Esca. results versus other physical and chemical data

X-ray photoemission spectra yield quantities of very direct interest in physics and chemistry. In this paper the relations of these spectra to other data and concepts are discussed. Both initial-state and final-state properties may be studied: the former are treated first. Charge distributions in molecules alter the effective (Coulomb plus exchange) potential experienced by core electrons in molecular ground states, there by shifting their binding energies. The shifts can be calculated by $\text{ab}$$\text{initio}$ methods or more directly by using potential models based on intermediate-level molecular-orbital theories such as INDO. One version, the ground-state potential model (GPM) yields good predictions of core-level shifts among atoms in similar environments. Alternatively, the measured shifts may be used to derive charges on individual atoms in molecules. It is more difficult to derive charges in solids in this way, but a characteristic splitting in the more tightly-bound valence bands yields a direct measure of ionicity in simple binary compounds of the zinc-blende and rocksalt structures. Atomic orbital composition of molecular orbitals can be deduced from photoemission spectra. In solids such as diamond and graphite comparison of photoemission spectra with x-ray emission spectra yields the atomic-orbital composition of the valence bands. Turning to final-state properties, the spectra are dominated by relaxation effects. Again a simple approach—the relaxation potential model (RPM)—predicts core-level shifts well for cases in which the atomic environments are varied substantially. Among ammonia and the methylamines, for example, the N(ls) shifts are predicted correctly by RPM, while GPM reverses the order. For paramagnetic molecules RPM predicts electron charge transfer toward the positive hole but usually spin transfer away, in agreement with experiment. Extra-atomic relaxation in metals, a many-body effect, is manifest both as a contribution to the binding energy and as line-shape asymmetry. Delocalized valence electrons also show relaxation shifts that can be understood as polari zation of the electron gas toward the “Coulomb hole”. Auger lines show larger relaxation shifts. Comparison of core-level or Auger shifts in nonmetallic solids separately is questionable because there is no reference level, but intercomparison of the two is meaningful. Finally, core-level binding-energy trends in series of simple alcohols, etc., agree quantitatively with proton affinities and core-level shifts in other functional groups. This suggests extending the concept of Lewis basicity to include lone pairs of core electrons. Thus, core-level shifts measure the chemical reactivity—a quantity of great chemical importance that depends on both initial- and final-state properties—rather directly. Rela xation energies are shown to be the dominant cause of trends in the lowest ionization potentials of simple alcohols and amines.     

X-ray photoemission studies on InSb clusters

The changes in core-level electronic structure of In_1−xSb_x nano-clusters have been studied using X-ray photoelectron spectroscopy. Though, clusters with mean composition InSb show shallow and deep core-level binding energies similar to those of bulk InSb, the Sb and In rich versions show a negative BE shift for the excess element and a positive shift for the minority element. The observed BE shifts have been explained considering a core and surface shell model for the structure of the clusters and possible surface atom core-level shifts.     

Correlation of inner shell binding energies with shifts of charge transfer bands

Inner shell binding energies were determined for the elements in the crystalline compounds A^IIIX^VO₄ where A  B, Al, Ga, Fe and X  P, As with fourfold coordination of A, and in the corresponding dihydrates of Al having a sixfold coordination of Al. The O 1s binding energies for the dihydrates could be resolved into two bands arising from XO₄ -groups and from the water molecules respectively. The O 1s binding energies from the XO₄ groups decrease by about 0.6 eV in going from the anhydrous compounds to their dihydrates whereas the Al 2p binding energies remain constant within the limits of error. These results correlate surprisingly well with shifts of the first charge transfer band of trivalent iron in these compounds. The binding energy differences between phosphates and arsenates are also in good agreement with concurrent shifts of this charge transfer band. The consequences of these results for the concept of optical electronegativities are discussed.     

