Resonances core hole excited states

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. 

Autoionization spectra resulting from specific resonances can be obtained by electron-electron coincidence measurements (Haak et al. 1984 Ungier and Thomas 1983, 1984, 1985). To associate a fr.rgmentation pattern with a particular core hole excited state and a particular autoionization or Auger decay channel, a double-coincidence experiment must be done using electron impact excitation. The energy of the scattered electron must be determined, the energy of the emitted electron must be detennined, and the ions produced in coincidence with these two events must be determined. The difficulties inherent in these kinds of experiments have been aptly summarized by Hitchcock (1989), If you can do it by photons, don t waste your time with electron-coincidence techniques. ... 

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. 

It is clear that a core-hole represents a very interesting example of an unstable state in the continuum. It is, however, also rather complicated [150]. A simpler system with similar characteristics is a doubly excited state in few-body systems, as helium. Here, it is possible [151-153] to simulate the whole sequence of events that take place when the interaction with a short light pulse first creates a wave packet in the continuum, including doubly excited states, and the metastable components subsequently decay on a timescale that is comparable to the characteristic time evolution of the electronic wave packet itself. On the experimental side, techniques for such studies are emerging. Mauritsson et al. [154] studied recently the time evolution of a bound wave packet in He, created by an ultra-short (350 as) pulse and monitored by an IR probe pulse, and Gilbertson et al. [155] demonstrated that they could monitor and control helium autoionization. Below, we describe how a simulation of a possible pump-probe experiment, targeting resonance states in helium, can be made. 

The Auger effect is an important process in solid state spectroscopy. One can use resonant Auger spectra to study the nature of core excitation in ionic solids by examining the Auger structure, the nature of the core holes can be determined, as well as the splitting of the states by the ligand field. 

Auger processes of all kinds (spectator and resonant) are also observed in molecules [268, 269, 270, 271, 272, 273, 274]. In addition, because of the very rapid dissociation of doubly-excited states in molecular species, neutral dissociation can occur before the decay of the core hole, and has been detected in the total ion yield of the H2S molecule [275]. 

The final example in this set is the pair of elements Ge and Sn [352], for which the outermost d subshell absorption spectra lie above the doubleionisation limit. As a result of Auger broadening of the parent ion core, very few Rydberg members are observed. As already noted in section 6.8, series become rather short when the parent ion state (the core hole) which serves as the series limit is broadened by Auger processes. The resonances arising by inner-shell excitation become very diffuse, and little can be done by way of detailed spectroscopy except to observe the leading series members. 

In atoms and molecules, shakeup satellites, corresponding to internal electronic transitions, are routinely observed using photoelectron and resonant Raman spectroscopy. In particular, shakeup satellites can be observed in the two particle spectrum, i.e., when two holes are left in the final state of an atom after electron emission. Satellite s strength can be strongly enhanced in the presence of a resonant intermediate state. For example, in copper atoms, the incident photon can first excite the core 3p electron to the 4s shell the core hole then decays to the 3d shell through the Auger process (with electron ejected from 3d shell) leaving two 3d holes in the final state [48]. For recent reviews of extensive literature the reader is referred to Refe. [49,50]). 

Fig. 3. Schematic, one-electron view of resonant magnetic scattering at the Ljjj absorption edge. The linearly polarised incident photon promotes a Ipj/j core electron into an empty state above the Fermi level. In the lanthanides there are 5d states available in the dipole approximation, and unfilled 4f states available through a quadrupole transition. Magnetic scattering results when the virtually excited electron decays, thereby filling the core hole and coherently emitting an elastically scattered photon.

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