Fast electrons, energy loss

Local composition is very useful supplementary information that can be obtained in many of the transmission electron microscopes (TEM). The two main methods to measure local composition are electron energy loss spectrometry (EELS), which is a topic of a separate paper in this volume (Mayer 2004) and x-ray emission spectrometry, which is named EDS or EDX after the energy dispersive spectrometer, because this type of x-ray detection became ubiquitous in the TEM. Present paper introduces this latter method, which measures the X-rays produced by the fast electrons of the TEM, bombarding the sample, to determine the local composition. As an independent topic, information content and usage of the popular X-ray powder dififaction database is also introduced here. Combination of information from these two sources results in an efficient phase identification. Identification of known phases is contrasted to solving unknown stmctures, the latter being the topic of the largest fiaction of this school. 

Another mechanism for electron energy loss in matter is the emission of Cerenkov radiation. When a beam of fast moving charged particles with a velocity... 

A direct measurement of the electronic energy loss as a function of the impact parameter is a hard task to be performed from the experimental point of view and only a few experiments have been performed for fast light ions. Experiments in gas targets under single collision condition provide a more direct and precise comparison of the theoretical results with the experimental data. Here we compare the results of the coupled-channel method for collisions of protons with He as a function of the projectile scattering angle. 

This expression highlights an important distinction between EELS and optical experiment, namely that the virtual photon field associated with the fast electron is longitudinal ( 11 ) whereas the real photon field is transverse (E q) [3.23]. This difference does not affect the main point of the photon analogy namely that electron energy loss and optical experiments measure the same quantity e(a>) in most (and practically all the) cases. It does mean that the polarisation of the electric field acting on the atomic system is defined differently in both cases, a fact of obvious importance for anisotropic materials. 

Cerenkov radiation accounts for a very minor part of the energy loss of fast electrons. Its main importance is for monitoring purposes and establishment of a reference time, since it is produced almost instantaneously with the passage of the particle. Katsumura et al. (1985) have observed a very fast rise of solute fluorescence attributable to the Cerenkov effect the G value for this process is estimated to be -0.02. 

A fully realistic picture of solvation would recognize that there is a distribution of solvent relaxation times (for several reasons, in particular because a second dispersion is often observable in the macroscopic dielectric loss spectra [353-355], because the friction constant for various types or modes of solute motion may be quite different, and because there is a fast electronic component to the solvent response along with the slower components due to vibration and reorientation of solvent molecules) and a distribution of solute electronic relaxation times (in the orbital picture, we recognize different lowest excitation energies for different orbitals). Nevertheless we can elucidate the essential physical issues by considering the three time scales Xp, xs, and Xelec-... 

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