X-ray Photoemission Line Research Articles

Ammonia adsorption on iron phthalocyanine on Au(111): Influence on adsorbate–substrate coupling and molecular spin

The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer.

Carbon 1s X-ray Photoemission Line Shape Analysis of Highly Oriented Pyrolytic Graphite: The Influence of Structural Damage on Peak Asymmetry

C 1s XPS spectra of various highly oriented pyrolytic graphite (HOPG) surfaces, untreated, as well as those treated by keV Ar+ beam bombardment and low-energy O2, N2, Ar, and H2O plasmas, have been systematically studied by comparing two XPS peak-fitting procedures. These procedures treat the spectrum as either (1) the overlap of several symmetric component peaks or (2) a single asymmetric peak. The results indicate that, in the case of HOPG, the asymmetry parameter defining the single peak is directly related to the extent of damage to the alternant hydrocarbon structure of the HOPG surface, as manifested by its correlation with the symmetric peak component due to the damaged HOPG structure.

X-ray line shapes of metals: Exact solutions of a final-state interaction model

By means of model calculations for an independent-electron metal, we obtain exact line shapes for the photon absorption, emission, and photoemission spectra of core states, including electronic relaxation. In all cases we find an x-ray edge anomaly. For the absorption and emission spectra this anomaly is superposed on a continuum resembling Elliott exciton theory. We display how the spectra evolve from the exciton limit to the free-electron limit as the final-state interaction strength is decreased or the Fermi energy increased. We compare the spectra obtained for different final-state interactions and find that different types of interactions produce different spectral shapes. Away from threshold the absorption and emission profiles show an enhancement of the free-electron result, as predicted by the screened-exciton theory. Our results offer potential explanations for (i) incompatibilities between threshold exponents and exponents extracted from other data, (ii) the occurrence of nearly symmetric x-ray photoemission lines, and (iii) the lack of mirror symmetry of absorption and emission edges.

Madelung Potentials and Disorder Broadening of Core Photoemission Spectra in Random Alloys

We present the first experimental evidence for ``disorder broadening'' of core level x-ray photoemission line shapes in alloys. The broadening is attributed to variations in electrostatic potential. A model in which the charge on a site is assumed to be proportional to its number of unlike neighbors has recently been shown to greatly improve the calculated total energies of disordered alloys. This model predicts a broadening a factor of 2 larger than observed. We discuss this result in terms of short range order and a generalized charge model.

Temperature dependence of the X-ray photoemission line shape and of the hopping rate in a marginal Fermi liquid

The authors study the spectral properties of a localized particle (a deep core level, or a heavy particle hopping in a solid), coupled to conduction electrons that are described by a marginal Fermi liquid hypothesis. The main result is that, in this model, the core level line shape in an X-ray photoemission experiment shifts with temperature. The spectrum is consistent with the decrease of the hopping rate with decreasing temperature obtained by Zhang et al. (1992). In the presence of a distribution of energy levels for the hopping particle, the temperature dependence of the hopping rate is found to be less pronounced. The observability of both the shift in the X-ray photoemission spectrum and the decrease of the hopping rate depends on the strength of the interaction. A rough estimate of the value of the interaction parameter indicates that the effect might be observable in X-ray photoemission experiments.

Adsorption and reaction of CO 2 on Ni{110}: X-ray photoemission, near-edge X-ray absorption fine-structure and diffuse leed studies

Using three different techniques X-ray photoemission (XPS), near-edge X-ray absorption spectroscopy (NEXAFS) and diffuse LEED we have studied the adsorption and reaction of CO, on Ni(ll0). In agreement with previous angle-resolved photoemission (ARUPS) and vibrational electron energy loss (EELS) data both a linear, physisorbed molecule and a bent, chemisorbed species CO,Sare found. An evaluation of the XPS line intensities shows that the stoichiometry of the chemisorbed species is 1 to 2 in carbon and oxygen. The polarisation dependence of the NEXAFS indicates that the physisorbed molecule lies with its axis parallel to the surface and that the molecular plane of the CO;species is perpendicular to the surface. There is no clear preferential azimuthal orientation. This is in agreement with a diffuse LEED analysis where equal numbers of bent molecules adsorbed on atop sites and oriented along the (100) and (110) directions gave the best fit to the data.

X-ray spectra of model binary alloys A1-xBx.

