Core-hole Screening Research Articles

Shake-up excitations and core-hole screening in Eu systems

X-ray photoemission (XPS) spectra of the Eu $3d$ and Eu $4d$ core levels in Eu metal and in divalent and trivalent Eu compounds exhibit splittings into two components, caused by the occurrence of $4{f}^{6}$ and $4{f}^{7}$ final-state configurations. The observed phenomena are described in terms of final-state shake-up and shake-down processes accompanied by a decrease, respectively, increase of the $4f$ occupation number. An extension of the Kotani-Toyozawa model to $f$ and $d$ states gives a consistent description of both processes.

Dynamic screening of a core hole. II. Application to EXAFS

For pt.I see ibid., vol.9, p.1189 (1979). The dynamic screening model describing the relaxation of the conduction electrons after the absorption of an X-ray photon in a simple metal is extended to the physical situation of EXAFS. It is used to calculate the potential of the central atom as a function of the velocity of the photoelectron and to derive the corresponding phaseshifts delta l. They show a transition between a neutral-like behaviour at low energy and an ionic-like one at high energy. This transition takes place in the whole energy range (above about 1 keV) and not only around the threshold for plasmon emission. The authors deduce from these results the accuracy of the determination of distances that can be expected when using ionic phase-shifts in a metal. This precision is found to depend on two parameters: (i) the distance of the neighbouring atom; and (ii) the energy Emax of the higher part of the EXAFS spectrum. The problem of the transferability of phaseshifts is thus discussed in a more quantitative way. In addition, they obtain the reduction of the amplitude of the EXAFS oscillations due to plasmon losses; this amplitude is greater than the one which is predicted by the mean free path factor e-2R lambda /, because of the transient processes of the screening mechanism.

XPS/UPS study of the oxidation of La and LaNi 5 and of the electronic structure of LaNi 5

The VB and the core levels of clean and oxygen exposed bulk samples of La and LaNi 5 were investigated by means of photoelectron spectroscopy using MgKα (XPS) and HeII (UPS) radiation. The HeII VB of La only shows 1.3 eV FWHM. The VB of LaNi 5 is mainly derived from the VB of Ni. The low binding energy satellite of the La3d core level (4f 1 final state) is much stronger in LaNi 5 than in La, reflecting the better screening of the 3d core-hole by filling of f states. The spectra indicate clearly that La oxidizes and not Ni upon the exposure of LaNi 5 to small amounts of oxygen.

Interaction between Closed-Shell Systems and Metal Surfaces

The relationship between the van der Waals description of the binding of rare-gas atoms to metal surfaces and the description using a local approximation for exchange-correlation effects is discussed. The local-density treatment is shown to provide a good account of recent data on atomic binding energy, dipole moment, and core-level binding-energy shift. Charge rearrangement maps are used to analyze bond formation and to discuss the validity of the image picture of core-hole screening.

Core-electron excitation edges in metallic Ni, Cu, Pt, and Au

The shapes of core excitation edges with ${E}_{B}<150$ eV are reported for Ni, Cu, Pt, and Au. The spectra are interpreted as arising from the interference of excitations of core electrons into conduction-band states of $d$ symmetry with excitations of $d$ electrons into continuum states of $f$ symmetry well above the vacuum level. In all cases excitation of core electrons into $d$ conduction-band states hybridized into the $\mathrm{sp}$ conduction bands is important, but in the metals Ni and Pt, which have partially filled $d$ bands, the excitation of core electrons into unhybridized $d$ states outweighs the contributions of the hybridized $d$ states at the Fermi edge. The line shapes due to the spectral densities resulting from excitations into the unhybridized part are accurately described by Fano interferences due to configuration interaction between a spin-orbit component of a core-hole state and the $d$-electron continua of partial waves indexed by the same total angular momentum. Off-diagonal interactions between the core-hole spin-orbit components appear to be unimportant, as are contributions from many-body screening effects. No theory is available for the interference involving the hybridized $d$-band states. The energies of the edges in electron-energy-loss or x-ray-absorption measurements are shown to be the same within experimental error (about 0.2 eV) as the corresponding values of fully screened core-electron binding energies referenced to the Fermi level. These latter values were determined by Wertheim and coworkers, who fitted the threshold peaks in x-ray-photoemission spectra with a many-body screening theory. It is shown that such agreement between the core-excitation thresholds measured by electron-energy loss or by x-ray absorption and the fully screened binding energies implies that the screening of core holes in Ni and Pt is due almost entirely to the $d$ conduction electrons.

