Core hole sites

Studies have demonstrated that the electronic relaxation depends upon the location of the core hole site and the configuration of the core hole excited state. The important feature is that the core hole is localized on a specific atom and this localization is projected onto the valence electrons in the decay process. Molecular Auger spectra thus present a view of molecular electronic structure from the perspective of particular atoms in a molecule. The spectra therefore can serve to identify particular molecules and functional groups, to distinguish between localized and delocalized bonding, and to measure orbital atomic populations for various atoms in a molecule (Rye and Houston 1984). This localization and the projection onto the valence... 

Relative Ion Yields as a Percentage of Total Yield for Different Core Hole Sites... 

The differences in the mass spectra for different core hole sites can be understood qualitatively in terms of the atomic populations and the overlap populations of the valence molecular orbitals. The atomic populations of an orbital give a quantitative measure of the electron density on a particular atom and indicate which orbitals are involved in the autoionization or Auger decay process (Jennison 1978). The overlap populations provide a quantitative measure of the bonding properties of these orbitals. Bonds will tend to be broken if the orbitals involved in the autoionization are bonding orbitals bonds will tend to remain if the orbitals are anti-bonding or nonbonding orbitals. 

The efficient screening approximation means essentially that the final state of the core, containing a hole, is a completely relaxed state relative to its immediate surround-ing In the neighbourhood of the photoemission site, the conduction electron density of charge redistributes in such a way to suit the introduction of a core in which (differently from the normal ion cores of the metal) there is one hole in a deep bound state, and one valence electron more. The effect of a deep core hole (relative to the outer electrons), may be easily described as the addition of a positive nuclear charge (as, e.g. in P-radioactive decay). Therefore, the excited core can be described as an impurity in the metal. If the normal ion core has Z nuclear charges (Z atomic number) and v outer electrons (v metallic valence) the excited core is similar to an impurity having atomic number (Z + 1) and metalhc valence (v + 1) (e.g., for La ion core in lanthanum metal, the excited core is similar to a Ce impurity). 

Figure 5 shows DOS from self-consistent field, full multiple scattering calculations as implemented in the FEFF8 code33 on the supported Ptio cluster illustrated. The support is mimicked by the 3 water molecules as shown the O atoms represent the O atoms in the support, and the H atoms effectively terminate the cluster and represent the Si or A1 atoms such as that in a zeolite. The DOS are shown from calculations with and without a core-hole on the 3 different site symmetries (surface, edge, and center). 

Figure 6 The calculated AVB (solid lines) from FEFF8 for the same 3 sites on the Ptio cluster indicated in Figure 4, obtained with (dark) and without (light) the core-hole. Also shown are the components A lo (dotted lines) and Apo% (dashed lines).

A different interpretation is favoured for the chromium on polyimide data by Jordan et al. [21], who propose that initial attack of chromium and charge transfer occurs on the carbonyl moiety. Since the charge transferred from the metal is distributed over the planar PMDA moiety of the polyimide, the core level spectra by themselves will not allow a distinction between the two models proposed. However, a careful analysis of the shake up features in the Cls, Ols and Nls core hole spectra might reveal the initial binding site when the amplitude distributions of the LUMO s and HOMO S in the system relative to the created core hole are considered. 

The following picture is emerging from this research. Core electron excited states have finite lifetimes and unique characters. The electronic and chemical relaxations are coupled and depend upon the atomic and electronic structure of the molecule, the atomic site of the core hole, and the configuration of the core hole excited state. Experiments leading to an understanding of these dependences are just beginning to be done. 

In cases where the yield of molecular ions is higher than 10% and where the fragmentation pattern depends upon the atomic site of the core hole, the dissociation processes clearly depend upon the electronic structure of the molecule and the details of the electronic relaxation, i.e. not all pathways produce essentially the same result. The mechanism then may involve vibrational dissociation or electronic or vibrational predissociation as well as direct dissociation. Even in these cases, some of the electronic relaxation channels may rupture all the bonds in a molecule and high-kinetic-energy fragments can be produced. Such channels sometimes are labeled a Coulomb explosion, but this terminology should not be confused with the more specific use of the term that is proposed above. 

The first evidence that fragmentation of a molecule in the gas phase could be localized to some extent around the site of the core hole was reported for acetone (Eberhardt et al. 1983a). Partial ion yield spectra were obtained, and the pi resonance was a prominent feature in only the and spectra. The pi resonance is associated with excitation of the Is electrons of the carbon atom in the carbonyl group. The Is to pi excitation therefore appears to cause the molecule to break around the CO group, producing C, O, and neutral CH3 groups. 

It is clear that the chemical reactions induced by core electron excitation are atom-specific. They generally depend upon the atomic site of the core hole and the electronic configuration of the core hole excited state. In this sense, the chemistry is selective. The chemistry can be selected by tuning the photon energy to produce particular core hole excited states. In another... 

The shift in measured by XPS core level binding energies between a rare gas such as xenon in the gas phase and adsorbed on a surface results from a combination of chemical shift, local potential at the site of the adsorbate, and stabilization of the photoionization core hole by polarization of the substrate electron density. As discussed in reference (13), the contribution due to substrate polarization is related to the surface electronic polarizability and can be isolated from the other contributions to a good approximation by measurements of the xenon gas phase and adsorbed phase "Auger parameter", a. a is defined as the difference between the (Jkl) core level Auger electron kinetic energy, K (jkl), and the (j) core level photoelectron kinetic energy, K (J). 

These mixed-valenee systems have been discussed by several authors with respect to final state effects in their core level spectra [12,35]. A distinction is generally made between the cases where the extra electrons occur as itinerant conduction band electrons (metallic case) or whether they are completely localized to single sites. In the former case, the core ionization of one site will in itself lead to the creation of localized levels (by pulling down from the conduction band), whose occupancy in the final state of the ionization process will depend statistically on the conduction electron density. Thus, the final state localized level may be either filled or empty. The net result is a loss of direct correlation between observed relative peak intensities and the number of inserted electrons per formula unit (x value), since the population of the two possible core-hole states is considered to be entirely a final state phenomenon. On the other hand, for the case of complete single-site localization (non-metaUic case) the relative intensities are expected to truly represent the relative number of the two possible valence states before ionization and will not be affected by effects due to the final state of the core ionization process. 

A brief final comment should be added concerning X-ray emission spectroscopy. The physical basis of X-ray emission is more complex than the absorption one, but in the context of catalysis, its application is mainly restricted to the obtention of high resolution XANES spectra, not limited by the core-hole broadening effect of the electron excited. This implies the measurement with high energy resolution (using secondary monochromators) of specific radiative decay paths. As an example of use within the field of catalysis, we mention that this approach has been used to attempt to distinguish CO absorption site (e.g. on-top, bride, or three-fold) on nanosized Pt clusters supported on alumina, methane partial oxidation with Pd catalysts, or CO oxidation with Pt. ... 

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