The x-ray photoemission, absorption, and emission spectra of a one-dimensional tight-binding model for a binary metallic alloy ${A}_{1\mathrm{\ensuremath{-}}x}$${B}_{x}$ are evaluated in a change of mean-field model. The combined effects of disorder and multielectron excitations are included. The extent of the asymmetric tails of the x-ray photoemission lines depend on the local character of the Fermi-energy states in the vicinity of the core hole; this effect could explain the long-standing mystery of why Na 1s lines in ${\mathrm{Na}}_{\mathrm{x}}$${\mathrm{WO}}_{3}$ are symmetric. Features in the absorption and emission spectra reminiscent of the anomalous ramplike thresholds observed for absorption by rare-gas atoms in alkali-metal hosts are also found.

Study of plasmon structure in XPS spectra of silver and gold

Measurements of the low-energy satellites of the core X-ray photoemission lines of silver and gold are presented. The results are compared with electron energy loss spectra and dielectric measurements. Interpretation of the results on the basis of plasmon excitation is given and compared with earlier interpretations.

Model calculations of the spectra of rare-gas impurities in alkali hosts

The spectra of rare-gas impurities in alkali hosts are evaluated for a change of mean-field (ΔMF) model. The model produces absorption and emission edge anomalies and asymmetric X-ray photoemission line shapes, but the threshold shapes are different from those observed by Tilton, Phelps and Flynn. The resolution of the discrepancy between theory and experiment appears to lie beyond this ΔMF model.

Many-body processes in x-ray photoemission line shapes from Li, Na, Mg, and Al metals

X-ray photoemission from all the accessible core levels in Li, Na, Mg, and Al have been measured with Al $K\ensuremath{\alpha}$ radiation from metal films evaporated in ultrahigh vacuum, and the asymmetric line shapes of the core-level spectra were analyzed in terms of the Doniach-\ifmmode \check{S}\else \v{S}\fi{}unji\ifmmode \acute{c}\else \'{c}\fi{} (DS) function. Observed lifetimes were compared with theoretical calculations and showed generally good agreement for the $1s$ and $2p$ levels of Na, Mg, and Al but poor agreement for the $2s$ levels. The Li $1s$ lifetime was found to be considerably longer than predicted from simple atomic calculations, but was in good accord with a more recent many-body calculation. Singularity indices were determined from data extending to within half the Fermi energy from the main peak. The range of validity of the DS function and the possible line shape interferences from other sources were carefully examined, showing that the electron-hole pairs measured in the asymmetic tails are an intrinsic property of the bulk-metal spectrum and that the DS function gives an excellent representation of the data over a wide range of energies. The magnitude and trend of the singularity indices for Na, Mg, and Al were interpreted in terms of the partial screening charges associated with the hole-state atom and were found to compare favorably with several recent calculations. Detailed investigation of temperature-dependent phonon broadening magnitudes and zero-point and phonon cutoff energies were carried out for Li, indicating that a recently proposed interference effect may be responsible for the anomalously large line-shape broadening in that metal. At present, however, no theory adequately accounts for the observed Li $1s$ linewidth. For Na, Mg, and Al, on the other head, enhanced broadening effects were not observed and recent calculations of phonons broadening agreed well with our measurements. The Li $1s$ binding energy was observed to be temperature dependent, and its magnitude and trend were explained by a combination of anharmonic lattice expansion and the heretofore unappreciated temperature dependence of the electronic screening energy. The surface plasmon energy of clean Li was found to be in marked disagreement with that predicted from the empirical bulk plasmon energy and simple free-electron theory and still remains to be explained. The validity of the sudden approximation used in the interpretation of the electron-hole pair and phonon spectra observed in this work is also discussed.

Model‐independent constraints on many-electron X-ray photoemission asymmetries

Abstract The many-electron orthogonality catastrophe index for free-electron metals, δ ═ 2(2j+ 1)(δj/π)2, is shown to satisfy constraints including : (i) 0–05 ≤ δ ≤ 0–11, (ii) dδ/dr δ>0, and (iii) consequences of the requirement that δ, the L2 3 threshold exponent α0, and the impurity resistivity be related to the same phase shifts. Values of δ extracted from asymmetric X-ray photoemission lines of Na, Mg, A1, Li, Cs, Cu, Ag, Au, Cd, In and Sn violate one or more of these general constraints. The theoretical and experimental implications of these results are discussed.

Phonon broadening of x-ray photoemission line shapes in solids and its independence of hole state lifetimes

Phonon broadening of x-ray photoemission line shapes in solids and its independence of hole state lifetimes

Influence of many-body effects and intra-atomic exchange interactions on the shape of x-ray emission and photoemission lines of3dmetals

The asymmetry and width of the x-ray emission and x-ray photoemission lines of iron transition metals are discussed in terms of many-body final-state effects and of the exchange interaction between a core hole and unpaired $3d$ electrons.

