Core-state excitation

Core hole, 34 210 core-hole lifetime, 34 215 Core level shift, C(ls), 29 13-14 Core-state excitation, 34 204 Correlation data, structure effects, 29 159-160 Correlations, adsorptivity, 29 189-190 Co9Sg, structure, 40 222 CoSiOj powders, Fischer-Tropsch synthesis, 39 288-289... 

As the region near an X-ray absorption edge is scanned in energy, the ejected photoelectron sequentially probes the empty electronic levels of the material. Theoretically, interest in core-state excitation has developed considerably since the work of Mahan (179) and Nozieres and De Dominicis (219) on the singular response of the conduction electrons (in metals) to the sudden potential created by removal of a core electron. The resulting electron-hole pair excitations lead to a threshold edge asymmetry. 

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

To determine the BEs (Eq. 1) of different electrons in the atom by XPS, one measures the KE of the ejected electrons, knowing the excitation energy, hv, and the work function, electronic structure of the solid, consisting of both localized core states (core line spectra) and delocalized valence states (valence band spectra) can be mapped. The information is element-specific, quantitative, and chemically sensitive. Core line spectra consist of discrete peaks representing orbital BE values, which depend on the chemical environment of a particular element, and whose intensity depends on the concentration of the element. Valence band spectra consist of electronic states associated with bonding interactions between the... 

In the levels for which the oscillator strengths are shown in Tables 4 to 6 the atomic core state is the excited, D, whilst in the remaining tables the levels correspond to the ground state of the core, P. After a careful inspection of the tables a few remarks can... 

In inverse photoemission (and in case of high electron excitation energy) a final core state containing an additional electron (instead of a hole) may be created (for f-states fN -> f " ). Thus, and in the same hnes as for photoemission, a multiphcity of non... 

The model employed for the core-level excitation runs as follows a) the process of photoemission is a two-step process one electron is photoemitted from a conduction state at the Fermi level, and one electron is promoted from a core to a valence state of the solid ... 

Good examples are the core hole excited states of homonuclear molecules. When one electron is removed from a core orbital, the original Dooh symmetry is lowered to C v The D h group can be decomposed into two CooV components related by a C, or Cs operation, so it is fair to consider that the core-hole excited states are described by resonance between the two structures. The adiabatic subsystems have, by definition, zero overlap in the real space. Their interaction is defined only in complex space through the explicit overlap between the many-electron states. 

The proper theoretical study of core-hole excited states has been one of the most challenging problems in molecular quantum mechanics. The difficulty stems from the fact that these states lie above the continuum part of the spectra, and their attainment as high-energy roots in an ordinary configuration interaction calculation is impossible [46]. However, core-hole excited states play an important role in the identification of chemical species by X-ray based spectroscopic techniques. Additionally, there are many interesting effects that are particular to this region of the spectra and their study are the object of active research [47]. 

AES Auger Electron Spectroscopy Core-hole excitations are created, usually by 1-10 KeV incident electrons, and Auger electrons of characteristic energies are emitted through a two-electron process as excited atoms decay to their ground state. AES gives information on the nearsurface chemical composition. 

The calculation of PAES intensities largely reduces to the calculation core annihilation probabilities for positrons in the surface state [11]. This follows from the fact that almost all of the core hole excitations of the outer cores relax via Auger emission and that almost all of the positrons incident at low energies become trapped in a surface state before annihilation. First-principles calculations of the positron states and positron annihilation characteristics at metal and semiconductor surfaces are based on a treatment of a positron as a single charged particle trapped in a "correlation well" in the proximity of surface atoms. The calculations were performed within a modified superimposed-atom method using the corrugated-mirror model of Nieminen and Puska [12]. 

The interpretation of the X-ray spectra has also caused problems. The spectra have been determined from experiments in which an X-ray is absorbed, lifting an electron from a core state to some excited state. That excited state may be in the conduction band or it may be a lower energy state with the electron bound to the core hole. Different workers have interpreted the same peaks in different ways, leaving the field explained unsatisfactorily. Recently, Pantelides (1975b) included... 

In earlier papers [6-8] we have proposed a procedure for evaluating core ioniza-tion/excitation chemical shifts in molecules from computed core ionization/excitation energies for the relevant isolated atom in neutral and valence-ionized states and from computed charge transfer relative to this atom within the molecule. The atomic calculations involved relaxation, possibly correlation and, when appropriate, relativity and other effects, while in the molecule one could use any approximate method (possibly involving effective core potentials) yielding reliable charges. 

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