Electron lenses 606 Subject

Optical Materials. The polyphosphazene skeleton is electron-rich, which means that it provides a refractive index increment compared to conventional saturated organic backbones. In addition, the macromolecular substitution synthesis aUows highly unsaturated organic side groups to be linked to the skeleton in ways that allow the refractive index, the color, the liquid crystalline, and nonlinear optical characteristics of the polymer to be finely tuned. Thus, the use of these polymers in opto-electronic (photonic) switches and lens systems is a subject of growing interest. 

Methods for the experimental measurement of the intensities of the diffracted beams will be described in Chapter 7, and methods for deriving relative phases of these beams for recombination will be discussed in Chapter 8. The result of these mathematical calculations, which simulate the action of a lens, will be all the information that is needed for the calculation of a three-dimensional electron-density map. This is a map in which peaks are situated at or near atomic positions. In this way, measurements of the diffraction pattern lead to an image of the molecules or ions and their arrangement in the crystal under study. Details of the calculation of the electron density maps that reveal the atomic arrangement will be described in Chapter 9. For more information on each aspect of diffraction by crystals, the reader is referred to the many texts on the subject listed in the Preface to this book. 

A particle-sampling mass spectrometer could provide a useful approach to measuring inhalation exposure to pollutants in a wide variety of environments. One example17 employs an aerodynamic lens that samples very fine particles and creates a beam that can be modulated to give a crude time-of-flight mass distribution. The particles impinge on a hot surface, causing vaporization of constituents that can be ionized (by electron impact or photoionization) and subjected to mass analysis by any of several kinds of mass analyzers. 

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