Diffraction, by electrons

As we have seen, the electron is the easiest probe to make surface sensitive. For that reason, a number of hybrid teclmiques have been designed that combine the virtues of electrons and of other probes. In particular, electrons and photons (x-rays) have been used together in teclmiques like PD [10] and SEXAFS (or EXAFS, which is the high-energy limit of XAES) [2, Hj. Both of these rely on diffraction by electrons, which have been excited by photons. In the case of PD, the electrons themselves are detected after emission out of the surface, limiting the depth of sampling to that given by the electron mean free path. 

The object of this paper is to discuss some of these problems. We start with the evidence for the new system of ionic radii. X-rays are diffracted by electrons in principle therefore X-ray diffraction should always locate the few outer electrons involved in bonding, but in fact this requires sophisticated treatment of meticulous measurements on crystals of high symmetry [6—9). But it has long been clear that some ionic crystals show round each ion an electron density which is approximately spherical but falls away to a very low background the following Table shows a typical example. 

In Chapter 6 it was pointed out that X rays are diffracted by electrons. Yel on page 528 h is staled that the anionic electrons of complexed cesium electride do nol show" in the structure determination. Discuss this apparent paradox. (Him Why is it hard to locale hydrogen atoms in m X-ray crystallographic determination/)... 

In X-ray diffraction (XRD), X-rays are diffracted by electrons surrounding the nuclei in atoms in a crystalline or polycrystalline solid. 

Neutron diffraction can be used to calculate electron deformation densities. This is achieved by making use of the fact that X-rays are diffracted by electron clouds and neutrons by nuclei. Thus, performing Fourier S5mthesis using the function yF ... 

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