Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | X-ray Characterization of Nanoparticles 25 Energy keV a b Figure 2-6. a X-ray absorption spectrum of Au-L3 edge of a gold film. Note the modulation of the absorption coefficient above the edge b Pictorial representation of the interference process between ejected solid line and backscattered dashed line photoelectron waves that gives rise to the EXAFS oscillations. See text for explanations. seen as a low-energy electron diffraction process where the photelectron comes from an energy-selected atomic element. Concerning nanostructured materials many authors have tackled the problem of determining the inter-atomic distances by EXAFS 31-33 . This measurement is rather difficult to obtain in these systems by other means due to the intrinsic lack of 1 ong-rangc order in small particles. In the following sections we describe the basic process and the special features of EXAFS experiments on nanoparticles and illustrate these with a particular example. EXAFS In order to understand the physical origin of EXAFS oscillations wc must first remember that the probability for a core electron to absorb an X-ray photon depends on both the initial and final states. Above the edge the final state can Ire described by an outgoing spherical wave originating at the absorbing atom. Ulis wave may Ire seat-tered by neighboring atoms resulting in an interference pattern see big. 2-6h . The final state will depend on both outgoing and scattered wave phases which in turn will depend on the electron wavevector k or equivalently on the ejection energy. Hence the exact position of neighboring atoms can affect the probability of exciting a core electron and gives rise to the oscillatory behavior of the absorption coefficient as a function of photoelectron energy. Mathematically the interference term arising from scattering by a single neighbor can be expressed as A fe sin 2fer fe where k is the modulus of the wavevector r is the distance between absorbing and neighbor atoms and f k .