KLL Auger and x-ray photoelectron spectra of phosphine,some phosphorus halides and the other hydrides isoelectronic with argon

KL₂L₃(¹D₂) Auger and 1s photoelectron energies have been measured for molecular hydrides isoelectronic with Argon (HCl, H₂S, PH₃ and SiH₄). In addition a detailed comparison of Auger and photoelectron shifts in a series of phosphorus halides (vs phosphine) has been undertaken using additional P_2p binding energies. The potential model is better able to predict 1s binding energy shifts with either ground state or relaxation corrected models than the 2p shifts. These latter values seem also to be reduced by shielding effects. In general, fluorides are better predicted than chlorides. Auger shifts correlate linearly(but not in a 1:1 relationship) with 1s photoelectron shifts throughout the isoelectronic series and also in the case of the phosphorus fluorides and phosphine. The two potential models, however, provide poor prediction of Auger shifts.     

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu_1?xAu_x and Yb_1?xAu_x. Furthermore, these shifts are different from the chemical shifts of the bulk 4f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.     

Surface core-level binding energy shifts in alloys

A simple theory for the core-level binding energy shifts at the surfaces of binary alloys A_xB_1?x is presented. Results are given for the surface core-level shifts of Ni in Ni_xCu_1?x alloys and Pd in Pd_xAg_1?x alloys. It has been shown that the surface core-level shifts may depend sensitively on surface segregation.     

Benford’s law predicted digit distribution of aggregated income taxes: the surprising conformity of Italian cities and regions

Core-level photoemission spectra of the Fabre salts with X = SbF₆ and PF₆ were taken using hard X-rays from PETRA III, Hamburg. In these salts TMTTF layers show a significant stack dimerization with a charge transfer of 1e per dimer to the anion SbF₆ or PF₆. At room temperature and slightly below the core-level spectra exhibit single lines, characteristic for a well-screened metallic state. At reduced temperatures progressive charge localization sets in, followed by a 2nd order phase transition into a charge-ordered ground state. In both salts groups of new core-level signals occur, shifted towards lower kinetic energies. This is indicative of a reduced transverse-conductivity across the anion layers, visible as layer-dependent charge depletion for both samples. The surface potential was traced via shifts of core-level signals of an adsorbate. A well-defined potential could be established by a conducting cap layer of 5 nm aluminum which appears “transparent” due to the large probing depth of HAXPES (8–10 nm). At the transition into the charge-ordered phase the fluorine 1s line of (TMTTF)₂SbF₆ shifts by 2.8 eV to higher binding energy. This is a spectroscopic fingerprint of the loss of inversion symmetry accompanied by a cooperative shift of the SbF₆ anions towards the more positively charged TMTTF donors. This shift does not occur for the X = PF₆ compound, most likely due to smaller charge disproportion or due to the presence of charge disorder.     

A new charge-correction method in X-ray photoelectron spectroscopy

The 2p_{$\text{3}\text{2}$} binding energy (242.3 eV) of Ar implanted in insulating materials is available to correct for charging shifts. Argon ions hav materials SiO₂ and soda glass. In each case the charging shift for Ar 2p_{$\text{3}\text{2}$} electrons agrees exactly with those for core-level elec The charge-corrected binding energies of the insulating materials permit the identification of atomic chemical states. Ion-induced reduction of the ins investigations.     

A CNDO/2 study of σ- and π- relaxation energies and core electron binding energy shifts in some simple molecules

A relaxation potential model has been used to study relaxation energies and shifts in core electron binding energies for some substituted alkenes and carbonyl compounds in terms of σ- and π-contributions.The conclusions from this study may be summarized as follows:(A) For the series R₁R₂C^*CH₂ and R₁R₂C^*O: (i) The total shifts vary in a regular manner, similar to the shifts in saturated systems; (ii) The variation in σ-relaxation energies is greater than the variation in π-relaxation energies.(B) For the series R₁R₂CCH₂ and R₁R₂CO: (i) There is little variation in the σ-relaxation energies; (ii) The π-relaxation energies show moderately large variations and the higher values are associated with unsaturated substituents which can donate π-electrons to the core-ionized atom via the conjugated π-system; (iii) The π-relaxation energies in the fluorinated systems are similar to those in the unsubstituted molecules; and (iv) The large -ve π-shift in the fluorinated systems results from a ground state build up of electron density at the site of core ionization.     

