The prediction of core-level binding-energy shifts from CNDO molecular orbitals

A theory is described for calculating core-level binding-energy shifts with potential models that employ “intermediate-level” molecular-orbital wave functions. The relaxation-energy term in atomic core-level binding energies is considered first. The ground-state potential model (GPM) and relaxation-potential model (RPM) are developed for calculating core-level binding energy shifts in molecules from CNDO wavefunctions. It is shown that neglect of certain two- and three-center integrals in these models limits their accuracy when unlike molecules are compared. The models are modified by calculating r^?1 integrals, to be sensitive to bond directions of p orbitals. The pp′ modification, in which a subset of the neglected integrals is retained to recover invariance to coordinate transformations, is thereby necessitated. The GPM approach yields shifts in very good agreement with experiment when comparisons are restricted to similar molecules. The RPM version gives better agreement especially over wider classes of molecules. It also provides relaxation energies V_R that can be combined with ab initio orbital energies to give binding energies. Several applications of these potential models are discussed.     

Interatomic internal photoemission peaks in (Auger) electron spectra

Internal photoemission (IPE) features appearing in (Auger) electron spectra have two distinguishing properties: (1) they obey optical selection rules and (2) IPE intensities increase rapidly with incident excitation electron energy. IPE and Auger electrons from elemental materials have similar energies, giving complicated mixed spectra; however, core-level interatomic IPE peaks (from compounds) can appear by themselves, as shown here using SiO ₂. The IPE process has high photon utilization efficiency, and core interatomic IPE shows promise for certain specialized applications discussed in the text.     

Energy shift of photoemission spectra for organics-passivated CdSe nanoparticles: The final-state effect

Size-dependent energy shift of photoemission spectra with respect to bulk sample has been examined for colloidally prepared CdSe nanoparticles with a series of particle sizes. The core-level shifts are well described by a theoretical calculation based on a final-state effect model, whereas an additional initial-state effect due to quantum confinement is required to elucidate the valence-band edge shifts. The results indicate that the interaction between the photohole and the dielectric background in the final state has to be considered in photoemission measurements for organics-passivated nanoparticles. The calculated results in the literature appear to overestimate the initial-state effect compared to our experimental observation.     

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.     

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.     

Delayed onset of 4d photoemission relative to the giant 4d photoabsorption of La

For the giant 4d photoabsorption of La, both the total photoabsorption spectrum and the N_4.5-derived Auger emission intensity spectrum increase significantly above hν ? 112 eV, with spectral peaks at hν = 118 and 119 eV, respectively. However, the predominant 4d photoemission partial cross section shows a delayed onset of ～ 4 eV, with a peak at hν = 121 eV, while the 5s, 5p, and 5d partial cross sections all show a strong resonant enhancement at lower energies, with spectral peaks at hν = 116.6 eV. These results are compared with a recent many-body calculation for Ce. The photon energy dependence of the La 4d_{$\text{5}\text{2}$}/4d_{$\text{3}\text{2}$} photo-emission branching ratio is consistent with a “final-state model.”     

Electron core-hole interaction and its induced ionic structural relaxation in molecular systems under x-ray irradiation

The experiments on perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride and 16FCuPc on Ag(111) surfaces by the normal incidence x-ray standing wave are poorly reproduced by first-principles ground state calculations but well reproduced by considering initial- and final-state effects, which include the response of the valence electrons to screen a core hole created by x-ray photoemission and further induced ionic structural relaxation. This study shows that the initial- and final-state effects due to screening are of great importance in characterizing molecules on metal surfaces in the normal incidence x-ray standing wave experiments.     

Photoemission from single crystal xenon

Normal exit photoemission spectra from the (111) surface of a xenon single crystal indicate that the valence level emission is considerably broader than expected on the basis of band structure calculations. Similar observations from polycrystalline samples have lead Parrinello $\text{et al}$ to propose an excitonic screening mechanism which would cause an apparent increase in the spin-orbit splitting at the Γ point. The details of the spectra reported here show, however, that the effect could simply be due to a dispersion of the bands stronger than that predicted theoretically. Anomalous intensity effects at $\text{h}\text{?}\text{ω = 21.2}\text{eV}$ are explained in terms of electron-electron scattering.     

Calculation of bremsstrahlung isochromat spectra from NI(001)

First principles calculations of momentum resolved bremsstrahlung isochromat spectra from ferromagnetic Ni(001) are performed for a photon energy $\text{h}\text{?}\text{ω = 9.7}$ eV and electrons incident in the ΓXUL mirror plane. The results obtained within the framework of an inverse one-step model of photoemission using an ab initio self-consistent bandstructure potential show very good agreement with recent experimental data due to Desinger et al..     

The He(I) photoelectron spectroscopy of heavy group IV—VI diatomics

The He(I) photoelectron spectra of the Group IV—VI diatomics GeO, GeS, GeSe, GeTe, SnS, SnSe, and SnTe are presented. The outermost valence structure of these molecules is similar to that observed in the lighter series CO, CS, etc. of this valence isoelectronic group; in each case a relatively sharp peak is assigned to ionization from the nominally non-bonding 3σ orbital and a broader band to ionization from the bonding 1π orbital. At higher binding energies the spectra exhibit several peaks where only a single peak is expected, from the (2σ)^?1 hole state. This structure is assigned to correlation peaks resulting from configuration interaction among hole states of ²Σ⁺ (Ω = $\text{1}\text{2}$) symmetry. Semi-empirical CNDO—MO calculations have been performed for these molecules, and the results are used to interpret the observed trends. In addition, a simple molecular orbital model is employed to estimate the importance of spin—orbit coupling in the valence electronic structure of the heavy IV—VI ions.     

