Core-hole Screening Research Articles

Density-functional theory calculations of the electron energy-loss near-edge structure of Li-intercalated graphite

We have studied the structural and electronic properties of lithium-intercalated graphite (LIG) for various Li content. Atomic relaxation shows that Li above the center of the carbon hexagon in a AAAA stacked graphite is the only stable Li configuration in stage 1 intercalated graphite. Lithium and Carbon 1 s energy-loss near-edge structure (ELNES) calculations are performed on the Li-intercalated graphite using the core-excited density-functional theory formulation. Several features of the Li 1 s ELNES are correlated with reported experimental features. The ELNES spectra of Li is found to be electron beam orientation sensitive and this property is used to assign the origin of the various Li 1 s ELNES features. Information about core-hole screening by the valence electrons and charge transfer in the LIG systems is obtained from the C 1 s ELNES and valence charge density difference calculations, respectively.

Core-Hole Screening as a Probe for a Metal-to-Nonmetal Transition in Lead Clusters

Metal clusters serve as model systems to study basic problems of electronic correlation. Vacuum ultraviolet light from the free-electron laser FLASH ionizes 5d electrons from mass-separated negatively charged clusters, thus transiently leading to core-ionized neutral systems. Shielding of the core hole affects the electron binding energy. From the strong deviation from expectations of the metallic droplet and jellium models we conclude on reduced electronic shielding once the cluster size falls below about 20 atoms. This suggests a metal-to-nonmetal transition, in agreement with previous local density approximation calculations.

Charge transfer and polarization screening in organic thin films: phthalocyanines on Au(100)

Core hole screening effects at organic/metal interfaces were studied by core level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). The comparison of energetic shifts in XPS and XAES enables the estimation of electronic relaxation energy (screening ability). Magnesium phthalocyanine (MgPc) and zinc phthalocyanine (ZnPc) evaporated on single crystalline Au(100) were used as model molecules. Two different features in the Mg KLL spectra can be clearly separated for (sub-)monolayer coverages, while only minor changes of the shape of Mg 1s are observed. Applying a dielectric continuum model, the major screening mechanism cannot be described sufficiently by polarization screening due to mirror charges, significant contributions by charge transfer screening have to be considered. In contrast, small screening effects in the bulk material can be explained by surface polarization.

Electronic Structure of Nd2-xCexCuO4 and La2-xSrxCuO4 Deduced from Cu 2 p Photoemission and Photoabsorption

The electronic structures of Nd 2- x Ce x CuO 4 and La 2- x Sr x CuO 4 are studied using numerical diagonalization calculations for cluster models, Cu 5 O 16 , Cu 7 O 20 , and Cu 8 O 16 . The Fermi energy of Nd 2 CuO 4 used in the experiment is assumed to be pinned at the bottom of the upper Hubbard band, while that of La 2 CuO 4 is assumed to be pinned at the top of the Zhang–Rice singlet band. This assumption enables us to consistently understand the doping dependence of the Cu 2 p photoemission and photoabsorption spectra in Nd 2- x Ce x CuO 4 and La 2- x Sr x CuO 4 on the basis of the so-called “crossing the gap” picture. The present calculation of Cu 2 p photoemission spectrum also shows that core hole screening caused by charge transfer between neighboring CuO 4 plaquettes, in other words, nonlocal screening (NLS), is important in Nd 2 CuO 4 as well as in La 2 CuO 4 .

X-ray photoemission studies on rare gas bubbles in aluminium with annealing temperature

The annealing temperature dependence of the core-level spectra of the rare gas bubbles implanted in the sub-surface region of Al(111) has been investigated by x-ray photoelectron spectroscopy. A systematic change in the binding energy of the rare gas core-level has been observed, as a function of annealing temperature. The change in binding energy is explained by the increase in bubble size with annealing temperature. Al conduction electron screening of the rare gas core-hole decreases in the photoemission final state with increase in bubble size. We have also observed the decrease in concentration of the rare gas with annealing temperature. The Ne 1s spectra show two components separated by about 1.2 eV and change in intensity with temperature, which is possibly related to a near-surface and a sub-surface distribution of Ne bubbles. The near-surface component gets desorbed faster than the sub-surface component with annealing.