XPES studies of oxides of second- and third-row transition metals including rare earths

X-ray photoelectron spectroscopy has been employed to investigate oxides of second- and third-row transition metals, including those of rare earths. Systematics in the spin—orbit splittings and binding energies of core levels of the metals are described. In most of the cases studied, the dependence of the spin—orbit splittings on the atomic number Z is given by the relation ΔE = a(Z - Z₀)⁴, where a is the quantum defect parameter and Z₀ is the effective screening. Core-level binding energies are found to increase with the oxidation state of the metal. Most of the core-level binding energies are related to the atomic number Z by the expression E = x(Z - Z₀)², giving rise to linear plots of ln E versus ln Z. Specific features of individual oxides, with respect to satellites, multiplet structure, configuration mixing, and other properties are also discussed. The spectra of PrO₂, Pr₆O₁₁, TbO₂ and Tb₄O₇ are reported for the first time.     

Surface core-level shifts for Ge(100)−(2 × 1)

Using surface-sensitive photoemission techniques, Ge 3d core-level binding energies for surface atoms of Ge(100)?(2 × 1) are found to be smaller than the bulk values by 0.41 eV. The surface atoms with shifted core-level binding energies correspond to one full (100) atomic layer. A surface core-exciton resonance is observed in the partial-yield measurements. The empty surface state involved in this excitonic transition, without binding-energy correction, is located at the valence-band maximum.     

Quantum chemical and NMR investigations on structure of surface complexes

The semiempirical quantum chemical CNDO/2 method is used to calculate models of specific interaction between benzene, toluene, and butene molecules, respectively, and ions or hydroxyls representing active sites of adsorption on zeolitic surfaces. From energy minima of full potential curves the stabilization energies of the surface complexes have been obtained. On the basis of proposed complexes theoretical carbon-13 NMR chemical shifts of adsorbed molecules are calculated. The theoretical results are in rather good agreement with the experimental ones, confirming the conception of surface complexes. Moreover, experimental paramagnetic shifts of surface complexes containing Co²⁺ ions are tried to interprete in a quite similar way.     

The solvation and hydrophobic interaction of non-polar molecules in water in the approximation of interatomic potentials: The Monte Carlo method

The problem of calculating thermodynamic functions of solvation and hydrophobic interaction of non-polar molecules in water is considered, and the functions of ‘switched off’ interaction which allow one to compare calculated and experimental data are introduced and discussed. Expressions for the free energies, entropies and internal energies of solvation and hydrophobic interaction are deduced by statistical mechanics, and the possibility of computing these quantities from information about interatomic potential functions is considered. An application of the Monte Carlo method with the Boltzmann transition probabilities between the states of a Markov chain has allowed us to evaluate configuration integrals and thermodynamic functions of solvation and hydrophobic interaction. For solvation, the calculated values are in good agreement with the experimental data if the periodic boundary conditions are observed, and in moderate agreement should these be neglected. For hydrophobic interaction, the plot of F _HI(R ₁₂) is built and the potentials of average force for two methane molecules, as well as two hard spheres, is evaluated.     

Binding energies and radii of α-cluster-type nuclei from calculations based on the strictly restricted dynamics model

The properties of α-cluster-type nuclei with 4≤A≤80 are studied by employing the microscopic strictly restricted dynamics model (SRDM). The SRDM parameter set found via a fit to the experimental and theoretical values of nuclear binding and excited-level energies, classified according to the ground-state SU ₃ configurations, includes a nuclear-radius parameter r ₀ entering into the expression R=r ₀ A ^1/3 [fm], as well as the parameters of the effective central NN-interaction potential. The use of the microscopic SRDM allows one to obtain additional information about nuclei along the Z=N line, including binding energies, radii, and spectra of low-lying levels. The calculated nuclear binding energies and nuclear root-mean-square radii 〈r ²〉^1/2 are in reasonable agreement with their experimental counterparts.     