Plasmon energy gain and double ionization satellites in the Be Auger spectrum

Two relatively weak, higher energy satellites are observed at 18 and 38 eV above the Be KVV Auger spectrum. The lower energy satellite is assigned to coupling of energy from bulk plasmon de-excitations $\text{(}\text{h}\text{?}\text{ω ～ 18 eV)}$ with Auger electrons and the higher energy event to Auger electrons ejected from Be atoms with doubly ionized K levels.     

LECTRONIC STRUCTURE STUDIES OF Bi2Sr2CaCu2-xSnxO8+δ SYSTEM

Photoemission measurements have been carried out for Bi₂Sr₂CaCu_2-xSn_xO_8+δ system with conventional x-ray photoemission spectroscopy for core-level spectra and synchrotron radiation photoemission spectroscopy for valence band. With Sn doping, all core levels shift differently in binding energy, and the intensity near fermi energy becomes smaller in valence hand. From the experiment, we can deduce that the shifts of all core levels and valence hands may involve some other mechanisms, such ms electrostatic effects, in addition to binding energy referencing effects. We argue that the chemical environment plays a crucial role in the electronic structure of high-temperature superconductors.     

Addendum

Accurate valence band dispersions E$\text{k}$ of LaB₆ have been determined along three main symmetry lines ΓM, ΓX, and XM using one single crystal (001) surface by a simple new photoemission technique of general utility. Using direct interband transitions, E$\text{k}$dispersions are mapped out by suitably scanning polar emission angle and the photon energy. Detailed results agree well with recent Xα-APW energy band calculations.     

Rule of corresponding Auger spectra

The normalized core-valence-valence Auger spectra A(?) of the simple metals Be, Mg, and A$\text{l}$, when plotted as a function of ?/2λ, where λ is the free-electron Fermi energy, are all nearly the same. Li, with λ=4.1±0.1 eV, also has the same spectrum. This suggests that the Auger spectra of simple metals depend primarily on the electron gas density and are almost independent of details of the band structure.     

Core-level satellite structure in photoemission spectra of transition metals

A theory of the satellite structure of the core-level photoemission spectrum of transition metals is presented. It is applied to the $\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level of metallic nickel. The calculation is carried out for both the paramagnetic and ferromagnetic states and is based on two scattering parameters, one intraband and one interband. Agreement with experiment is very good.     

The absorption of Ag on Ge(100)-(2 × 1)

The initial stages of interface formation for Ag deposited onto Ge(100)-(2 × 1) were studied with high-energy electron diffraction and high- resolution photoemission. The surface core-level energies for clean Ge(100)-(2 × 1) were not changed with the deposition of about one monolayer of Ag, indicating that there was no chemical reaction or atomic intermixing. The Ag nucleated at a coverage of about $\text{1}\text{3}$ monolayer and showed three-dimensional growth for higher coverages.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

An angle-resolved photoemission study of the chemisorption of chalcogens on Cu(100): II. Cu(100) + c(2 × 2)O

Starting with the three-step direct-transition model of ARPES for bulk materials, which was examined in the preceding paper, we propose a framework for describing changes in the photoemission spectra due to chemisorption. Normal emission ARPES data for Cu(100) with a c(2 × 2)O overlayer were obtained in the photon energy range hv = 11 to 34 eV. These spectra have been compared within the proposed framework with those obtained from clean Cu(100). Changes were found in the Cu emission features which could be explained by the relaxation of momentum conservation perpendicular to the surface in the optical excitation step and by the relaxation of momentum conservation parallel to the surface in the escape step. These changes include a photon energy dependent broadening of the d-band peak and the preferential attenuation of the sharp direct-transition feature associated with the sp-band. Some evidence for a surface resonance at the top of the d-bands has been obtained. Changes in the spectrum of scattered electrons were related to modifications of evanescent final states. A 1.3 eV wide band derived from the oxygen p_x,y-orbitals was deduced from spectra obtained at normal emission and along the $\text{Γ}\text{X}$ and $\text{Γ}\text{M}$ lines of the surface Brillouin zone. On the other hand, no emission was clearly detected from the oxygen p_z-orbitals. Oxygen induced emission above the Cu d-bands was observed and attributed to antibonding states. This emission was directed towards the bulk [011] directions.     

Low binding energy satellites in the 3d X-ray photoelectron spectra of rare earths (R) in RMn2Si2 and RGa2

The 3_d core-level X-ray photoelectron spectra of rare earths (R) in RMn₂Si₂ (R = Pr, Nd, Sm, Gd) and RGa₂ (R = Pr, Nd, Sm) have been recorded. A structure has been detected on the low binding energy side of each of the 3d_{$\text{5}\text{2}$} and 3d_{$\text{3}\text{2}$} photoemission peaks. The structure identified as a shake-down satellite shows no chemical shift. The energy separation of the satellite has been measured with respect to the main 3d_{$\text{5}\text{2}$} peak. The present results have been examined in the light of recent experimental and theoretical reports.     

Electronic structure of bases in DNA duplexes characterized by resonant photoemission spectroscopy near the Fermi level

The electronic structure of bases in DNA duplexes was investigated by resonant photoemission spectroscopy near the Fermi level, in order to specify charge migration mechanisms. We observed a kinetic energy shift of N-KLL Auger electrons and an intensity enhancement of valence electrons on the resonant photoemission spectra for both poly(dG).poly(dC) and poly(dA).poly(dT) DNAs. These directly show the localized unoccupied states of the bases. We conclude that the charge hopping model is pertinent for electric conduction in a DNA duplex, when electrons pass through the unoccupied states.