Surface chemistry of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru on polycrystalline Ta

The adsorption and desorption of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru [DER] on polycrystalline Ta have been studied by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). DER exposures to Ta at 140 K result in primarily molecular adsorption and desorption, while a minor surface reaction occurs at defect sites. Monolayer DER desorbs between 278 and 297 K with increasing coverage, exhibiting a first order, zero coverage desorption energy of 2.3 eV. Multilayer DER desorbs between 272 and 263 K, most likely with fractional order kinetics, and exhibits a zero coverage desorption energy of 0.9 eV. XPS Ru 3d binding energies increase with increasing coverage due to core hole screening in the monolayer regime and increasing sample charging as the DER overlayer becomes thicker in the multilayer regime. DER exhibits a three-dimensional (3D) “hit and stick” growth mode in which random 3D structures form due to the lack of adsorbate mobility at 140 K. DER exposures to Ta between 298 and 773 K result in minor decomposition resulting primarily in adsorbed hydrocarbon species on the surface. When the Ta is pre-covered with atomic iodine, DER dissociation is significantly decreased while adsorption is increased.

Chemical potential shift ofFe3−xVxSistudied by hard x-ray photoemission

Core-level photoemission spectra of ${\text{Fe}}_{3\ensuremath{-}x}{\text{V}}_{x}\text{Si}$ alloys with inequivalent ${\text{Fe}}_{\text{I}}$ and ${\text{Fe}}_{\text{II}}$ sites are investigated via hard x-ray photoemission spectroscopy over the entire doping range $x=0--1$. All the measured $1s$ core-level peaks are found to shift to higher binding energy with increasing V concentration. First-principles, all electron charge- and spin-self-consistent electronic structure computations within the framework of the local-spin-density approximation are used to interpret the experimental results. The measured size of energy shift in going from $x=0$ to 1 is consistent with the corresponding theoretical value for the ${\text{Fe}}_{\text{II}}$ and $\text{Si}\text{ }1s$ core levels, whereas for the ${\text{Fe}}_{\text{I}}$ and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a ``pseudogap'' or a region of reduced density of states in the vicinity of the Fermi energy.

Charge Transfer and Polarization Screening at Organic/Metal Interfaces: Distinguishing between the First Layer and Thin Films

We studied the core hole screening at organic/metal interfaces using combined photoemission spectroscopy (PES) and X-ray excited Auger electron spectroscopy (XAES) as a function of the organic layer thickness. As a model system for organic semiconductor/metal interfaces, magnesium phthalocyanine was evaporated onto gold foil. It was found that the screening of double hole final states is remarkably increased for molecules directly at the interface, whereas further layers are effected very weakly. The screening mechanism is discussed in terms of both charge transfer and electronic polarization.

Singularities in core-level spectra near metallic surfaces

Calculations of the screening shift and dynamical singularity index of the structureless core hole placed near and inside the metallic slab are presented, where the screening mechanism is described using the quantum-mechanical response function calculated in the local density approximation. Core-hole screening shifts show oscillations which are compared with the experimental results. Spatial dependence of the static singularity index, besides experimentally observed enhancement at the surface, shows some new features (bulk oscillations) which were not yet experimentally observed. The dynamical singularity index is used to calculate complete core hole spectrum, including singular line shapes and bulk and surface plasmon losses, and the results explain very well some observed features in the experimental photoemission spectra.

An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films

Crystalline and amorphous nanoparticles of silicon in thin silica layers were examined by transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy (XPS). We used XPS data in the form of the Auger parameter to separate initial and final state contributions to the Si2p energy shift. The electrostatic charging and electron screening issues as well as initial state effects were also addressed. We show that the chemical shift in the nanocrystals is determined by initial state rather than final state effects, and that the electron screening of silicon core holes in nanocrystals dispersed in SiO2 is inferior to that in pure bulk Si.

First-principles Analysis of Core Level Shifts

We review first-principles theoretical calculations of core level shifts including the final state core hole screening effect. By using pseudopotentials with core holes, it is possible to calculate accurately the shifts of core levels which vary depending on their chemical environments. We point out the validity and limitation of the scheme by showing some examples.

Dynamical core-hole screening in the x-ray absorption spectra of hydrogenated carbon nanotubes and graphene

We have calculated the electronic structure and the x-ray absorption (XA) spectrum of a hydrogenated single graphite plane, in order to simulate recent experimental results on hydrogenated single wall carbon nanotubes (SWCNT) as well as hydrogenated graphene. We find that the presence of H induces a substantial component of $s{p}^{3}$ bonding and as a result the $\ensuremath{\pi}$ and ${\ensuremath{\pi}}^{*}$ components to the electronic structure vanish. We have calculated a theoretical x-ray absorption spectrum using a multiband version of the Mahan-Nozi\`eres--De Dominicis theory. By making a fitting of the XA signal of C atoms that have H attached to them and C atoms without H in the vicinity we obtain a good representation of the experimental data and we can draw the conclusion that in the experiments [A. Nikitin et al., Phys. Rev. Lett. 95, 225507 (2005)] some 35--50 % H have been absorbed in the SWCNT.