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4f_7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4f_7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4f_7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born-Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4f_7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Core photoelectron spectroscopy of some acetylenic molecules

Carbon 1s binding energies have been measured for CH₃CCH, CH₃CCCH₃, CF₃CCH and CF₃CCCF₃ and compared to a verified value for acetylene. Assignments are based on the application of a CNDO potential model with relaxation corrections which is quite successful in predicting binding energy shifts and upon qualitative considerations. Substitution of CF₃ groups shifts the acetylenic C 1s binding energy from 291.2 (HCCH) to 292.2 in CF₃CCH and 292.7 eV in CF₃CCCF₃. The unequal substitutional shifts are probably due to a saturation of substituent effect expected in competitive situations. With reservations arising from uncertainties in assignment due to lack of resolution, it appears that acetylenic C 1s binding energies decrease [to 290.7 (av.) in CH₃CCH and to 290.1 eV in CH₃CCCH₃] upon replacement of H by CH₃ groups. Although the decrease in acetylenic binding energies agrees with the chemical notion that CH₃ groups are electron donating with respect to unsaturated portions of the molecule, theoretical calculations available in the literature indicate that actual electron withdrawal or donation does not occur in these differently substituted molecules. The shifts of apparent binding energy correlate reasonably well with a ground state potential model which accounts for the effect of the charge on the adjacent atoms as well as on the photoionized atom. Even better correlation is obtained if the atomic potentials are corrected for electronic redistribution (relaxation) effects which occur during the photoionization process, and it is suggested that relaxation effects make a significant contribution to shifts of apparent binding energies. Surprisingly ground state potential and relaxation corrected potential calculations with the CNDO method suggest a large difference in C 1s binding energies of the two acetylenic carbon atoms in CH₃CCH which is not verified experimentally nor mirrored by calculations on CF₃CCH. The CH₃ binding energies are 291.8 eV in CH₃CCH and 291.3 eV in CH₃CCCH₃, both higher than values assigned to CH₄ or C₂H₆.     

Simple local NN potentials involving forbidden states and polarization in ed scattering

A simple NN potential is proposed in an analytic form. The parameters of this potential are fixed by fitting the effective range, the scattering length, and the deuteron binding energy. The phase shifts for np scattering at energies ranging up to 500 MeV and the properties of the deuteron are calculated with the resulting parameters. The effect of the πNN coupling constant on the potential parameters and on the accuracy in describing various properties of NN interaction is explored. The results of the present calculations are found to be in good agreement with experimental data, with available results from NN partial-wave analyses, and with the results of calculations with Nijmegen potentials. The tensor polarization t ₂₀ in elastic electron-deuteron scattering is analyzed by using some NN interactions.     

An XPS—AES study of gaseous xanthates and related sulfur-containing compounds

Core XPS spectra for carbon, oxygen, and sulfur and KLL Auger spectra for sulfur in CH₃OCS₂CH₃, (CH₃OCS₂)₂, CS₂, and OCS have been measured and relaxation shifts determined for the sulfur atoms by combining the S 1s measurement with the S KLL (¹D₂) measurement.Relaxation shifts of the sulfur atoms were also estimated from CNDO results for the neutral and core-ionized molecules using the equivalent cores approximation. The results are in qualitative agreement with the measurements, but exaggerate the relaxation by about 100%.The results show that the bonding of the (CH₃O-) group in the two xanthate compounds is very similar. The ionization energies of the S and -S- atoms within the xanthate molecules differ from each other by 1.5 eV; this difference arises almost entirely from the initial-state charge distribution rather than from final-state relaxation. However, the ionization energies of similarly bonded sulfur atoms are nearly the same. The effect of the oxygen atom on the bonding of the carbon and -S- atoms in the (-CS₂-) group in the xanthate compounds is to increase the (C 1s-S 2p_{$\text{3}\text{2}$}) ionization energy difference from the value reported for aliphatic disulfides.     

X-ray photoelectron spectroscopic study of the adsorption of N2 and NO on tungsten

X-ray photoelectron spectroscopy (ESCA) has been used in a study of N₂ and NO adsorbed on a polycrystalline tungsten ribbon. The sample was flash cleaned under ultrahigh vacuum conditions, and cooled to either 300 K or 100 K for the adsorption studies. Large chemical shifts, as great as 8 eV, were observed between the N (1s) spectra associated with the weakly chemisorbed γ-nitrogen states and the strongly chemisorbed β-nitrogen states. Chemical shifts in both the N(1s) and O(1s) spectra suggest that NO is largely non-dissociatively chemisorbed at 100 K. In general, the binding energies of N(1s) and O(1s) electrons in the adsorbed layers are smaller than the binding energies for the same atoms in small gaseous molecules. In addition, the binding energies associated with the weakly-bound states of NO and N₂ are invariably greater than the binding energies associated with strongly chemisorbed species.