Core Level Binding Energy Shifts Research Articles

Layer-resolved shifts of photoemission and Auger spectra from physisorbed rare-gas multilayers.

Rare-gas multilayers adsorbed on Pd(111), Pd(001), and Al(111) were studied with photoemission and Auger spectroscopy. The Xe 4d core levels and the Xe N/sub 4,5/O/sub 2,3/O/sub 2,3/ Auger transitions show layer-resolved shifts for Xe multilayers on these three substrates. A simple model explains the experimental results satisfactorily. The core-level binding-energy shifts relative to the gas phase are dominated by the final-state hole-screening effect. The Auger kinetic-energy shifts relative to the gas phase, dominated by the difference in initial- and final-state hole-screening energies, are about minus three times the corresponding core-level shifts. The screening energies were calculated using a jellium model for the substrate and a dielectric continuum model for the rare-gas adlayer. Surface-induced shifts of core-level binding energies and Auger kinetic energies for bulk solid Xe(111) were also observed; the theoretical shifts are in very good agreement with the experimental values. The valence levels for multilayers of Ar, Kr, and Xe on Pd(001) were studied. They showed qualitatively similar shifts as the core levels, but the results cannot be explained accurately by our model, because the valence excitations are not as localized as the core excitations. Using spacer layers of Kr and Xe with various thicknesses, the effect of metallicmore » substrate screening on the valence-level shifts of Ar was demonstrated. The issues related to a proper energy-reference level and the question of ionic versus neutral excited states will be discussed.« less

The Analysis and Understanding of Ionisation Phenomena: Ab initio Molecular Wavefunctions

Two distinctly different kinds of information can be obtained from ab initio wavefunctions. The v~lue of observables can be computed, quite accurately for relatively small systems, but this only complements experiment. However, the analysis and interpretation of wavefunctions can lead to conceptual understanding of the significance of measured values. Ab initio wavefunctions are essential because they provide a unique and sound basis for determining the importance of various mechanisms. Three cases where this analysis has lead to new understanding will be described. The shift to higher binding energy (BE) of the 50" orbital relative to the hr orbital for chemisorbed CO is shown to be dominantly an electrostatic effect; it does not indicate the chemical bonding. The shift of core level BEs with the size of metal clusters is shown to be an initial state effect which depends on the coordination of the ionised atom; differential final state effects may not be large. The satellite structure for core ionisation of the NO dimer is due to both intra- and inter-unit screening. In all� these cases, the correct mechanisms could only be established with accurate ab initio wavefunctions.

Core-level binding-energy shifts due to end effects in polymers: A Hartree-Fock Green's-function study.

Hartree-Fock Green's-function studies of end effects on the core-level structure of metallic and insulating quasi-one-dimensional model polymers reveal additional core peaks outside the bulk bands. In the metallic case, shifts to both lower (\ensuremath{\sim}-150 meV) and higher (\ensuremath{\sim}+50 meV) binding energies are observed, whereas in the insulating case, split-off peaks occur only at the lower-binding-energy side (\ensuremath{\sim}-150 meV). It is shown that a positive or negative net valence population alone does not determine the direction of the shift. The binding-energy changes are determined by a detailed balance between the energy loss due to a decrease in the electron-nuclear attraction and the energy gain due to a decrease in the electron-electron repulsion experienced by the core electrons of the end atoms. This can probably also explain why for some metal surfaces, shifts towards lower, and for others, shifts towards higher, binding energies are found. In the valence region of the investigated lithium chains, the ends do not produce localized end states.

Core-level binding energies and Auger electron energies in epitaxial rare-gas layers on graphite (001)

Layer-dependent shifts of core-level binding energies and Auger electron kinetic energies are reported for epitaxial multilayers for Xe, Kr, and Ar adsorbed on graphite (001). Despite negligible changes of the substrate work function, clearly resolved shifts are observed, which are described by layer-dependent hole screening effects. As with metallic substrates, the Auger electron energies shift three times more than the core-level binding energies, but the magnitudes of the shifts are reduced due to the small electron density in graphite.

Core-level binding energies and Auger electron energies in epitaxial rare-gas layers on graphite (001)

Abstract Layer-dependent shifts of core-level binding energies and Auger electron kinetic energies are reported for epitaxial multilayers for Xe, Kr, and Ar adsorbed on graphite (001). Despite negligible changes of the substrate work function, clearly resolved shifts are observed, which are described by layer-dependent hole screening effects. As with metallic substrates, the Auger electron energies shift three times more than the core-level binding energies, but the magnitudes of the shifts are reduced due to the small electron density in graphite.