Sensitivity to hole doping of CuL3resonant spectroscopies: Inelastic x-ray scattering and photoemission ofLa2−xSrxCuO4

We present Cu ${L}_{3}$ edge resonant inelastic x-ray scattering (RIXS) and resonant x-ray photoemission spectroscopy (RXPS) of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ ($x=0.0$, 0.13, and 0.27). RIXS is found to be much more sensitive to the hole doping level than resonant photoemission. RIXS allows us to determine the average energy of $dd$ excitations of both undoped and hole doped sites. The trade-off between the Raman and non-Raman spectral features depends on the doping level strongly in RIXS but weakly in RXPS. These experimental findings are consistently interpreted within a cluster model: The role of nonlocal core hole screening in the intermediate state is shown to be crucial. Nonlocal screening has an impact also on the interpretation of the ${L}_{3}$ x-ray absorption spectra in cuprates.

Al Kα and Cu Kα 1 excited XPS of vanadium oxide and VF 3 powders: Measurement of the V 1s – KLL Auger parameters

Al Kα ( hν = 1486.6 eV) excited XPS shows that powder samples of V 2O 3, V 2O 5 and VF 3 are surface contaminated and that the V 2O 3 can be cleaned by heating in vacuum at 400 °C. The greater sampling depth of Cu Kα 1 ( hν = 8047.8 eV) excited XPS allows measurement of the bulk V 1s – KL 2L 3 Auger parameters (APs) for these materials. The APs of VF 3 and V 2O 5, relative to V metal, fall into the range of values expected for metal fluorides and oxides with non-local final state core-hole screening, whereas the AP of V 2O 3 is significantly closer to that of V metal. We ascribe this to a greater final state valence orbital occupation following photionisation in V 2O 3, part of which results from metal-like screening.

Manifestation of screening effects andA−Ocovalency in the core level spectra ofAsite elements in theABO3structure ofCa1−xSrxRuO3

We investigate the evolution of $\mathrm{Ca}\phantom{\rule{0.2em}{0ex}}2p$ and $\mathrm{Sr}\phantom{\rule{0.2em}{0ex}}3d$ core level spectra in ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ru}{\mathrm{O}}_{3}$ using photoemission spectroscopy. Core level spectra in this system exhibit multiple features and unusual evolution with the composition and temperatures. Analysis of the core level spectra in conjunction with the band structure results indicates final state effects due to different core hole screening channels. Such screening in the photoemission final states can be attributed to the large $A\text{\ensuremath{-}}\mathrm{O}$ covalency in these systems. Changes in the core level spectra with temperature and composition suggest significant modification in $A\text{\ensuremath{-}}\mathrm{O}$ $(A=\mathrm{Ca}∕\mathrm{Sr})$ covalency in Ca-dominated samples, which gradually reduces with the increase in Sr content and becomes insignificant in $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$. This study thus provides a direct evidence of cation-oxygen covalency and its evolution with temperature, which may be useful in understanding the unusual ground state properties of these materials.

Photon energy dependence of final state screening in a dilute electron gas system: A synchrotron radiation photoemission study of β-PbO 2

Photoemission spectra of thick films of β-PbO 2 have been measured over a range of exciting photon energies between 40 eV and 150 eV. There is evidence for occupation of conduction band states above the top of the main valence band to give a degenerate electron gas and the spectra terminate in a sharp metallic Fermi edge. Structure associated with screening of shallow 5d core holes by the mobile conduction electrons observed in Al Kα excited X-ray photoemission spectra is suppressed in the lower energy spectra and the Pb 5d core level appears as a simple spin-orbit doublet.

Resonant inelastic x-ray scattering from magnetic systems with angular resolution and polarization analysis of the scattered beam: Results on metallic Co, Fe, and Co ferrite at theL3,2edges

