Core excitations

Dissociative electron attachment (DEA) occurs when the molecular transient anion state is dissociative in the Franck-Condon (FC) region, the localization time is of the order of or larger than the time required for dissociation along a particular nuclear coordinate, and one of the resulting fragments has positive electron affinity. In this case, a stable atomic or molecular anion is formed along with one or more neutral species. Dissociative electron attachment usually occurs via the formation of core-excited resonances since these possess sufficiently long lifetimes to allow for dissociation of the anion before autoionization. 

From a theoretical point of view XES furthermore provides a very strong basis for the evaluation of methods for population analysis, i.e., the decomposition of the molecular orbitals into atomic contributions [10]. Many different schemes subdividing the charge density into contributions assigned to respective atoms have been proposed, but the lack of means to directly measure the atomic populations in different orbitals has made all techniques somewhat arbitrary and a matter of taste. Due to the strongly localized character of the intermediate core-excited state, however, in combination with the direct dependence of the XES transition probability on the amount of local -population (assuming a Is core hole), XES provides a very sensitive tool to directly measure this atomic population. In all, we have an atom-specific tool, which can be used to address important questions regarding... 

XAS, on the other hand has a core-excited final state for which the effect of the core-hole must be taken into account. To obtain the full spectrum, i.e., valence, Rydberg and continuum excitations, we use the Slater transition-state approach [22,23] with a half-occupied core-hole. This provides a balanced description of both initial and final states allowing the same orbitals to be used to describe both initial and final states and all transitions are obtained in one calculation [23,24]. Details of the computational procedure can be found in the original papers as referenced in the following sections. In the present chapter, the focus is on the surface chemical bond and the spectra, measured or calculated, will mainly be used to obtain the required information on the electronic structure. 

Core excitation spectroscopy of such cyclic diaminocai benes, -silylenes and -germylenes I--------------1... 

The paper by Jungen and Atabek [10] also contains a sketch of the extension of the theory to include additional purely electronic interactions, such as the mixing of different partial waves l by the nonspherical field of the molecular core and/or interactions with core-excited Rydberg channels. This was not implemented at the time, however. 

A laser method which has been used to study autoionizing states of alkaline earth atoms is the isolated core excitation (ICE) method first used by Cooke etal. to study the autoionizing 5pn( states of Sr.11 It has since been used to study autoionizing states of Mg, Ca, and Ba as well.12-14... 

Using resonant effects in core-level spectroscopic investigations of model chromophore adsorbates, such as bi-isonicotinic acid, on metal-oxide surfaces under UHV condition, even faster injection times have been tentatively proposed [85]. The injection time is observed to be comparable to the core-hole decay time of ca. 5 fs. It is also possible to resolve different injection times for different adsorbate electronic excited states with this technique. While the core-excitations themselves provide a perturbation to the system, and it cannot be ruled out that this influences the detailed interactions, the studies provide some of the first local molecular, state-specific injection time analysis with good temporal resolution in the low femtosecond regime. The results provide information about which factors determine the injection time on a molecular level. 

Apen,E.,Hitchcock,A.P.,and Gland, J. L. (1993). Experimental studies of the core excitation of imidazole, 4,5-dicyanoimidazole, and s-triazine. J. Phys. Chem. 97, 6859-6866. 

Hitchcock, A. P., Urquhart, S. G., and Rightor, E. G. (1992). Inner shell spectroscopy of benz-aldehyde, terephthalaldehyde, ethylbenzoate, terephthaloyl chloride, and phosgene Models for core excitation of poly(ethylene terephthalate). J. Phys. Chem. 96,... 

Ishii, I., and Hitchcock, A. P. (1987). A quantitative experimental study of the core excited electronic states of foramide, formic acid, and formyl fuoride. J. Chem. Phys. 87, 830-839. 

Measurements on enhanced El deexcitation transitions in light and heavy nuclei are discussed. In light nuclei these appear only from core-excited... 

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