Surface and bulk core level binding energy shifts in PtAu alloys

The surface core level shift of the 4 f 7 2 level in a Pt2%Au and a Pt15%Au alloy has been investigated using high resolution synchrotron radiation excited photoelectron spectroscopy. The bulk and surface components in the recorded spectra have been extracted using a curve fitting procedure. The chemical shifts and the bulk-to-surface intensity ratios are consistent with a strong enrichments of Au to the alloy surfaces. The chemical shifts on nonsegregated surfaces have been measured on these alloys and on a Pt90%Au alloy using conventional XPS.

Surface and bulk core level binding energy shifts in Pt-Au alloys

Surface and bulk core level binding energy shifts in Pt-Au alloys

Cluster properties: AB initio theoretical studies of alkali and alkaline-earth clusters

The results of ab initio molecular-orbital calculations for Li, Be, Mg, and Ca clusters are presented. The wavefunctions are analyzed to explain trends in cluster properties with respect to cluster size, and two important general features are identified. For the alkaline earths, s → p hybridization is closely correlated to cluster stability. The coordination of an atom is also shown to be important for the equilibrium bond distance and for initial-state core-level binding-energy shifts.

X-ray photoelectron spectroscopy and infrared adsorption spectroscopy - correlations

Since the early seventies, a correlation has been known to exist between changes in intramolecular bond vibrational frequencies (as measured by IAS) and core level binding energy shifts (as measured by XPS). It is possible to explain such correlations by relating both shifts to the partial atomic charges which exist on the bonding pair of atoms. For a series of related molecules, for example, the charge on a common atom may vary and may be related to the core level shifts on that atom. Equally, Liesegang and Lee have shown that changes in this charge correspond with variations to the bond stretching frequency. This model has also been recently extended to include correlations between XPS shifts and rocking vibrational mode variations. In both applications, however, the elastic properties of bonds (springs) are considered. In the former, spring stretching properties are linked to the partial charges, while in the latter case molecular flexing involving a cantilever-like vibration permits a link with partial charges and their effect on rocking frequency. Experimental data will be presented for comparative purposes to illustrate the predictions of the model.

A core-level binding-energy shift analysis of CO, H, and O adsorption on Cu–Ni surfaces

The equivalent core approximation (Z+1) is used with a Born–Haber cycle analysis to analyze the Ni 2p3/2 surface core-level binding-energy shifts which occur upon adsorption of CO, H, and O on Ni(100). The analysis gives values for the heats of desorption of these gases from Cu–Ni alloy surfaces. Perhaps more importantly, it also provides a quantitative determination of how chemisorption of CO, H, and O modify the heats of surface segregation for Cu–Ni surfaces.

X-ray photoelectron and Auger-electron forward scattering: A new tool for studying epitaxial growth and core-level binding-energy shifts

X-ray photoelectron and Auger-electron forward scattering: A new tool for studying epitaxial growth and core-level binding-energy shifts

Core level shift calorimetry

The binding energies of deep lying core electrons of closed inner atomic shells are sensitive to changes in chemical bonding. For metallic systems the shifts of core level binding energies as function of the alloy composition can be related to partial solution energies of the alloy and therefore these shifts can be used to determine the formation energy of a binary alloy. It is shown for the alloy systems AgPd, NiCu, AuCu, PdNi and PdCu that there is good agreement between the thermochemical data measured directly and those extracted from the core level binding energy shifts as measured by photoelectron spectroscopy. In addition partial solution energies of ternary alloys can be obtained from these data. The same ideas can also be used to obtain thermochemical data of metal surfaces and interfaces.

Summary Abstract: A core-level binding-energy shift analysis of N2 on Ni(100)

Summary Abstract: A core-level binding-energy shift analysis of N2 on Ni(100)