We present experimental results on resonant inelastic x-ray scattering (RIXS) in perpendicular geometry excited with circularly polarized x rays at the Co ${L}_{3,2}$ edges in Co ferrite $(\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4})$ and Co metal and at the Fe ${L}_{3,2}$ edges in Fe metal. In this geometry with magnetic samples the incident x-ray beam is perpendicular to the magnetization and the magnetic dichroism is only present in RIXS. We study the $3s$-RIXS channel terminating in an inner shell $3s$ hole and extra $3d$ electron $(2{p}^{6}3{d}^{n}\ensuremath{\rightarrow}2{p}^{5}3{d}^{n+1}\ensuremath{\rightarrow}2{p}^{6}3{s}^{1}3{d}^{n+1})$. The work is based on the measurement of the spectra vs incident photon energy and integrated over the scattered photon energy. We present data resolved in linear polarization of the scattered beam for scattering at 45\ifmmode^\circ\else\textdegree\fi{} and data summed over the outgoing polarizations and taken vs the scattering angle $\ensuremath{\varphi}$. The results show common features in both metals and ionic system: the dichroism scales as $\mathrm{sin}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\varphi}$ while the spectral shape remains constant. The dichroism is maximum when the outgoing linear polarization is in the scattering plane and zero when the outgoing linear polarization is perpendicular to the scattering plane. The presence of the dichroism is due to the breaking of the mirror symmetry of the geometry consisting of the helicity vector of the incident radiation, the sample magnetization, and the linear polarization of the scattered beam. This is expected, in general, when the scattering occurs without rearrangement in the intermediate state. It is noteworthy that this occurs also in the metallic systems, where the scattering takes place with rearrangement due to a spin-dependent screening of the intermediate state core hole. This shows that this rearrangement does not require a more complex vector system in its description. The results show the relation between the selection of the scattering angle and the measurement of the polarization of the scattered radiation. We also present results from a geometry where the dichroism is absent in the emission integrated over outgoing polarization but where it is observed when this polarization is selected.

Study of PEDOT:PSS-SnO2 nanocomposite film as an anode for polymer electronics

Poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate)(PEDOT:PSS) is a candidate material for applications in molecular electronics, such as organic field effect devices, organic photovoltaics, and organic light emitting devices. The properties of 3.5–4.0 nm sized SnO2 nanoparticles doped PEDOT:PSS films were investigated for anode application. Sheet resistance was decreased and rms roughness was slightly increased with the incorporation of SnO2 nanoparticles. However, the connectivity of conducting grains was improved by the plasticizing effect of surface –OH groups of SnO2 nanoparticle. Using photoemission spectroscopy and near edge X-ray absorption fine structure (NEXAFS), the electronic structure of the films is studied comparatively on the C 1s NEXAFS, secondary electron emission cut off, and valence band spectra. The start of electron emission retarded and valence band maximum was increased in the PEDOT:PSS-SnO2 nanocomposite films. These changes in the electronic structure resulted from emitted electron screening of core-hole in the PEDOT:PSS energy band and electron donation of SnO2 nanoparticles.

Large magnetic circular dichroism in resonant inelastic x-ray scattering at the MnL-edge of Mn-Zn ferrite

We report resonant inelastic x-ray scattering (RIXS) excited by circularly polarized x-rays on Mn-Zn ferrite at the Mn L2,3-resonances. We demonstrate that crystal field excitations, as expected for localized systems, dominate the RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms of spin-resolved partial density of states, which has been the standard approach for RIXS dichroism. We observe large dichroic RIXS at the L2-resonance which we attribute to the absence of metallic core hole screening in the insulating Mn-ferrite. On the other hand, reduced L3-RIXS dichroism is interpreted as an effect of longer scattering time that enables spin-lattice core hole relaxation via magnons and phonons occurring on a femtosecond time scale.

Competition between relaxation and decay of a core-level electron ionized state in a charge transfer system

In a weak charge transfer (CT) system, such as an adsorbate–substrate system a core-level electron ionized state in an adsorbate observed by photoelectron spectroscopy (PES) as the smallest binding energy state, may be a metastable state. On time scale of core–hole decay it may relax further to the most stable state by CT core–hole screening via transfer of an electron from the substrate to an unoccupied adsorbate discrete level pulled down below the Fermi level of the substrate by an attractive core–hole potential. The latter state is a CT shakedown state, which is not observable by PES but its presence is visible in a decay spectrum, such as Auger-electron spectroscopy (AES) spectrum. The CT shakedown state resembles a resonantly excited core–hole state lying below the core–hole state of the smallest binding energy. If this is so, the AES spectrum measured at far above the core-level electron ionization threshold has then an additional spectral feature, which resembles the autoionization spectrum by the core–hole decay of the resonantly excited core–hole state. Such an example is the N 1s AES spectrum of N 2 molecule physisorbed on graphite reported by Björneholm et al. [Q. Björneholm, A. Nilsson, A. Sandell, B. Hernnäs, N. Mårtensson, Phys. Rev. Lett. 68 (1992) 1892]. The competition between relaxation and core–hole decay of a core-level electron ionized state in a weak CT system is formulated by a many-body theory and discussed. The present theory provides a theoretical foundation of the interpretation by Björneholm et al. They determined the CT time from the intensity ratio of the AES spectrum by the decay of the CT shakedown state to that by the decay of the core–hole state observed by PES as the smallest binding energy state. However, the CT time cannot be determined, unless the branching ratio of the former decay rate is equal to that of the latter one.