Excitation of core

Inelastic scattering processes are not used for structural studies in TEM and STEM. Instead, the signal from inelastic scattering is used to probe the electron-chemical environment by interpreting the specific excitation of core electrons or valence electrons. Therefore, inelastic excitation spectra are exploited for analytical EM. 

The primary features in Eig. 2.7 are peaks arising from excitation of core-level electrons according to Eq. (2.1). At the low-energy end, two intense peaks are found at... 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

X-ray absorption near edge structure (XANES) The X-ray absorption spectrum, as for EXAFS, may also show detailed structure below the absorption edge. This arises from excitation of core electrons to high level vacant orbitals, and can be used to estimate the oxidation state of the metal ion. 

In Sections 1.2 and 1.3, we focused on transitions between valence orbitals spanning a range of 5 eV ( 40,000cm-1). Here we consider excitations of core... 

Resonant excitation of core levels involves the use of tunable soft- and hard-X-rays. This energy regime has widely been used since decades for a variety of investigations on atomic, molecular, surface and condensed matter physics, where synchrotron radiation is currently the primary X-ray source for experimental work in this field. Earlier work on resonant excitation of core levels has already shown that there are numerous unique properties, such as e.g. element specific excitations. Methods for determining structural properties, as well as site-selective fragmentation, promised important applications with respect to fundamental and applied research on size effects of matter. 

NEXAFS is a synchrotron-based spectroscopic tool routinely used as a complementary technique with XPS for surface characterizations. This method probes the adsorption of X-rays by the excitation of core (K-shell) electrons into unoccupied electronic states near the ionization limit. Subsequent emission of Auger electrons results in the formation of an NEXAFS electron yield the observed spectmm. Because the source of Auger electrons can extend only up to 10 nm and the spectral peak positions and intensities are directly related to the nature of unoccupied electronic states, NEXAFS spectroscopy provides an important tool for studying stmctural and chemical features of various surface thin films and coatings (Hemraj-Benny et al., 2006 Hahner, 2006). 

Evidence for the presence of core holes in sputtered atoms/ions can be identified in the spectra from several techniques, with the most direct being that of Ion-induced Auger Electron Spectroscopy (lAES). This is understood as lAES spectra arises from the de-excitation of core holes formed as a result of atomic collisions induced through energetic ion impact on the solid s surface of interest. De-excitation in the sputtered population is, however, only noted in spectra collected at close to glancing angles (reasons are outlined in Section 3.2.1.2). This is noted as ... 

S Kinetic Emission Model The Kinetic Emission model proposed by Joyes (.loyes 1963, 1969a, 1969b) was developed to explain the yield trends exhibited by the positive multiply charged atomic ions from elements lighter than Phosphorus. This mechanism describes the formation of multiply charged ions as being formed as a result of the de-excitation of core holes present in the sputtered atomic/ionic populations, i.e. an auto-ionization process. 

Fig. 5. Fluorescence and Auger processes for de-excitation of core hole states.

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