Auger Energy Shifts for Metallic Elements

A previous treatment of core-level binding-energy shifts for metals is extended to Auger energy shifts. In comparison to single core-hole shifts, the new information given by the Auger shift is contained in the shift of the two core-hole final state. An accurate expression for the shift of the two core-hole ionization energy between the free atom and the metal is derived. In the actual calculation of the shift a (Z + 2) equivalent core approximation is applied, but where important corrections are included. Comparison between theory and experiment is performed for the metallic elements Na-Al, Ni-Ga, Pd-Sn, Au, Tl-Bi, Ba and Yb, and in all cases a good agreement is obtained. A rather detailed compilation of atomic and solid phase double-hole energies is presented in the Appendices. To elucidate the break-down of the concept of metallic screening, non-conducting elements like Si, Ge-Se, Sb and Te are also considered. In addition, atom-metal two core-hole energy shifts are calculated for the 4d transition series and an abrupt change in the energy shift is found when we proceed from the d elements to the elements beyond. The influence of the atomic structure on the shift is considered for the 4dn5s2, 4dn+15s and 4dn+2 configurations. The metallic renormalization of the atomic core-hole Coulomb correlation energy is calculated for the 4d elements and the chemical shift of this quantity is shown to be closely related to the change of the Auger parameter. The present formalism provides a most suitable framework for treating chemical shifts in metallic systems. As a special example the situation at the surface of a metal is considered and surface shifts are predicted both for the two-hole and the Auger energies. As for the single core-holes, these surface shifts change sign as we proceed through a transition series due to the bonding-antibonding division of the d-band.

ELECTRONIC STRUCTURE OF NiSi2

The electronic structure of NiSi2 was calculated by using the self-consistent LMTO method. Our result was found to be in resonable agreement with the results of AEUPS photoemission experiment and other calculations. The change in the Ni electron configuration was found to be responsible for the Ni3p3/2 core level binding energy shift; the decrease of Ni3d electrons weakened the screening effect for the Ni3p electrons. The result of calculation showed that the ionicity plays a minor role in chemical bond.

Electronic structure of C 6Ba studied by photoelectron spectroscopy

We have prepared stage 1 barium-graphite intercalation compounds by vapour phase reaction, and a new type of liquid phase reaction, whereby homogeneous large samples are obtained. The electronic structure of these samples has been studied by XPS and UPS. The measurements show strong similarities to the alkali-graphite intercalation compounds, but the conduction band seems to have a more complex structure with both graphite derived and barium derived bands. The C 1s core level binding energy shift indicates a charge redistribution towards the carbon site which is similar to that in C 6Li.

Coadsorption of potassium and CO on Pt(111)

The adsorption of CO on a K-covered Pt(111) surface was studied at 160 and 300 K as a function of K coverage and compared to CO adsorption on clean Pt(111). Different adsorbed states of CO could be distinguished by O(1s) and C(1s) binding energies. CO on clean Pt(111) was characterized by O(1s) binding energies at 532.7 eV (on-top site) and 531.0 eV (bridge site) whereas K-promoted CO was measured at 531.3 eV binding energy. The peak height of this K-influenced CO increases linearly with K coverage. The peak at 532.7 eV decreased in intensity with increasing K coverage. The corresponding C(1s) spectra showed a broad maximum at 286.9 eV for CO on Pt(111) and a single maximum at 285.8 eV for CO on Pt + K. UPS spectra of CO adsorbed on Pt + K showed changes in the 4σ and 5σ + 1π molecular orbital peaks which are consistent with an increased 4σ−1π energy separation, indicative of an increased C-O bond length. The work function increased for CO on Pt + K while the magnitude of this increase was proportional to K coverage for θ K < 0.17. A dipole moment of CO for the Pt + K surface of 1.8 D was evaluated from the work function change with CO coverage. Core level binding energy shifts, UPS data and work function changes (dipole moment) for CO on Pt + K indicate a large negative charge on the adsorbed CO which is interpreted in terms of enhanced back donation of electrons into the 2π ∗ anti-bonding orbital of CO. This K-influenced CO species is likely to be adsorbed in an on-top site on the Pt(111) surface. Its adsorption energy is increased relative to that for CO on Pt(111). No dissociation of CO on K covered Pt is observed.

Summary Abstract: Thermochemistry of interface and surface segregation and chemisorption from core level binding energy shifts

Summary Abstract: Thermochemistry of interface and surface segregation and chemisorption from core level binding energy shifts

Core level binding energy shifts for reconstructed GaAs(001) surfaces

With the use of surface-sensitive photoemission spectroscopy 3d-core level binding energy shifts have been determined for surface atoms in reconstructed MBE-grown GaAs (001) surfaces. An analysis of As(3d) shifts suggests that the C(4×4) structure is produced by chemisorption of arsenic on the As-rich 2×4 reconstructed surface.

XPS and UPS study of Pb monolayers on polycrystalline Pd

The deposition of up to 10 monolayers of Pb on polycrystalline Pd has been studied by XPS and UPS. Core-level binding energy shifts with coverage are reported. It is found that Pb forms initially a two-dimensional layer, followed by surface alloying at coverages above one monolayer.