XPS line shape asymmetry in the Pd-H system: The role of the electron screening

The asymmetry parameter of the 3 d doublet of Pd in the Pd-H system is measured by X-ray photoemission (XPS) at increasing hydrogen concentrations and it is found to decrease as a function of hydrogen content with a slope change when the complete filling of the 4 d band occurs. We discuss a simple scattering model in the Born approximation limit. An explanation of the experimental behaviour is provided in terms of s-like electron density alone, which gaves a bandwidth in accordance with that derived from the band calculations of Switendick.

CHARGE DENSITY WAVE PHASE TRANSITIONS OBSERVED BY X-RAY PHOTOEMISSION

X-ray photoemission (XPS) measurements of the binding energies of the Ta 4f core levels in TaS2, TaSe2, and Ta0.97Hf0.03S2 at temperatures between 14 K and 371 K are reported. The presence of charge density waves (CDW's) produces inequivalent potentials at different Ta sites which split the 4f XPS lines ; the splitting is a measure of the CDW amplitude and represents an order parameter for the CDW-lattice system. The form of the components of the split XPS lines allows the CDW-lattice configuration to be deduced, and since the potential gradients at inequivalent Ta sites are different, the unequal widths of the components yields a measure of thefluctuating nature of the CDW-lattice system. The results indicate the existence of CDW fluctuations in commensurate phases, and that in some incommensurate and quasi-commensurate phases the CDW's cannot maintain long range coherence. In 1 T-Ta0.97Hf0.03S2 the Hf impurities pin the incommensurate CDW's into domains.

Optical and Photoelectrical Studies of the Energy Levels of CoX42-in K2CoX4and RbCoX3(X≡Cl or Br)

It has been found that the optical absorption spectra of CoX 2 (X≡Cl or Br) evaporated on substrates of KX or RbX change drastically when samples are annealed above 70°C. An analysis of X-ray photoemission lines of core levels indicates that the change is caused by the formation of a new compound as K 2 CoX 4 or RbCoX 3 . The chemical reactions, CoX 2 +KX→K 2 CoX 4 and CoX 2 +RbX→RbCoX 3 , proceed fast above 80°C. The major results include the intense crystal-field bands in the 1.5–2 eV region arising from 3 d electrons of Co 2+ occupying tetrahedral sites and the p → d charge-transfer bands above 4 eV, as well as the X-ray photoemission spectra of valence electrons. The f -values of the d → d transitions are of the order of 10 -3 . All of the observed spectra are interpreted in terms of the localized excitation in a CoX 4 2- complex-ion.

Many-electron screening in narrow-band metals; XPS in layered dichalcogenides

The cut-off in the many-body lineshape of X-ray photoemission lines from metals has been observed in the narrow band metals 2H-NbSe 2, 2H-TaSe 2 and Fe x TaS 2. Simple considerations show that the lineshape will always change its character for excitations greater than the Fermi energy.

The spherical-solid model: An application to x-ray edges in Li, Na, and Al

In order to treat highly localized excitations in a solid a simple spherical model has been developed. The Coulomb potential from the nucleus at the site of the excitation is treated exactly, while the potentials from the surrounding sites are turned into pseudoion pontentials. Only the spherical average of these potentials is retained and separate self-consistent calculations for the ground state and for the excited state are made. These states are mistreated by the model but the difference accurately describes the large effects of polarization involved. The positions of the x-ray edges are obtained with an accuracy of 0.5 eV for singly ionized levels and 1 eV for doubly ionized levels. The phase shifts relevant to the Nozi\`eres-De Dominicis theory of x-ray absorption and emission edges and x-ray photoemission line shapes are also calculated. We obtain a noticeable many-body enhancement of the ${L}_{2,3}$ edge in Na while the many-body effect in Li and Al is negligible. Our exponents agree with those extracted from recent x-ray photoemission experiments, but they do not agree with those extracted from x-ray emission or absorption spectra. We argue that the exponents taken from the photoemission experiment are more reliable but no definite conclusion concerning the validity of the present many-body theory should be drawn until the experimental situation has become more clear.

Many-Electron Effects in Core-Level X-Ray and Electron Spectroscopies from Na, Mg, and Al

X-ray photoemission line shapes of $2s$ and $2p$ electrons from Al, Mg, and Na have been analyzed in terms of the theory of Mahan and of Nozi\`eres and De Dominicis. The resultant singularity indices, 0.115, 0.13, and 0.19, respectively, require that all x-ray threshold exponents, save that for the Na $K$ edge, be positive.