Core-hole decay

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. 

The EAPFS excitation cross section is monitored by fluorescence from core - hole decay (also known as SXAPS). 

Monoenergetic photons excite a core hole. The modulation of the absorption cross section with energy at 100 - 500 eV above the excitation threshold yields information on the radial distances to the neighbouring atoms. The cross section can be measured by fluorescence as the core holes decay or by attenuation of the transmitted photon beam. EXAFS is one of the many fine -structure techniques. 

A more surface - sensitive version of EXAFS where the excitation cross - section fine structure is monitored by detecting the photoemitted electrons (PE-SEXAFS), Auger electrons emitted during core - hole decay (Auger-SEXAFS), or ions excited by photoelectrons and desorbed from the surface (PSD-SEXAFS). 

Fine structure experiments are often carried out with synchrotron sources, since the initial electron state is better defined for photoemission than for electron excitation. When core-hole decay is detected by Auger or secondary electron emission, the technique is surface sensitive. Core-hole decay can also be detected by fluorescence, or by adsorption of the incident photon beam. These methods are not intrinsically surface sensitive, but they are useful when the source atoms are exclusively located at the surface. 

There are three main detection modes for EAPFS within the appearance potential spectroscopy (APS) technique./31/ First, one may monitor soft-x-ray emission due to the decay of the core hole left by the primary process. This is called SXAPS-EAPFS (Figure le). Second, it is also possible to monitor Auger electrons due to the same core-hole decay, as in AEAPS-EAPFS and AMEFS-EAPFS, cf. Figure If. Third, one may measure the remaining total intensity of... 

Higher-order processes contribute in both the excitation of the core electron and relaxation of the core hole. These multi-electron processes are significant because of the strong perturbation caused by the creation or the annihilation of a core hole. In these processes, additional electrons are excited (shake-up) or ionized (shake-off). Two electron processes, double autoionization and double Auger decay, were mentioned above. The final states reached in core hole decay may be excited states and also may autoionize. It is clear that excitation of a core electron and the relaxation of the core hole provide many paths leading to multiple-electron excited states. These states have a unique chemistry relative to the single-electron excited states produced by arc lamp, laser, or vacuum ultraviolet (VUV) excitation. 

The detection of fluorescence provides a high-resolution probe of the electronic decay channels and serves to detect neutral and ionic products in excited states. This technique has been implemented in only a few cases. Techniques for detecting ground-state neutral products of core hole decay also need to be applied. 

The absorption can also be measured by recording core-hole decay products in the case of diluted systems. The inner shell photoionization process can be described as a two-step process. In the first step the photon excites a core hole-electron pair, and in the second step the recombination process of the core hole takes place. There are many channels for the core hole recombination. These channels can produce the emission of photons, electrons or ions, which can be collected with special detectors. The recombination channel that is normally used to record bulk x-ray absorption spectra of dilute systems is the direct radiative core-hole decay producing x-ray fluorescence lines. In Fig. 3 a beam line with an apparatus to record absorption spectra in the fluorescence mode is represented schematically. 

SXAPS and AEAPS spectra may differ largely because of the core hole decay mechanisms following the excitation of the core electrons in these spectroscopies. In SXAPS X-ray emission is slow and core hole production and de-excitation are only weakly coupled. On the other hand, in AEAPS Auger decay is fast and the excitation and decay are strongjy coupled. This may lead to some broadening of structure in AEAPS. Dose et al. [94] have observed in solid Ni the smearing of the threshold slope and structure in the AEAPS spectrum as compared to the SXAPS spectrum. APS is, however, not limited to solid metals only. With proper experimental arrangement it could be extended to the study of liquid metals, as has been demonstrated by Dose et al. [94]. 

The widths and profiles of Miv-v absorption lines jnovide important information on the 4f potential barrier which shields 4f electrons fiom the external electrons in a R solid. This effect is more or less visible in the widths and profiles of absorption lines. The lines in the Mv spin-orbit group are almost symmetric and are well represented by a Lorentzian shape. The Miv lines are asymmetric and exhibit a Fano-like profile. Final states with either 3ds/2 or 3ds/2 core holes decay primarily by Auger transitions. The fluorescence decay is small (Connerade and Kamatak 1981). States built on the 3d3/2 hole state have an additional decay channel, leading to autoionization if they interact with... 

---