Core level excitations

E of the incident polarized X-ray relative to the axis of the a and n orbitals. This, coupled with the fact that sharp core level excitations for elements C, N, O, and F occur in the soft X-ray spectral region, makes NEXAFS an ideal technique for probing molecular orientations of organic molecules. 

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 ... 

The electron-energy-loss spectroscopy (EELS) was performed in transmission with a primary beam energy of 170 keV in a purpose-built UHV spectrometer described in detail elsewhere [5]. For the valence level excitations and elastic scattering (electron diffraction) data the momentum resolution of the instrument was set to 0.04 A 1 with an energy resolution of 90-140 meV. The core level excitations were performed with a momentum and energy resolution of 0.2 A"1 and 90-140 meV, respectively. All EELS experiments were conducted at room temperature. 

Fig. 9 Resonant photoemission data from a film of Ce2 C72 recorded at photon energies crossing the Ce-N4 3 core-level excitation spectrum. The numbers refer to the energy positions as indicated in the right panel of Fig. 8 and are 1 94 eV 2 107.2 eV 3 110.2 eV 4 123 eV (at the maximum of the giant resonance, spectrum shown in black) 5 150 eV...

Now the question arises, as before, of the degree to which the rare earth s valence electrons (and in particular 5d) are populated by hybridisation with the electron-rich 7T-MO system of the C72 host molecule. Figure 9 shows resonant photoemission spectra of Ce2 C72, with photon energies selected to span the N4>5 core level excitation spectrum, as indicated in Fig. 8. 

Because of these problems, observations of the screening of the core-hole and the creation of excitons in core level excitation has become a favored technique for observing the metal-insulator transition the most utilized here has been core-level X-ray photoelectron spectroscopy (XPS)5. 

The variations in the detected intensity as a function of energy are orders of magnitude larger than the fine-structure modulation in core-level excitation cross-sections. 

This yields sensitivity to the surface within a photo-electron mean free path length of the photoexcited atom, which can be chosen with chemical selectivity. Compared to ARUPS, ARXPS is a more convenient approach for the extraction of structural information, since core-level excitations are more easily described than valence-level excitations. Angle-resolved detection is normally used to obtain the most complete structural information, which is interpreted in a procedure very similar to that of LEED with full simulation of the electron emission intensity as a function of energy or angle. 

Core-level excitation from a closed sub-shell results in the most intense and typically narrow photoelectron Unes. The width of a core level measured at half its maximum height, fuU width at half maximum (FWHM), is usually defined as the convolution of the width of the photon source, the natural line-width and the analyzer resolution. AU transitions (except from s levels) give rise to doublets due to spin-orbit coupling, in which the spin of the unpaired electron left in the orbital can couple in a paraUel or an anti-parallel manner with its orbital angular momentum. As all the relevant elements (except H) show at least one transition detectable by this technique, and the core levels of different elements are usually readily distinguishable, XPS is widely used for elemental analysis in the sampling depth of the topmost atomic layers. 

Another technique used for the study of core level excitation is electron energy loss spectroscopy (EELS). Experimentally an electron beam is incident on a target and the energy loss of the reflected or transmitted electrons is analyzed. 

There have been several articles which review EELS studies of solids [8.1-8.5]. For the EELS study of oxide superconductors, most of the research has been concentrated on the oxygen core level excitations, where the information about the density of hole carriers and their crystallographic confinement can be obtained [8.9]. Unlike the core level EELS studies, only a few low loss EELS studies have been reported for oxide superconductors [8.6, 8.10, 8.11]. 

These unique properties of core level excitation spectroscopies are briefly reviewed. The fundamentals of core level spectroscopies, such as X-ray absorption, X-ray absorption fine structure, inner-shell photoionization, electronic and radiative relaxation, and fragmentation in the regime of core level excitation, are outlined along with their characteristics to size effects... 

The attenuation of an X-ray beam by matter, in the typical photon energy regime of core level excitations (typically 100 eV E 100 keV), is given by the Beer-Lambert rule ... 

Various experimental approaches have been used in the past to correlate r with (N). These include electron diffraction, mass spectrometry, and molecular beam scattering experiments. The latter approach is especially suitable for the low ( )-regime. The typical value range that is available for core level excitation on krypton clusters in our experiment is (N) < 1700, if the correlation of Farges et al. is used. Somewhat lower values are obtained, if the more recent, but likely more realistic calibration of Karnbach et cd. is applied (cf. Table 1). ... 

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