Lenses electron

After the skimmer, the ions must be prepared for mass analysis, and electronic lenses in front of the analyzer are used to adjust ion velocities and flight paths. The skimmer can be considered to be the end of the interface region stretching from the end of the plasma flame. Some sort of light stop must be used to prevent emitted light from the plasma reaching the ion collector in the mass analyzer (Figure 14.2). 

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

An electron gun produces and accelerates the electron beam, which is reduced in diameter (demagnified) by one or more electromagnetic electron lenses. Electromagnetic scanning coils move this small electron probe (i.e., the beam) across the specimen in a raster. Electron detectors beyond the specimen collect a signal that is used to modulate the intensity on a cathode-ray tube that is scanned in synchronism with the beam on the specimen. A schematic of the essential components in a dedicated STEM system is shown in Figure 2. 

Ruska, E. (1980) The Early Development of Electron Lenses and Electron Microscopy (Hirzel, Stuttgart). 

Figure 2. Total ionization source of Rapp et al59 where F is the filament electron lenses are labeled 1, 2 and 3 guard plates G ion collector C1 and field plate C2 electron collector shield S electron collector cylinder T and electron collector plate P.

Electron interference devices, 22 169 Electron ionization source, 15 652-653 Electron lenses, 24 78 Electron microprobe analysis (EMA), 24 78, 109... 

In the optoelectronic X-ray image intensifier (Fig. 86), [5.427], the X-ray phosphor screen (input screen) is in direct optical contact with a photocathode that converts the luminance distribution of the X-ray screen into an electron-density distribution. The liberated electrons are accelerated in an electric field between the photocathode and an anode (20-30 kV) and are focused by electron lenses onto a second phosphor screen (output screen), where conversion of the electron image to a visible image takes place. 

High-resolution cryo-EM data can be collected in two forms as electron images (69) or as electron diffraction patterns. Cryo-EM images contain information on both amplitude and phase, which can be analyzed after Fourier transformation. The quality of the amplitude data can be improved if combined with electron diffraction data, which contains only amplitude information. In this way, EM overcomes one of the main difficulties in XRC. In XRC, only diffraction patterns are obtained. X-rays caimot be used to form an image of the crystal therefore, the phase information is lost. In contrast, electron microscopes contain electron lenses that can capture phase information. 

The most frequently-used lenses in electron optics are aperture lenses or tube (cylindrical) lenses. There is a substantial literature on the design and characteristics of electron lenses and on charged-particle optics. Harting and Read (1976), Hawkes and Kasper (1988) and Wollnik (1987) are especially useful. 

The resolution in TEM is limited by lens aberrations. In contrast to optical microscopy, where by serially ordering concave and convex lenses, aberrations can be compensated and hence the wavelength of the radiation is resolution limiting in TEM lens aberrations cannot be compensated since concave electron lenses are not feasible. The objective lens is the crucial part for image defining the microscope s resolution. The quality of the objective lens is described by the constants of spherical Cs(" 0.5-3mm) and chromatic aberration Cc ( 1-2 mm). Recently, microscopes equipped with complex correctors for the spherical aberration have become available. ... 

Acceleration once formed the ions are immediately extracted from this part of the instrument to be focussed and accelerated by a series of electronic lenses, to increase their kinetic energy. 

Electrospray ionization is similar in effect to the thermospray technique and is useful for similar applications. The difference resides in the use of a high electric field to nebulize the sample solution (or sample and eluant), creating droplets with excess electric charge. As the droplet solvent evaporates during traverse of a desolvation chamber, charge transfers to the analyte molecules and these are released as gaseous ions. A further refinement in this technique is the use of electronic lenses to direct ions more efficiently into the mass spectrometer. Because the analyte is not subject to heating, there is also less possibility for thermal decomposition of complex lipid components. 

FIGURE 15.7 Left, a photograph of a filament assembly. The filament is sandwiched between repeller and electron lenses. The electron beam direction is reversible by adjusting the voltages of the repeller and electron lenses. Right, a photograph of a filament assembly with the electron lens removed so that the filament assembly design is seen to be similar to that of a normal filament. 

Taylor, H.S., A theory of the catalytic surface, Proc. R. Soc. London (A), 108, 105-111, 1925. Ruska, E., The Early Development of Electron Lenses and Electron Microscopy, S. Hirzel Verlag, Stuttgart, 1980. 

E. Munro, Computer-aided Design of Electron Lenses by the Finite Element Method, Image Processing and Computer Aided Design in Electron Optics, P. W. Hawkes (ed.). Academic Press, London, 1973. 

Hawkes PW (ed.) (1982) Magnetic Electron Lenses. Berlin Springer. 

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