X-ray Absorption Spectroscopy (XAS)
- Practical Electron Microscopy and Database -
- An Online Book -

https://www.globalsino.com/EM/

 This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers. ================================================================================= Much of the spectral information obtainable from EELS is similar to that given by synchrotron-XAS (x-ray absorption spectroscopy), so that EELS in TEM has been referred to as a synchrotron in electron microscopes [1]. The electric field of an incident high energy x-ray can eject an electron of condensed matter. Page4777 lists the comparison between XAS and EELS. One important difference from EELS is that x-rays transfer all their energy by ejecting electrons from their initial state in the material. Irradiating by the x-ray, the electrons in the sample feel a force parallel to the polarization, resulting in a change of momentum. This force is due to a coupling between the electron and the x-ray (of angular frequency ω), given by,         ----------------------------- [4778a] where,           ε -- Unit vector specifying the polarization of the x-ray;           d -- Dipole operator, given by;           E0 -- Magnitude of the x-ray electric field.         ----------------------------- [4778b] where,           -e -- Charge of the electron. The cross-section for x-ray absorption from the ground state |ψ0> of the Hamiltonian H0 by applying Fermi's golden rule can be given by, [2]         ----------------------- [4778c] or,         ----------------------- [4778d]         ----------------------- [4778e] where,           ħ -- Reduced Planck constant;           c -- Speed of light. The most common way of performing XAS is to let x-rays penetrate a thin foil with typical thickness of 10 - 100 µm and then measure the fraction of the incident beam intensity penetrating the foil as a function of the energy of the incident x-rays. Therefore, an x-ray source, such as synchrotrons, that is intense over a large energy range is needed. For energies below an absorption edge, the x-rays penetrate rather easily without absorption because the Pauli exclusion principle prevents excitation, while above the absorption edge threshold the x-rays have sufficient energy to eject core electrons to empty states above the Fermi level. X-ray absorption extended fine structure (EXAFS) analysis generally uses inner-shell edges with binding energies of order 10 kV that provide a range of 1000 - 2000 eV of fine structure information. EXAFS is particularly useful for amorphous and highly disordered materials. The first use of density functional theory (DFT) for the calculation of X-ray absorption spectra was done by Müller et al. using a linearized augmented plane waves method in the late 70s [3].                 [1] Brown L M 1997 A synchrotron in a microscope Proc. EMAG97 (Cambridge) (Inst. Phys. Conf. Ser. 153) pp 17–21 [2] Fermi, E., Nuclear Physics. University of Chicago Press, 1950. [3] Müller, J.E., Jepsen, O., 1978. Systematic structure in the K-edge photoabsorption spectra of the 4d transition metals: theory. Phys. Rev. Lett. 40 (11), 720 - 722.

=================================================================================

The book author (Dr. Liao) welcomes your comments, suggestions, and corrections, please click here for submission. You can click How to Cite This Book to cite this book. If you let Dr. Liao know once you have cited this book, the brief information of your publication will appear on the “Times Cited” page. This appearance can help advertise your publication.