Initial-state screening effects in metal Auger spectra: Be

The Auger spectrum of beryllium metal is presented and analyzed. This analysis suggests that the valenceband screening of the initial-state core hole alters the observed line shape from that predicted on the basis of band structure alone. Decomposition of the spectrum into components describing the angular momenta of the final-state holes indicates that the screening charge is largely $s$-like. A rigorous description of the screening-charge effect is presented together with an approximate theory which preserves the analysis in terms of band structure. The approximate theory is in qualitative agreement with the results of ab initio calculations on a 13 atom cluster.

Bonding, shake-up energies and shake-up intensities in Cr(NO)4

We report the results of SCF-xα-multiple scattering calculations of the electronic structure of Cr(NO)4 in its neutral ground state and in several positive-ion states. This allows a description of the bonding of NO to the Cr atom, explains how the screening of core holes (created in a XPS measurement) is effected and yields the N1S shake-up spectrum. There is good agreement with the experimental valence region photoemission spectra peak positions and with the shake-up peak positions and intensities.

Anomalous auger shifts in chemisorption on tungsten

Substrate Auger line energy shifts for monolayer gas adsorption are readily detected. The observed shifts in tungsten are incompatible with predicted chemical-shift trends and hence relaxation effects must be considered. It is proposed that transition-metal s electrons play a decisive role in core-hole screening. The measurement of substrate Auger line energy shifts on gas adsorption may thus provide information on the contribution of these electrons to chemisorption.

Dynamic screening of core hole. I. Semiclassical model

Creation of a deep hole and a fast electron in a metal by an X-ray photon produces a dynamic relaxation of the surrounding electrons of the Fermi sea which is treated here in a semiclassical way. The authors emphasise the contribution of the plasmons which should be predominant far from threshold. They construct an energy-dependent effective potential acting on the photoelectron. They show the existence of a characteristic time t0 (a fraction of the plasmon period) necessary for the creation of plasmons: at a given distance r from the hole, the potential seen by the photoelectron of velocity nu is a statically screened potential when r/ nu >t0; when r/ nu <t0 it presents a percentage of ionic character which is calculated. Finally, they discuss the notions of mean free path and effective time of transition.

Core-hole screening in lanthanide metals

The multicomponent structure of the $3d$ x-ray photoemission spectra of the light rare-earth metals is elucidated in terms of recent x-ray absorption and appearance-potential spectroscopy results for these materials. Both multiplet splitting due to $3d\ensuremath{-}4f$ exchange and satellite structure due to two competing screening mechanisms are identified. The main component of the $3d$ line arises when the core hole is screened by $5d$ conduction electrons; the satellite from a $4{f}^{n}\ensuremath{\rightarrow}4{f}^{n+1}$ process. The satellites in the metals occur at lower binding energies because the empty $4f$ level falls below the Fermi energy. This interpretation is supported by an "equivalent cores" argument.

Core level shifts in the3dtransition metals and tin

We present estimates of various contributions to the free-atom-metal binding-energy shifts, $\ensuremath{\Delta}{E}_{B}$, for the $2s$ and $2p$ core levels of the iron series elements and, for comparison, the $3{d}_{\frac{5}{2}}$ level of Sn. Our results indicate that the sharp break in the experimental $\ensuremath{\Delta}{E}_{B}$ values occurring between Ni and Cu is due to variation in differences of $d$-electron count, i.e., $d$ configuration, between atom and metal. Ley et al. who neglected the $d$-configuration differences, attributed the break in $\ensuremath{\Delta}{E}_{B}$ to a change in extra-atomic screening. Through free-atom calculations we find extra-atomic screening energies of 6-8 eV for Ti through Zn, with no break between Ni and Cu, and \ensuremath{\sim} 10 eV for Sn. In conjunction with charge-renormalization and dipole-term estimates, these results suggest that intra-atomic screening of core holes in metals is an important factor influencing free-atom-metal binding-energy shifts.