6‐311++G(2df,2pd) level of theory, a total of 62 vibrational states is found when J = 0. The complete vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants for these vibrational states are also reported. The reasonable dissociation limit for this state is deduced using the calculated results at present. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Rapid evaluation of the binding energies in hydrogen-bonded amide-thymine and amide-uracil dimers in gas phase

The binding energies and the equilibrium hydrogen bond distances as well as the potential energy curves of 48 hydrogen‐bonded amide–thymine and amide–uracil dimers are evaluated from the analytic potential energy function established in our lab recently. The calculation results show that the potential energy curves obtained from the analytic potential energy function are in good agreement with those obtained from MP2/6‐311+G** calculations by including the BSSE correction. For all the 48 dimers, the analytic potential energy function yields the binding energies of the MP2/6‐311+G** with BSSE correction within the error limits of 0.50 kcal/mol for 46 dimers, only two differences are larger than 0.50 kcal/mol and the largest one is only 0.60 kcal/mol. The analytic potential energy function produces the equilibrium hydrogen bond distances of the MP2/6‐311+G** with BSSE correction within the error limits of 0.050 Å for all the 48 dimers. The analytic potential energy function is further applied to four more complicated hydrogen‐bonded amide–base systems involving amino acid side chain and β‐sheet. The values of the binding energies and equilibrium hydrogen bond distances obtained from the analytic potential energy function are also in good agreement with those obtained from MP2 calculations with the BSSE correction. These results demonstrate that the analytic potential energy function can be used to evaluate the binding energies in hydrogen‐bonded amide–base dimers quickly and accurately. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

On the evaluation of dynamic X-ray diffractometric data for kinetic and structural purposes

Possible errors made due to incorrect evaluation of time- or temperature-resolved X-ray data (TXRD) are described. The extraction of compositional information, e.g., for kinetic purposes, can be highly erroneous when neglecting the influence of the changing mass absorption coefficient and a proper conversion of mass fractions to molar fractions. When structural data are computed, care should be given to a possible volume change of the sample. The correct way of evaluation and the extent of these errors is demonstrated for the thermal decarboxylation of calcium carbonate.     

Rapid evaluation of the binding energies between peptide amide and DNA base

The binding energies and the equilibrium hydrogen bond distances as well as the potential energy curves of 20 hydrogen‐bonded amide–base dimers are evaluated from the analytic potential energy function established in our laboratory recently. The analytic potential energy function is used to calculate the N? H···N, N? H···O?C, C? H···N, and C? H···O?C dipole–dipole attractive interaction energies and C?O···O?C, N? H···H? N, and N? H···H? C dipole–dipole repulsive interaction energies in the 20 dimers composed of DNA bases adenine, guanine, cytosine, or thymine and peptide amide. The calculation results show that the potential energy curves obtained from the analytic potential energy function are in good agreement with those obtained from MP2/6‐311+G** calculations by including the basis set superposition error (BSSE) correction. For all the 20 dimers, the analytic potential energy function yields the binding energies of the MP2/6‐311+G** with BSSE correction within the error limits of 0.50 kcal/mol for 19 dimers, only one difference is larger than 0.50 kcal/mol and the difference is only 0.61 kcal/mol. The analytic potential energy function produces the equilibrium hydrogen bond distances of the MP2/6‐311+G** with BSSE correction within the error limits of 0.030 Å for all the 20 dimers. The analytic potential energy function is further applied to four more complicated DNA base‐peptide amide systems involving amino acid side chain and β‐sheet. The values of the binding energies and equilibrium hydrogen bond distances obtained from the analytic potential energy function are also in good agreement with those obtained from MP2 calculations with the BSSE correction. These results demonstrate that the analytic potential energy function can be used to evaluate the binding energies in hydrogen‐bonded peptide amide–DNA base dimers quickly and accurately. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

Summary of ISO/TC 201 Standard: XX ISO 18118: 2004 – Surface chemical analysis – Auger electron spectroscopy and X‐ray photoelectron spectroscopy – Guide to the use of experimentally determined relative sensitivity factors for the quantitative analysis of homogeneous materials

ISO 18118 provides guidance on the measurement and use of experimentally determined relative sensitivity factors for the quantitative analysis of homogeneous materials by Auger electron spectroscopy (AES) and X‐ray photoelectron spectroscopy (XPS). This article provides a brief summary of this International Standard. Copyright © 2005 John Wiley & Sons, Ltd.     

A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the Cope rearrangement of 1,5-hexadiene and its cyano derivatives

A novel interpretation of the chameleonic and centauric models for the Cope rearrangements of 1,5-hexadiene (A) and different cyano derivatives (B: 2,5-dicyano, C: 1,3,4,6-tetracyano, and D: 1,3,5-tricyano) is presented by using the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) on the reaction paths calculated at the B3LYP/6-31G(d,p) level. The progress of the reaction is monitorized by the changes of the ELF structural stability domains (SSD), each being change controlled by a turning point derived from CT. The reaction mechanism of the parent reaction A is characterized by nine ELF SSDs. All processes occur in the vicinity of the transition structure and corresponding to a concerted formation/breaking of C(1)-C(6) and C(3)-C(4) bonds, respectively, together with an accumulation of charge density onto C(2) and C(5) atoms. Reaction B presents the same number of ELF SSDs as A, but a different order appears; the presence of 2,5-dicyano substituents favors the formation of C(1)-C(6) bonds over the breaking of C(3)-C(4) bond process, changing the reaction mechanism from a concerted towards a stepwise, via a cyclohexane biradical intermediate. On the other side, reaction C presents the same type of turning points but two ELF SSD less than A or B; there is an enhancement of the C(3)-C(4) bond breaking process at an earlier stage of the reaction by delocalizing the electrons from the C(3)-C(4) bond among the cyano groups. In the case of competitive effects of cyano subsituents on each moiety, as it is for reaction D, seven different ELF SSDs have been identified separated by eight turning points (two of them occur simultaneously). Both processes, formation/breaking of C(1)-C(6) and C(3)-C(4) bonds, are slightly favored with respect to the parent reaction (A), and the TS presents mixed electronic features of both B and C. The employed methodology provides theoretical support for the centauric nature (half-allyl, half-radical) for the TS of D.     

Accurate X-ray diffraction data required for proper evaluation of bond valence sums and global instability indexes: redetermination of the crystal structures of diamond-like Cu2CdSiS4 and Cu2HgSnS4 as a case study

Our calculations of the global instability index (G) values for some diamond-like materials with the general formula I₂–II–IV–VI₄ have indicated that the structures may be unstable or incorrectly determined. To compute the G value of a given compound, the bond valence sums (BVSs) must first be calculated using a crystal structure. Two examples of compounds with high G values, based on data from the literature, are the wurtz–stannite-type dicopper cadmium silicon tetrasulfide (Cu₂CdSiS₄) and the stannite-type dicopper mercury tin tetrasulfide (Cu₂HgSnS₄), which were first reported in 1967 and 1965, respectively. In the present study, Cu₂CdSiS₄ and Cu₂HgSnS₄ were prepared by solid-state synthesis at 1000 and 900 °C, respectively. The phase purity was assessed by powder X-ray diffraction. Optical diffuse reflectance UV/Vis/NIR spectroscopy was used to estimate the optical bandgaps of 2.52 and 0.83 eV for Cu₂CdSiS₄ and Cu₂HgSnS₄, respectively. The structures were solved and refined using single-crystal X-ray diffraction data. The structure type of Cu₂CdSiS₄ was confirmed, where Cd²⁺, Si⁴⁺ and two of the three crystallographically unique S²⁻ ions lie on a mirror plane. The structure type of Cu₂HgSnS₄ was also verified, where all ions lie on special positions. The S²⁻ ion resides on a mirror plane, the Cu⁺ ion is situated on a fourfold rotary inversion axis and both the Hg²⁺ and the Sn⁴⁺ ions are located on the intersection of a fourfold rotary inversion axis, a mirror plane and a twofold rotation axis. Using the crystal structures solved and refined here, the G values were reassessed and found to be in the range that indicates reasonable strain for a stable crystal structure. This work, together with some examples gathered from the literature, shows that accurate data collected on modern instrumentation should be used to reliably calculate BVSs and G values.     

Mo(CO)3(NCMe)(PPh3)2: Synthesis, X-ray structure and evaluation of its catalytic activity for the homogeneous hydrogenation of olefins and their mixtures

The complex Mo(CO)₃(NCMe)(PPh₃)₂, was synthesized by the reaction of Mo(NCMe)₃(CO)₃ with two equivalents of PPh₃ and characterized by UV–Vis, IR, NMR and X-ray diffraction. This complex was used as a catalyst precursor for the hydrogenation of 1-hexene, styrene, cyclohexene and 2,3-dimethyl-1-butene and their mixtures under moderate conditions in homogeneous media. Under mild reaction conditions (T = 373 K, P = 60 atm), the substrates showed the following reactivity order: styrene > 1-hexene > cyclohexene > 2,3-dimethyl-1-butene. A quaternary equimolar mixture showed a different hydrogenation order: 1-hexene > cyclohexene > styrene > 2,3-dimethyl-1-butene; the presence of dibenzothiophene or mercury does not interfere with the activity of the catalyst.